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Eukaryotic Cell, May 2006, p. 836-848, Vol. 5, No. 5
1535-9778/06/$08.00+0     doi:10.1128/EC.5.5.836-848.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Unique Posttranslational Modifications of Chitin-Binding Lectins of Entamoeba invadens Cyst Walls

Katrina L. Van Dellen,1 Anirban Chatterjee,1 Daniel M. Ratner,1,2 Paula E. Magnelli,1 John F. Cipollo,1 Martin Steffen,3 Phillips W. Robbins,1 and John Samuelson1,4*

Department of Molecular and Cell Biology, Boston University Goldman School of Dental Medicine, Boston, Massachusetts 02118,1 Section of Infectious Diseases, Boston Medical Center, Boston, Massachusetts 02118,2 Departments of Genetics and Genomics,3 Microbiology, Boston University School of Medicine, Boston, Massachusetts 021884

Received 17 January 2006/ Accepted 3 March 2006


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ABSTRACT
 
Entamoeba histolytica, which causes amebic dysentery and liver abscesses, is spread via chitin-walled cysts. The most abundant protein in the cyst wall of Entamoeba invadens, a model for amebic encystation, is a lectin called EiJacob1. EiJacob1 has five tandemly arrayed, six-Cys chitin-binding domains separated by low-complexity Ser- and Thr-rich spacers. E. histolytica also has numerous predicted Jessie lectins and chitinases, which contain a single, N-terminal eight-Cys chitin-binding domain. We hypothesized that E. invadens cyst walls are composed entirely of proteins with six-Cys or eight-Cys chitin-binding domains and that some of these proteins contain sugars. E. invadens genomic sequences predicted seven Jacob lectins, five Jessie lectins, and three chitinases. Reverse transcription-PCR analysis showed that mRNAs encoding Jacobs, Jessies, and chitinases are increased during E. invadens encystation, while mass spectrometry showed that the cyst wall is composed of an ~30:70 mix of Jacob lectins (cross-linking proteins) and Jessie and chitinase lectins (possible enzymes). Three Jacob lectins were cleaved prior to Lys at conserved sites (e.g., TPSVDK) in the Ser- and Thr-rich spacers between chitin-binding domains. A model peptide was cleaved at the same site by papain and E. invadens Cys proteases, suggesting that the latter cleave Jacob lectins in vivo. Some Jacob lectins had O-phosphodiester-linked carbohydrates, which were one to seven hexoses long and had deoxysugars at reducing ends. We concluded that the major protein components of the E. invadens cyst wall all contain chitin-binding domains (chitinases, Jessie lectins, and Jacob lectins) and that the Jacob lectins are differentially modified by site-specific Cys proteases and O-phosphodiester-linked glycans.


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INTRODUCTION
 
The infectious form of Entamoeba histolytica, a protozoan parasite that causes amebic dysentery and liver abscesses, is the quadranucleate cyst (10, 15, 25). Because E. histolytica does not encyst in axenic culture, cyst formation has been studied in Entamoeba invadens, a reptilian pathogen that also forms quadranucleate cysts (7, 31). E. invadens converts to chitin-walled cysts within 2 days when subjected to osmotic shock and/or glucose deprivation (30). The E. invadens cyst wall resembles the E. histolytica cyst wall, and E. invadens cysts are able to excyst readily when placed in full medium. Cyst formation is blocked by the addition of inhibitors of Cys or Ser proteases, although the mechanism of inhibition is unclear (21, 23). E. histolytica has 20 lysosomal Cys protease genes, 8 of which are transcribed by trophozoites in culture, while E. invadens has multiple Cys proteases (4, 23).

The most abundant protein in the cyst wall of E. invadens is the Jacob lectin (EiJacob1), a secreted glycoprotein that contains five tandemly arranged chitin-binding domains (CBDs) (9). Each EiJacob1 CBD contains six conserved Cys residues and numerous conserved aromatic amino acids. EiJacob1 was identified by sequencing the largest of >20 spots on two-dimensional (2-D) gels containing proteins isolated from purified E. invadens cyst walls. The finding that quadranucleate, "wall-less" cysts are formed when encystation is induced in the presence of galactose suggested a two-lectin model of E. invadens cyst wall formation. A plasma membrane galactose lectin binds sugars on Jacob, and the Jacob lectin, in turn, cross-links chitin fibrils (9, 16).

Entamoeba chitinases, which may be involved in remodeling the walls of encysting parasites or in degrading the walls of excysting parasites, have a unique, N-terminal CBD that contains eight conserved Cys residues (6, 29). The same eight-Cys CBD is also present in a family of E. histolytica lectins named Jessie, which have an unknown function. The six-Cys and eight-Cys CBDs of Entamoeba are reminiscent of, but not homologous to, the Cys-rich CBDs found in insect chitinases and peritrophic membrane proteins and in plant lectins, such as wheat germ agglutinin (26, 35).

E. histolytica trophozoites have two types of glycans on their surfaces. Asn-linked glycans (N-glycans) are built upon a truncated lipid precursor that contains 7 sugars (Man5GlcNAc2) rather than the 14 sugars present in precursors from animals, plants, and fungi (Glc3Man5GlcNAc2) (22). E. histolytica N-glycans include unprocessed Man5GlcNAc2 as well as complex glycans containing Gal and Glc (our unpublished data). E. histolytica trophozoites also have proteophosphoglycans, which are glycosylphosphatidylinositol-anchored peptides with O-phosphodiester-linked glycans (O-P-glycans), on their surfaces (18). The O-P-glycans, which are immunogenic, contain Gal at the reducing end and are extended by Glc.

