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Eukaryotic Cell, October 2003, p. 841-846, Vol. 2, No. 5
1535-9778/03/$08.00+0 DOI: 10.1128/EC.2.5.841-846.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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Institute of Molecular Biology, Academia Sinica,1 Institute of Genetics, National Yang-Ming University, Taipei, Taiwan, Republic of China2
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On the other hand, methylation of the histones, in particular at Lys-9 of H3, also plays essential role in the modification of the chromatin structure (20, 42). The Pre-SET and SET domains within a group of histone methyltransferases are responsible for their intrinsic activities of Lys methylation (18). Interestingly, genetic evidence from studies of Neurospora spp. suggests that DNA cytosine methylation is downstream of the H3 methylation (37). Consistent with this, an Arabidopsis CpNpG DNA methyltransferase (CMT3) was shown to interact with the H3 Lys-9-binding protein, HP1, and thus provides a physical link between the sequential events of histone and DNA methylation in the plants (17). A similar event has recently been suggested to exist in the mammals (11).
An MBD motif has been located in the human histone H3K9 methylase SETDB1 (34). This implies that DNA methylation might also be located upstream of histone methylation. The following questions could be asked. How many eukaryotic MBD proteins also contain motifs required for histone modification, in particular the H3K9 methylation? How widespread are these proteins in different species? When the conserved MBD motif was used as the bait for search (1), a total of eleven, three, five, two, and twelve MBD-containing genes were found to exist in the genomes of humans, tunicates (Ciona intestinalis), D. melanogaster, Caenorhabditis elegans, and Arabidopsis thaliana, respectively. The same number of MBD proteins as in humans is found in the mouse genome, each representing the respective human ortholog (32). In contrast, none could be identified in the Archaea, protozoa, and fungus genomes (Table 1). Archaea has been placed as a distinct domain from the bacteria and eukaryotes in evolution, with their metabolic features closer to bacteria and information processing machinery closer to the eukaryotes (4). Quite a few Archaea species have been fully sequenced. Although putative methyltransferases have been proposed in Methanococcus janaschii (5), one of the fully sequenced archaeal species, no MBD protein could be encoded by its genome. Several protozoan genome projects are ongoing as well, and that of the human parasite Plasmodium falciparum has been finished (13). As found in Trypanosoma cruzi (31), DNA methylation has been observed in Plasmodium falciparum (27). However, its genome does not encode any MBD protein. On the other hand, of the two distinct classes of fungi, Saccharomyces cerevisiae lacks methylated cytosines (28), as well as the MBD proteins, whereas Neurospora crassa has well-documented DNA methylation (9) but does not encode MBD proteins. The findings of these genomic analyses of the distribution of MBD proteins are summarized in Table 1. As shown, there is no clear-cut relationship between the genome sizes of different species and the numbers of MBD proteins they encode. Neither do the latter reflect the extents to which the individual genomes are methylated.
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TABLE 1. Classification of MBD proteins found in databasea
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1 helix and the hairpin-loop mainly present in these proteins (Fig. 2). The Drosophila protein CG10042 possesses one MBD motif and one plant homeodomain (PHD) finger. PHD is required for activity of the histone acetyltransferases (19). This suggests that CG10042 might transact signals between DNA methylation and histone acetylation.
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FIG. 1. Domain architecture of MBD proteins. The multiple MBD proteins are grouped according to the arrangement of different motifs within their sequences. The different motifs and domains are indicated by colored boxes. MBD, PWWP, bromodomain, PHD, and Pre-SET/SET are described in the text. DDT is a domain conserved in several transcription and chromatin remodeling factors (8). Note that of the five previously identified mammalian MBD proteins (15), only MBD2 is shown here as a representative. h, human; d, Drosophila; c, C. elegans. The accession numbers are as follows: MBD2, AF072242; KIAA1887, NM_052897; KIAA1461, AB040894 (this polypeptide misses a start site); BAZ2A, NM_013449; BAZ2B, NM_013450; CLLD8, AF334407; SETDB1, NM_12432; dMBD2/3, AE003683; six-banded, SD04244; CG10042, AE003695; Toutatis, AF314193; CG30426, AE003465; R05D3.11, NM_066447; and ZK783.4, NM_066272.
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FIG. 2. Sequence alignment of the MBD motifs. The alignment is constructed by using the Clustal W program and then examined manually, and the sequences are arranged in the order shown in Fig. 1. The numbers above the alignment correspond to the amino acid positions in human MBD2. Identical amino acids are boxed and shaded in dark gray; similar residues are boxed and shaded in light gray. Functionally important residues in the MBD motifs of MBD1 and MeCP2 are shown above the alignment. The definitions of L1, 1, and hairpin-loop indicated on the MBD2 sequence are derived from the study of MBD1 (26).