The goal of the present study was to answer a number of questions concerning the E. invadens cyst wall, as follows. First, how many genes encoding Jacob lectins, Jessie lectins, and chitinases are present in the E. invadens genome, and are they encystation specific like EiJacob1 and E. invadens chitinase 1? Second, what are the identities of the >20 uncharacterized spots present on two-dimensional gels containing E. invadens cyst wall proteins? Are they mostly Jacob lectins, which cross-link chitin, or are there other chitin-binding proteins in the cyst wall? Can other proteins be identified by mass spectrometry (MS) of tryptic fragments of cyst wall proteins? Third, what are the posttranslational modifications (e.g., glycosylation or proteolytic processing) of Jacob lectins and other proteins present in the E. invadens cyst wall? What is the endopeptidase(s) that cleaves between CBDs of Jacob lectins?


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MATERIALS AND METHODS
 
Database searches and sequence assembly. The E. invadens genome has been extensively sequenced, so complete genes were most often present within large contigs assembled at The Institute for Genomic Research (http://www.tigr.org/tdb/e2k1/eha1/) (31). E. invadens Jacob lectins were identified in TBLASTN searches of the strain IP-1 genomic DNA sequence using the EiJacob1 lectin sequence (GenBank accession number AF175527 [GenBank] ) (1, 9). Five novel E. invadens Jacob lectin genes were complete (EiJacob2 to EiJacob6), while EiJacob7 was missing the 3' end. Five complete E. invadens Jessie lectins (EiJessie1a, -1b, and -1c and EiJessie3a and -3b) were identified with the sequences for E. histolytica Jessie lectins 1 to 3 (GenBank accession numbers AF401986 [GenBank] to AF401988 [GenBank] ) (1, 29, 31). PCR was used to link the 5' and 3' portions of the EiJessie3a gene. Two additional complete E. invadens chitinase genes (E. invadens chitinases 2 and 3) were identified with the E. invadens chitinase 1 sequence (GenBank accession number AAB52724 [GenBank] ) (1, 6, 31).

Signal peptides and transmembrane helices of E. invadens proteins were predicted using SignalP and TMHMM, respectfully (14, 19). Jessie lectin and chitinase sequences were aligned using ClustalW (27).

RT-PCR analysis of Jacob expression in encysting E. invadens. E. invadens strain IP-1 was induced to encyst by transferring cells from TYI-S-33 to 47% low-glucose (LG) medium (8, 9, 30). For RNA preparations, cells were harvested immediately after the transfer (0 h) and after 22 h in 47% LG medium. Total RNA was isolated using an RNAqueous-4PCR kit (Ambion, Austin, TX) and was treated with DNase I for 1 h at 37°C. First-strand cDNA synthesis was done with RETROscript (Ambion), using oligo(dT) primers. Reverse transcriptase (RT) was omitted from negative control reactions. PCR was performed for 25 cycles of 30 s at 95°C, 30 s at 50°C, and 1 min at 72°C, using AmpliTaq Gold (Applied Biosystems, Foster City, CA). PCR primers, all of which were tested with E. invadens genomic DNA, are listed in Table 1.


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  		TABLE 1. Primers used for RT-PCR analysis

Mass spectrometry of tryptic fragments of cyst wall proteins. E. invadens strain IP-1 was induced to encyst, and purified cyst walls were obtained by previously described methods (9). Previous transmission microscopy had shown that E. invadens cyst wall preparations are, for the most part, clean of membranes or organelles (9). E. invadens cyst wall proteins were released in 1% sodium dodecyl sulfate (SDS), separated by 1-D SDS-polyacrylamide gel electrophoresis (SDS-PAGE), and stained with Coomassie blue. The entire set of cyst wall proteins or prominent protein bands was cut out and digested in the gel with trypsin (24). Reverse-phase chromatography was carried out using a nano-HPLC pump and autosampler (Surveyor and MicroAS, respectively; Thermo Finnigan, San Jose, CA) on a 10-cm by 100-µm ID MAGIC C18 reverse-phase capillary column (Michrom, Auburn, CA) at the Boston University Proteomics Core Facility and the MIT CCR Biopolymers Laboratory. Peptides were separated using gradients of 2% to 98% acetonitrile over 30 to 200 min in the presence of 0.1% formic acid (36). Peptides were analyzed using an LTQ ProteomeX ion-trap mass spectrometer (Thermo Finnigan, San Jose, CA), and mass spectra were compared to tryptic digests of predicted E. invadens proteins (see below) by using SEQUEST (12). Tryptic peptides with a SEQUEST XCorr score of >1.75, 2.5, or 3.5 for a Z value of 1, 2, or 3, respectively, a peptide probability of <0.01, and a protein probability of <0.001, and proteins with two or more high-scoring fully tryptic peptides were considered present.

Because the E. invadens genome sequence is incomplete, it was necessary to manually curate the set of E. invadens proteins used to identify the tryptic peptides (31). The E. invadens proteins included all those present in GenBank, as well the Jacobs, Jessies, and chitinases identified here. In addition, >100 other E. invadens proteins which have signal peptides or transmembrane helices (Cys peptidases, Gal/GalNAc lectins, receptor kinases, etc.) were included. A set of >30 E. invadens cytosolic proteins included thioredoxins, actins and associated proteins, and tubulins. The mass spectrometry results for E. invadens cyst walls were also searched using predicted tryptic peptides from the E. invadens genome sequence (updated 24 January 2006), translated in all six frames (31).

Compositional analysis was performed using the BioWorks 3.2 protein area/height calculation. In short, this algorithm generates a reconstructed ion chromatogram by calculating the precursor mass from the raw data. The smoothed reconstructed ion chromatogram is used to integrate the peak areas for all peptides identified by the SEQUEST algorithm. These integrals are used to create an approximate composition of the sample, as determined by the percentage of the total integral comprising the integral of a given peptide.

Mass spectrometric identification of unmasked phosphopeptides. Isolated cyst wall proteins were treated with 40 mM trifluoroacetic acid (TFA) at 95°C for 10 min to cleave O-P-glycans. The subsequent proteins contained an O-phosphate modification at the serine or threonine that had been occupied by O-P-glycan. These proteins were digested with trypsin (as previously described), and the tryptic peptides were subjected to mass spectrometric identification. Phosphopeptides were identified with SEQUEST or X!Tandem (The Global Proteome Machine Organization) by searching for a neutral loss of 98 Da, 49 Da, or 33 Da from parent ions for a Z value of 1, 2, or 3, respectively. This neutral loss event created a dominant peak with the mass of the parent peptide less a phosphate and water (2, 12). Peptide identification was performed on the suppressed y and b ions when this characteristic neutral loss was observed.

Two-dimensional SDS-PAGE analysis of cyst wall proteins. For each gel, a 20- to 30-µl packed volume of purified walls was boiled in 2 volumes of SDS buffer (50 mM Tris-HCl, pH 6.8, 2% SDS, 5% ß-mercaptoethanol, 5% glycerol). Forty microliters of the supernatant from subsequent microcentrifugation was added to 360 µl of CHAPS buffer (7 M urea, 2 M thiourea, 2% CHAPS, 2% ampholytes, 65 mM dithiothreitol, Serdolit MB-1, and 0.01% bromophenol blue) and electrophoresed in a two-dimensional gel (20). 2-D gel electrophoresis was performed using an Investigator 2-D electrophoresis system (Genomics Solutions, Ann Arbor, MI) by K. Doud at the Harvard University NIEHS Center for Environmental Health Proteomics Facility. Precast gels contained ampholytes from pH 4 to 7 in the first (isoelectric focusing) dimension (Amersham Biosciences, Piscataway, NJ) and a gradient of acrylamide from 8 to 18% in the second (SDS-PAGE) dimension. Molecular weight markers used for the second dimension were broad-range SDS-PAGE standards (Bio-Rad, Hercules, CA). SYPRO Ruby protein gel stain (Molecular Probes, Eugene, OR) was used to detect proteins in 2-D gels. Glycoproteins were detected with Pro-Q Emerald 488 glycoprotein stain (Molecular Probes) (11). The stained gels were imaged on a Molecular Imager FX Pro Plus system (Bio-Rad), using a 532-nm diode-pumped solid-state laser (SYPRO Ruby) or a 488-nm argon-ion laser (Pro-Q Emerald 488).

N-terminal sequencing of cyst wall proteins. Cyst wall proteins were separated by 2-D gel electrophoresis and transferred to an Immobilon-P membrane (Millipore, Billerica, MA) using a Panther HEP-1 semidry electroblotter (Owl Separation Systems, Portsmouth, NH) and a three-buffer system (3). The blot was stained with Coomassie blue, and spots were excised for N-terminal protein sequencing by automated Edman degradation. Sequencing was performed by D. McCourt at Midwest Analytical (St. Louis, MO), using protein sequencers from Applied Biosystems. N-terminal sequences of 2-D spots were searched against peptide sequences predicted from the E. invadens strain IP-1 genomic sequence reads and scaffold assemblies.

Cleavage of a model peptide by endogenous and exogenous peptidases. A synthetic peptide (TPSVDKNEDI) labeled at the N terminus with fluorescein isothiocyanate (FITC) was made at the Harvard Biopolymers Laboratory. The peptide (molecular mass, 1,475.5 Da; 0.03 mg/ml) was digested with 20 µM trypsin (positive control) in 1 mM CaCl2, 50 mM Tris-HCl (pH 8), or 23 µM papain in 2.7 mM cysteine-HCl, 0.27 mM sodium EDTA (pH 4.5). Alternatively, the peptide was digested at pH 8 with a lysate of E. invadens trophozoites, which was prepared by sonicating amebae in the presence of 0.1% Triton X-100 in phosphate-buffered saline on ice. All assays were performed at 37°C with a shaking Thermomixer (Eppendorf AG, Germany). Protease inhibitors (E-64 and phenylmethylsulfonyl fluoride [PMSF]) were added to a final concentration of 2 mM. Reactions were stopped by adding 3 volumes of 100% methanol, and reaction mixtures were centrifuged through Microcon spin columns with a 3,000-Da cutoff (Millipore) at 10,000 x g for 10 min at 4°C to remove proteins that might interfere with mass spectrometric analysis. Peptide samples (<3,000 Da) were desalted using a ZipTip C18 reverse-phase microtip (Millipore). Mass spectrometry of the peptides was performed in reflector mode on a Reflex IV mass spectrometer (Bruker, Germany) with a laser power of 15 to 20% in the presence of 10 mg/ml of {alpha}-cyano 4-hydroxycinnamic acid matrix and peptide standards (Bruker, Germany).

Analysis of O-P-glycans released by mild acid hydrolysis. Purified cyst walls (12 mg) were extracted with 400 µl of 40 mM TFA for 12 min at 90°C to release O-P-glycans (17, 18). Residual cyst wall material was pelleted, washed with 100 mM Tris-HCl, pH 8.0, and boiled in 2 volumes of SDS buffer for 2-D gel analysis. The pH of the TFA supernatant was adjusted to 4.8, and 1 volume of methanol was added to precipitate proteins. After a 5-h incubation at –20°C, insoluble material was cleared by centrifugation, and the supernatant containing the released O-P-glycans was dried in a rotary evaporator. The O-P-glycans were labeled by converting their free reducing ends to alditols, as follows. The dried free O-P-glycans were incubated with 250 µl of 0.01 M NaOH-0.01 M NaB3H4 (50 µCi) for 6 h at 25°C. The reaction was stopped with acetic acid, and the excess of borate was removed by repeated distillation from 1% acetic acid-methanol.

The labeled reduced O-P-glycans were separated by BioGel P4 chromatography (110- by 1-cm column) in 0.1 M acetic acid-1% butanol. Fractions of 400 µl were collected, and aliquots were taken for scintillation counting. Peak fractions were recovered and dried. To determine the identities of the reduced sugars, samples were taken from each peak and hydrolyzed in 2 M HCl for 2 h at 90°C. After the samples were cooled down on ice, acid was removed by evaporation. The reduced radiolabeled sugars were chromatographed in a Dionex instrument, using a CarboPac MA1 column, in 300 mM NaOH for 30 min. The identities of the reduced sugars were determined by their comigration with internal alditol standards, including glucosaminitol, galactosaminitol, fucitol, arabitol, rhamnitol, xylitol, glucitol, mannitol, and galactitol. As a control, total O-P-glycans were released from E. histolytica trophozoites (18). Total acid hydrolysis and monosaccharide analysis were performed as described above for E. invadens cyst wall glycans.

Mass spectrometry was also used to characterize cyst wall glycans released with TFA. Briefly, released glycans were chromatographed on Biogel P4, and peaks were isolated, dried with a speed evacuation apparatus, and permethylated using methyl sulfoxide-NaOH (5). Matrix-assisted laser desorption ionization-time of flight MS was performed on permethylated glycans, using a Bruker Reflex IV instrument fitted with a nitrogen laser.

Nucleotide sequence accession numbers. Nucleotide and derived amino acid sequences for EiJacob2 to EiJacob7 have been submitted to GenBank with accession no. ABC59317 [GenBank] to ABC59322 [GenBank] . EiJessie1a to EiJessie1c were deposited with accession no. ABC59325 [GenBank] to ABC59327 [GenBank] . EiJessie3a and -3b have accession no. ABC59328 [GenBank] and ABC59329 [GenBank] , respectively. E. invadens chitinase 2 and E. invadens chitinase 3 have accession no. ABC59330 [GenBank] and ABC59331 [GenBank] , respectively.


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RESULTS
 
E. invadens genomic sequences predicted seven Jacob lectins, five Jessie lectins, and three chitinases. We were able to identify six new putative Jacob lectins (EiJacob2 to EiJacob7) in a TBLASTN search of the available sequence data, using EiJacob1 as a probe (Fig. 1) (1, 9, 29, 31). Each of the putative Jacob lectins contained multiple six-Cys CBDs, which showed highly conserved spacing of Cys residues as well as conserved aromatic residues that may participate in chitin binding (34). Each Jacob sequence predicted a signal peptide, while none of the Jacob lectins (or Jessie lectins and chitinases [described below]) had transmembrane helices (14). Four E. invadens Jacob lectins (EiJacob1 to EiJacob4) had five closely spaced six-Cys CBDs, while EiJacob5 had just three CBDs (Fig. 1A). Between the CBDs of these Jacob lectins were short, low-complexity, Ser- and Thr-rich spacers. Some of these spacers contained conserved sequences (e.g., TPSVDKS), which are the sites of endoprotease cleavage (see the proteomic data below).


Figure 1
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  		FIG.1. Primary structures of seven predicted E. invadens Jacob lectins, with locations of tryptic peptides and N-terminal sequences marked. (A) EiJacob1 to EiJacob5 contain short Ser- and Thr-rich spacers between CBDs. (B) EiJacob6 and EiJacob7 contain a large, low-complexity region between CBDs. Stop codons are marked with asterisks, while the incomplete sequence at the C-terminal end of EiJacob7 is marked by a pound symbol (#). Italics mark signal peptides. CBDs for which Cys residues are shown enlarged and in bold are numbered from the N to the C terminus. Conserved peptides (e.g., TPPVD) prior to proven or hypothetical endoprotease sites are also shown enlarged and in bold. Periods mark gaps in the alignments of CBDs, while spaces indicate sites of identified endoprotease cleavages. Single underlines mark tryptic peptides identified by mass spectrometry, while double underlines mark N-terminal sequences obtained by Edman degradation of proteins isolated from 2-D gels (see Fig. 5B). Shaded residues mark Ser residues shown by mass spectrometry to be sites of O-phosphodiester glycosylation (see Fig. 9).

Two novel Jacob lectins (EiJacob6 and EiJacob7) contained much longer low-complexity sequences between CBDs (Fig. 1B). These low-complexity sequences were rich in Ser, Glu, Lys, and His and contained several types of short internal repeats. EiJacob6 contained four predicted endoprotease sites between six tandemly arranged CBDs. The low-complexity sequences of EiJacob7, which were flanked by single CBDs, contained internal repeats that included eight predicted endoprotease sites. The long low-complexity sequences of these E. invadens Jacob lectins resemble those predicted for a second E. histolytica Jacob lectin that contains three CBDs (15, 29; unpublished data).


Figure 1
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  		FIG. 1— Continued.

The E. invadens genome predicted five Jessie lectins, three of which had a signal peptide, an N-terminal eight-Cys lectin domain, and little else (EiJessie1a to EiJessie1c) (Fig. 2). This was similar to the case for EhJessie1, EhJessie2, and a fourth predicted E. histolytica Jessie lectin (29; unpublished data). The other E. invadens Jessie lectins (EiJessie3a and -3b) had an N-terminal eight-Cys CBD, a Ser- and Thr-rich low-complexity spacer, and a conserved C-terminal domain, which is also present in EhJessie3 (29). The EhJessie3 C-terminal domain shares 17% identity with a hypothetical Clostridium botulinum protein predicted from genomic sequence data (1, 13, 29). The N terminus of this C. botulinum protein, which does not share significant sequence homology with EhJessie3, is likely a bacterial CBD that is also present in a putative glycosyl hydrolase from Clavibacter michiganensis and in a chitinase and chitodextrinase from Microbulbifer degradans (13; data not shown). These results suggested the likelihood that the unknown domain common to Entamoeba Jessie3 lectins and the putative C. botulinum protein is an enzyme involved in modifying chitin.


Figure 2
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  		FIG. 2. Primary sequences of E. histolytica and E. invadens Jessie lectins aligned with each other and with that of a predicted Clostridium botulinum protein. Sequences are marked as described in the legend to Fig. 1. The caret indicates that the C. botulinum protein, which is complete in the database, has been truncated here. A putative N-terminal CBD in the C. botulinum protein is not shown.

The E. invadens genome predicted two additional chitinases (E. invadens chitinase 2 and E. invadens chitinase 3), each of which contained a signal peptide but was lacking the N-terminal eight-Cys CBD present in E. invadens chitinase 1 (Fig. 3) (6, 29). Multiple chitinase activities have previously been identified in encysting E. invadens (6). The presence of the N-terminal eight-Cys CBD in E. invadens chitinase 1 is likely important because E. invadens chitinase 1 was abundant in the E. invadens cyst wall, while E. invadens chitinase 2 and chitinase 3 were not detected (see the proteomic data below).


Figure 3
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  		FIG. 3. Primary sequences of E. invadens chitinases with Cys-rich domains and tryptic peptides, marked as described in the legend to Fig. 1. While E. invadens chitinase 1 has an eight-Cys CBD, this domain is missing from E. invadens chitinase 2 and chitinase 3.

mRNA levels for most E. invadens Jacob lectins, Jessie lectins, and chitinases increased during encystation. To determine if mRNA levels for the predicted E. invadens Jacobs, Jessies, and chitinases increase during encystation, as previously shown for EiJacob1 and E. invadens chitinase 1 (6, 8), RT-PCR was performed using RNAs isolated from parasites collected 0 h and 22 h after transfer into encystation medium (Fig. 4). After 22 h in encystation medium, mRNA levels for most, but not all, of the Jacob lectins, Jessie lectins, and chitinases appeared higher than they were at 0 h, while mRNA levels for the housekeeping proteins actin and elongation factor 1{alpha} (EF1{alpha}) remained the same. No quantification of these changes was attempted, because Jacob lectins, Jessie lectins, and chitinases were directly identified in the cyst walls by proteomic methods. Low levels of some Jacob, Jessie, and chitinase mRNAs could be detected at 0 h, most likely because some E. invadens trophozoites spontaneously encyst in regular culture medium. These results suggested that Jacob, Jessie, and chitinase genes are expressed during encystation but did not determine the fate of the encoded proteins, which is addressed below.


Figure 4
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  		FIG. 4. RT-PCR analysis of Jacob and Jessie lectin expression in encysting E. invadens. Total RNA for RT-PCR analysis was isolated 0 and 22 h after transfer of cells to encystation medium. + and –, presence and absence, respectively, of reverse transcriptase in the first-strand cDNA synthesis reaction. EF1{alpha} and actin were examined as controls. These results are qualitative rather than quantitative.

Abundant E. invadens cyst wall proteins all contained chitin-binding domains. Proteins in purified cyst walls of E. invadens trophozoites were analyzed by mass spectrometry of tryptic peptides and by N-terminal sequencing of spots isolated on 2-D protein gels. Multiple repeats of mass spectrometry identified six of seven predicted Jacob lectins, some with many peptides (EiJacob1 to EiJacob3) (Fig. 1 and Table 2) (9). EiJacob1 to EiJacob3, each of which contains five six-Cys CBDs and short Ser- and Thr-rich spacers, also accounted for most of the spots on 2-D gels containing E. invadens cyst wall proteins (Fig. 5 and see below). EiJacob6 and EiJacob7, which contain long low-complexity sequences, were much less abundant (Fig. 1 and Table 2). EiJacob5 was not identified.


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  		TABLE 2. Composition of E. invadens cyst walls


Figure 5
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  		FIG. 5. 2-D gel separation of E. invadens cyst wall proteins for N-terminal sequencing. Two gels were run in parallel, using the same preparation of cyst wall proteins. The first gel (A) was stained with SYPRO Ruby, a fluorescent total protein stain. The second gel (B) was transferred to a polyvinylidene difluoride membrane and stained with Coomassie blue. The blotted area corresponds to the upper left quadrant of panel A (marked by the dotted line). Molecular weight markers used for SDS-PAGE appear on the left side of each gel. (B) The 23 circled spots were excised and subjected to N-terminal sequencing (see Fig. 1). Spots are labeled with their protein names (J1 = EiJacob1, J2 = EiJacob2, J3 = EiJacob3) or with an "X" if no N-terminal sequencing data could be obtained. Negative numbers in parentheses denote the numbers of CBDs missing from the N termini of proteins. Proteins which are not labeled with negative numbers were cleaved after their N-terminal signal sequences. The sample number of each spot appears as a subscript.

Mass spectrometry of E. invadens cyst walls identified (with many peptides) EiJessie3a and -3b, each of which has an N-terminal eight-Cys CBD and a C-terminal domain like that of a C. botulinum protein (Fig. 2 and Table 2) (13, 29). None of the Jessies with a single CBD (EiJessie1a to EiJessie1c) were identified. A partial explanation for this result may be the low levels mRNA expression for Jessie1a and Jessie1c (Fig. 4). However, we could not rule out the possibility that EiJessie1a to EiJessie1c (and E. invadens chitinase 2 and chitinase 3 [see below]) are much less abundant in the E. invadens cyst wall and thus were not identified by mass spectrometry.

E. invadens chitinase 1, which contains an N-terminal eight-Cys CBD, was identified with many peptides, while E. invadens chitinase 2 and chitinase 3 were absent from multiple mass spectroscopic studies of E. invadens cyst wall proteins (Fig. 3 and Table 2) (6). It is possible that E. invadens chitinase 2 and chitinase 3 are absent from the cyst wall, because they each lack an N-terminal CBD.

These results for chitinases, Jacobs, and Jessies showed that lectins with predicted CBDs comprise the vast majority of E. invadens cyst wall proteins (Table 2). Estimates of the relative abundance of each protein in the cyst walls (using peak heights of tryptic peptides) suggested that Jacob lectins, which are likely structural proteins that cross-link chitin fibrils, comprise ~30% of the cyst wall protein (Table 2). Chitinase and Jessie3 lectins, which may be enzymes involved in remodeling chitin, comprised ~70% of the cyst wall protein.

In contrast, we failed to identify >100 other secreted and membrane proteins, including 2 chitin synthases, 4 chitin deacetylases, 7 Gal/GalNAc lectin heavy and light subunits, Hsp70, 2 calreticulins, VIP36, 15 protein disulfide isomerases, 38 Cys proteases, 2 dipeptidyl proteases, 4 amylases, and 24 receptor kinases (31). Actin and actin-binding proteins (together comprising 7 to 10% of the total protein, depending on the run) were the major cytosolic contaminants in the E. invadens cyst wall preparations (Table 2). Because sequencing of the E. invadens genome is not complete, we cannot rule out the presence of other proteins in the E. invadens cyst wall for which genomic sequences are not available. Similarly, we cannot rule out the possibility that some of the chitin-binding lectins identified by mass spectrometry were present in secretory vesicles prior to disruption of cysts and then bound to the chitin in cyst walls during the purification procedure. It is likely that these chitin-binding proteins, which are encystation specific, were destined for the cyst wall anyway.

Jacob lectins were cleaved at conserved sites between chitin-binding domains by an endogenous Cys protease. E. invadens cyst wall proteins were separated in 2-D gels with a pH range of 4 to 7 in the first dimension. This gave better resolution of the large acidic proteins, where EiJacob1 had previously been identified by mass spectrometry (Fig. 5A) (9, 20). These 2-D gels resolved >50 E. invadens cyst wall proteins, which fell into two groups. Acidic proteins, which appeared to be the most abundant, formed elongated spots that varied in size from ~10 to ~100 kDa. Neutral proteins, which formed circular spots, varied from ~10 to ~65 kDa. Remarkably, mass spectrometry of an excised 100-kDa acidic spot and a 65-kDa neutral spot (large and small arrows, respectively, in Fig. 5A) showed that both were EiJacob1. We present evidence below that the differences in size and charge of the two EiJacob1s were secondary to differences in the abundances of O-P-glycans.

The N-terminal sequences of 19 E. invadens cyst wall proteins from 2-D gels revealed two remarkable findings (Fig. 5A and B). First, all of the N-terminal sequences obtained could be mapped to only three different Jacob lectins (Fig. 1 and 5B) (9). Eleven sequences matched EiJacob1, two matched EiJacob2, three matched EiJacob3, and three matched both EiJacob1 and EiJacob2, whose sequences resemble each other. It is not clear why we failed to identify EiJessie3 lectins and E. invadens chitinase 1, which were very abundant by mass spectrometry of tryptic peptides (Table 2).

Second, while E. invadens Jacob lectins were cleaved after the signal peptide, EiJacob1 to EiJacob3 were also cleaved at conserved sites in Ser- and Thr-rich spacers between CBDs (Fig. 1 and 5B). EiJacob1 was cleaved between its first and second CBDs (Fig. 5B, spots 11 to 14) and between its third and fourth CBDs (spot 15). EiJacob2 was cleaved between its third and fourth CBDs (spot 17). EiJacob3 was cleaved between its first and second CBDs (spot 18) and between its second and third CBDs (spot 19). The differences in the apparent molecular weights of groups of EiJacob1 spots with the same N-terminal sequence were likely due to missing C-terminal sequences in the smaller spots (e.g., compare spots 2 and 3 with spots 5 and 6 or spots 12 and 13 with spot 14).

The cleavages in the Jacob lectins occurred prior to Lys at sites resembling TPSVDK. Additional predicted sites for the endoprotease were also present in EiJacob2, EiJacob6, and EiJacob7 (Fig. 1). These results suggested that Jacob lectins, which contain as many as seven CBDs, may be cleaved at one or more conserved endopeptidase sites within the Ser- and Thr-rich spacers to make processed lectins with fewer CBDs. This cleavage likely did not occur during the purification of cyst walls, as the Cys protease inhibitor E-64 was added to cysts prior to sonication.

A model Jacob peptide (FITC-TPSVDKNEDI) which contained the conserved endoprotease site prior to Lys was cleaved at this site by an E. invadens lysate (Fig. 6A). This cleavage was inhibited by E-64 (Fig. 6B), strongly suggesting that Cys proteases are involved in site-specific cleavage (4, 23). Papain (a plant Cys protease) also cleaved the Jacob peptide at the same site as the amebic lysate and was inhibited by E-64 (Fig. 6C and D), while trypsin (a bovine Ser protease) cleaved the Jacob peptide after the Lys and was inhibited by PMSF (Fig. 6E and F). The latter result suggested that Ser proteases, which have been implicated in E. invadens encystation, likely do not cleave Jacob lectins (21).


Figure 6
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  		FIG. 6. Mass spectrometry of a model peptide (FITC-TPSVDKNEDI) incubated with an E. invadens lysate in the absence (A) or presence (B) of the Cys protease inhibitor E-64. The same peptide was incubated with papain in the absence (C) or presence (D) of E-64 or with trypsin in the absence (E) or presence (F) of the Ser protease inhibitor PMSF.

Jacob lectins were extensively modified by O-phosphodiester-linked glycans, which had deoxysugars at the reducing ends. Previous lectin-binding experiments showed that the acidic, high-molecular-weight EiJacob1 lectin is a glycoprotein (9). The Pro-Q Emerald 488 glycoprotein stain, which reacts with periodic acid-oxidized carbohydrate groups (11), stained the acidic proteins (including all three Jacob lectins) in 2-D gels of E. invadens cyst wall proteins but did not stain the more neutral EiJacob1 protein spots in the center of the gel (Fig. 7A). Subsequent staining of the gel with SYPRO Ruby showed that these previously unstained proteins were indeed present in the gel (Fig. 7B). The Pro-Q Emerald 488-stained glycans on Jacob lectins were most likely O-P-glycans, because the high-molecular-weight, acidic Jacob spots were nearly absent from 2-D gels containing E. invadens cyst wall proteins pretreated with 40% TFA under conditions that hydrolyze phosphodiester linkages (Fig. 7C) (17, 18). In contrast, the more neutral Jacob spots, which were not stained by Pro-Q Emerald 488, were not affected by mild acid treatment.


Figure 7
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  		FIG. 7. Detection and analysis of glycans of E. invadens cyst wall proteins separated in 2-D gels. (A) Untreated cyst wall proteins visualized at 488 nm after being stained with Pro-Q Emerald 488 glycoprotein stain. (B) Same cyst wall protein gel as that shown in panel A, but visualized at 532 nm after SYPRO Ruby staining, which stains all proteins. (C) E. invadens cyst wall proteins were treated with 40 mM TFA for 10 min at 100°C to remove O-P-glycans and were visualized at 532 nm after SYPRO Ruby staining. No staining of TFA-treated cyst wall proteins was observed at 488 nm after glycoprotein staining (data not shown). (D) BioGel P4 chromatography of cyst wall O-P-glycans released by mild acid hydrolysis. Glycans were reductively labeled with NaB3H4, so the y axis shows counts per min. The arrows indicate the elution positions of glucose oligosaccharide standards (linear ß-1,6-glucosyl glucosides, one to eight units long). Peaks A to D are further characterized in Fig. 8.

E. invadens cyst wall O-P-glycans, which were removed by TFA and labeled at their reducing ends with borotritide, were a heterogeneous mix (Fig. 7D). Total hydrolysis of these glycans followed by monosaccharide analysis of the alditols at the free ends showed two types of reduced sugars, eluting at 10.5 and 13 min. These alditols, which differed in their relative abundances depending on the particular peak analyzed (Fig. 8A and B), may be deoxysugars, based upon the following evidence. First, these alditols were neutral, as defined by their lack of affinity for ion-exchange resins. Second, the reduced sugar eluting at 13 min coeluted with an internal rhamnitol (6-deoxy-mannitol) standard. Third, although the reduced sugar eluting at 10.5 min did not comigrate with any of the available standards, mass spectrometry showed that peak D in Fig. 7D is composed of four hexoses and one deoxyhexose. Fourth, Entamoeba has at least two genes encoding putative nucleotide-sugar 4,6-dehydratases, which are involved in the synthesis of deoxysugars in the secretory pathway (data not shown). The alditols derived from the E. invadens cyst wall were also distinct from galactitol, which is the most abundant reduced sugar of O-P-glycans of the proteophosphoglycans of E. histolytica trophozoites (Fig. 8C) (18). While these results suggested that the reducing ends of cyst wall O-P-glycans are deoxysugars, further analysis will need to be performed to confirm their identities. Monosaccharides in peak J of Fig. 7D, which included glucitol and mannitol, were likely contaminants of E. invadens cyst wall purification.


Figure 8
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  		FIG. 8. Characterization of reduced sugars at ends of O-P-glycans of E. invadens cyst walls. The O-P-glycans shown in Fig. 7D were isolated from P4 columns, reduced with NaB3H4, hydrolyzed, and separated by a CarboPac MA1 column run. Alditol standards included glucosaminitol (gn), galactosaminitol (ga), fucitol (f), arabitol (ar), rhamnitol (rh), xylitol (x), glucitol (s), galactitol (d), and mannitol (m). (A) The reduced ends of the O-P-glycans in peaks A and B in Fig. 7D ran with a rhamnitol standard at 13 min. (B) The reduced ends of the O-P-glycans in peaks C and D did not run with a standard, but mass spectrometry of intact peak D showed a deoxysugar at the reducing end. (C) As a control, the reduced sugar from E. histolytica proteophosphoglycans ran with the galactitol standard (16).

The locations of a small, but likely representative, number of O-P-glycans on Jacob lectins were determined by mass spectrometric identification of phosphopeptides from cyst wall proteins treated with TFA to remove glycans (Fig. 1 and 9). Phosphorylated peptides were readily identified by their unique fragmentation patterns. Because the phosphorylated peptides were present in the TFA-treated samples but not in the untreated samples, each phosphate must have been present originally as an O-P-glycan. It is likely, however, that numerous phosphopeptides were missed because (i) there was limited sampling of peptides from the Ser- and Thr-rich spacers between CBDs and (ii) no attempts were made to enrich phosphopeptides prior to mass spectroscopy.


Figure 9
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  		FIG. 9. Identification of phosphopeptides from TFA-treated cyst wall proteins. (A) Sample tryptic peptide from EiJacob1 lectin with corresponding MS/MS cleavage pattern revealing y- and b-ion fragments (*, sites of phosphorylation). (B) MS/MS spectrum revealing neutral losses of 98 and 196 Da, corresponding to losses of one and two phosphates, respectively. (C) Suppressed peaks revealing y- and b-ion peptide fragments for identification of the peptide sequence by SEQUEST or X!Tandem.


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DISCUSSION
 
The results reported here suggest a model of the E. invadens cyst wall in which all of the abundant proteins are unique, encystation-specific lectins with either six-Cys CBDs (Jacob lectins) or an eight-Cys CBD (E. invadens chitinase 1, EiJessie3a, and EiJessie3b) (Fig. 10). The Jacob lectins, which show extensive gene duplication, have tandem arrays of CBDs that cross-link chitin fibrils and may protect chitin from glycohydrolases. The most abundant Jacob lectins (EiJacob1 to EiJacob3) also have Ser- and Thr-rich spacers between CBDs which undergo posttranslational modifications (discussed below). The less abundant Jacob lectins (EiJacob6 and EiJacob7) contain long, low-complexity sequences between CBDs. Insect peritrophins, which cross-link chitin fibrils in the wall around the blood meal, have tandem arrays of eight-Cys CBDs that are separated by low-complexity spacers (26). The spore coat proteins of Dictyostelium also have tandem arrays of cellulose-binding domains that are separated by low-complexity spacers (32). While Saccharomyces wall proteins have Ser- and Thr-rich domains that are extensively glycosylated, fungal wall proteins have not been shown to be lectins and are covalently bound to chitin and ß-1,3-glucan (36).


Figure 10
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  		FIG. 10. Best model of E. invadens cyst wall. Wall proteins are restricted to those that have six-Cys CBDs (Jacob lectins) or eight-Cys CBDs (Jessie3 lectins and chitinases). Multiple Jacob lectins vary in their numbers of six-Cys CBDs, secondary to differences in the numbers of CBDs encoded and to cleavages by Cys proteases in Ser- and Thr-rich spacers. Jacob lectins also vary in the presence or absence of O-P-glycans and long, low-complexity sequences. An N-terminal CBD is present in a single chitinase and in two Jessie3 lectins, which have a C-terminal domain of unknown function. Because chitinases and Jessie3 lectins were not identified on 2-D gels, it is not known whether these proteins are cleaved by proteases.

We were surprised by the large amounts of E. invadens chitinase 1, EiJessie3a, and EiJessie3b, which comprise ~70% of the total protein in the E. invadens cyst wall. These proteins likely do not cross-link chitin fibrils, because each has only a single eight-Cys CBD (29). Instead, E. invadens chitinase 1 may be involved in remodeling the chitin fibrils during cyst wall formation and/or degrading chitin fibrils during excystation. As discussed above, the unknown domain common to Entamoeba Jessie3s and the putative C. botulinum protein is also likely involved in modifying chitin (13). These results suggest that the walls of E. invadens cysts, like those of fungi, may be dynamic rather than static. Saccharomyces has numerous wall proteins (e.g., Bgl2 and Gas1) which are enzymes involved in remodeling ß-1,3-glucans (36).

The simple model of the E. invadens cyst wall is made somewhat more complicated by posttranslational modifications of E. invadens Jacob lectins (Fig. 10). Cleavage of Jacob lectins between CBDs, which was highly reproducible, occurred at conserved sites located within otherwise low-complexity Ser- and Thr-rich sequences (Fig. 1, 5, and 6). Studies with a model synthetic peptide strongly suggested that Jacob lectins are cleaved by endogenous Cys proteases, which have previously been shown to be involved with encystation and amebic invasion into host tissues (4, 10, 23, 25). Specific cleavages between CBDs of Jacob lectins may be a mechanism of regulating the thickness of the chitin wall during cyst construction and/or facilitate excystation. Giardia has a developmentally regulated Cys protease which is required for the proteolytic processing of a major cyst wall protein (28). A second developmentally regulated Cys protease is required for the excystation of Giardia. Future studies will determine whether there are encystation- or excystation-specific Cys proteases in Entamoeba and whether any of these proteases have CBDs, as shown for E. invadens chitinase 1 and EiJessie3s.

Jacob lectins in the E. invadens cyst wall had numerous O-P-glycans, which were released with TFA (Fig. 7 and 8). Why the same Jacobs appeared in heavily glycosylated forms and nonglycosylated forms is not clear, but the observation is remarkable. The O-P-glycans likely decorate Ser- and Thr-rich spacers between CBDs of Jacob lectins, in the same way that O-P-glycans are bound to Ser- and Thr-rich spacers between cellulose-binding domains of Dictyostelium spore coat glycoproteins (32). The Dictyostelium enzyme which adds O-P-glycans to Ser and Thr has not been characterized, but there are candidate enzymes that resemble bacterial enzymes that add O-P-glycans (33). Because E. invadens chitinases and Jessie lectins also contain Ser- and Thr-rich domains, it is likely that these cyst wall proteins also have O-P-glycans.

While the proteophosphoglycans of E. histolytica trophozoites have O-P-glycans terminating in glucose (18), the O-P-glycans on Jacob lectins were shorter and contained a deoxysugar at the reducing end rather than Gal. The O-P-glycans on Jacobs may protect cysts from degradation as they pass through the stomach and intestines. The cyst wall O-P-glycans may also be bound by the plasma membrane Gal/GalNAc lectin, as suggested by the formation of wall-less E. invadens cysts in the presence of excess Gal (9).

The E. invadens cyst wall model developed here has important implications for the pathogenesis of E. histolytica (Fig. 10) (10, 25). First, E. histolytica chitinases, Jacob lectins, and Jessie lectins, which have been shown to have chitin-binding activities in transfected E. histolytica (29), are likely abundant proteins in the E. histolytica cyst wall. Second, since E. histolytica chitinases, Jacobs, and Jessies also have Ser- and Thr-rich spacers between CBDs, these E. histolytica lectins are likely modified by O-P-glycans and may be cleaved by Cys proteases. Third, because E. histolytica chitinases, Jacobs, Jessies, and their attached O-P-linked glycans are unique, they appear to be good targets for new antiamebic diagnostics and/or vaccines.


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ACKNOWLEDGMENTS
 
We thank Katie Doud of the Harvard University NIEHS Center for Environmental Health Proteomics Facility for performing two-dimensional gel electrophoresis. We also thank David McCourt of Midwest Analytical for his N-terminal sequencing work and Dick Cook of the Cancer Institute at MIT for some of the mass spectrometry data. We thank Neil Hall of TIGR for providing assemblies of the E. invadens genome.

This work was supported in part by National Institutes of Health grants AI44070 (to J.C.S.) and GM31318 (to P.W.R.). Training support (to D.M.R.) was provided by the Training Program in Host Pathogen Interactions (5T32 AI052070). M.S. thanks the Whitaker Foundation Leadership Award to Boston University for financial support.


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FOOTNOTES
 
* Corresponding author. Mailing address: Department of Molecular and Cell Biology, Boston University Goldman School of Dental Medicine, 715 Albany St., Evans 426, Boston, MA 02118. Phone: (617) 414-1054. Fax: (617) 414-1041. E-mail: jsamuels{at}bu.edu. Back


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Eukaryotic Cell, May 2006, p. 836-848, Vol. 5, No. 5
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