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Finally, the last group contains the Pre-SET and the bifurcated SET domains consecutively located C terminal with respect to their MBD domains. Of the four proteins shown in Fig. 1, the coexistence of MBD and the Pre-SET/SET domains has been noted before for CLLD8, which is located in a region deleted in some B-cell chronic lymphocyte leukemia (24) and, as mentioned already, for the histone H3K9 methylase SETDB1 (34). Surprisingly, however, this pattern of coexistence extends into the flies and worms, as exemplied by the identification of Drosophila CG30426 and C. elegans R05D3.11. Sequence alignment of Fig. 3 shows that the Pre-SET and the bifurcated SET sequences (SET1 and SET2) of the group III proteins are highly homologous and that they also extend longer than the previously predicted boundaries of these domains.
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FIG. 3. Sequence alignment of Pre-SET/SET. Note that for either Pre-SET or the bifurcated SET (SET1 and SET2), the homologous sequences extend beyond the boundaries of the previously defined domains (the boxed regions).
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The MBD sequences shown in Fig. 2 reveal both similar and unique features, both among themselves and in comparison to the previously defined MBD structures (10, 25, 26, 40). Besides the unique insertion conserved mainly for the subgroup of group I proteins described in the earlier section, the group II and group III MBD proteins possess their distinct features as well. As shown in Fig. 2, the similarity is apparently higher by groups rather than by species: proteins within the same group tend to use identical or homologous residues. The hydrophobic core structure is essentially maintained with few exceptions, such as the residue phenylalanine present in CLLD8 and SETDB1 at the position corresponding to residue W159 of MBD2. Loop 1 (L1) is a critical component in DNA recognition. Many residues within L1 interacting with the DNA backbone are highly conserved among the five previously identified MBD proteins, MBD1 to MBD4 and MeCP2. A few of these residues have been confirmed by mutagenesis experiments to play significant roles in DNA binding. However, L1 is no longer conserved in these other MBD proteins, and their primary sequences vary greatly. In particular, there is a large insertion rich in asparagine (N) in fly dMBD2/3. Despite this divergence, one previous study showed that dMBD2/3
, an alternative spliced isoform of dMBD2/3 containing only the N-terminal half of the MBD including the asparagine-rich insertion, is still capable of preferentially recognizing methylated cytosines, albeit with a weak binding affinity (30). Apparently, the MBDs of MBD1, MBD2, MBD3, MBD4, and MeCP2 proteins together form a highly conserved and distinct class of the MBD motifs. On the other hand, how the variations of the primary sequences in the other MBD proteins affect their DNA-binding properties in vivo and whether, like MBD3, they have gained additional functions (33) await further investigations.
Relative to its small genome size, A. thaliana apparently reserves a large family of the MBD proteins (Tables 1 and 2). Interestingly, 10 of the 12 MBD polypeptides contain the MBD motif alone. Furthermore, unlike the animal MBD proteins, the locations of the MBD motifs within the Arabidopsis proteins are highly variable (Table 2).
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TABLE 2. MBD proteins in A. thalianaa
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Second, MBD proteins appear not to be required for gene regulation in some organisms, such as the protozoa and fungi. For the regulation of gene expression, there are at least two control mechanisms involving DNA methylation: one via the coupling of DNA methylation and MBD protein-mediated gene silencing (2) and another via the interplay between DNA methylation and histone methylation (12, 17, 37). It has been proposed that having both of the above-mentioned mechanisms could reinforce epigenetic silencing and create a self-propagating cycle to maintain a long-term transcriptional repression (3, 11). N. crassa is a popular model in the relevant studies, but apparently the former mechanism is missing in this organism due to the lack of MBD proteins. This finding is consistent with, but certainly not the sole explanation for, the observation that DNA methylation is dispensable in Neurospora (9, 21) but essential for the mammals (22). Also, methylation is not preferential at CpG dinucleotides in Neurospora (35) but occurs almost exclusively at CpGs in mammals. Finally, the methylated components in the genome of Neurospora reside mainly in the transposons. These distinct variations place Neurospora in a scenario different from that in mammals with respect to the function of DNA methylation.
Finally, quite a few MBD proteins also contain modules needed for histone acetylation, histone methylation, or binding to the modified histones. Although MBD proteins have been found to exist and function in multiple-subunit complexes, the existence of the multiple group II and group III MBD proteins suggests that there is a more economic way(s) that nature utilizes to link the functions and processes of DNA methylation and histone modifications. Perhaps more significantly, as mentioned above, both genetic data from fungi studies (37) and biochemical evidence from plant (17) have indicated that along the gene regulation/chromatin methylation pathway, histone (H3) methylation is upstream of DNA methylation (Fig. 4A; see also the reviews in references 3 and 38). However, the existence of the group II and III MBD proteins (Fig. 1) suggests that initiation of histone modification, in particular methylation, by DNA methylation could be a common regulatory pathway utilized in the animal kingdom. Furthermore, this regulation could be accomplished by single MBD protein molecule(s). Specifically, a protein containing both MBD and the Pre-SET/SET could first bind to a DNA region with methylated CpG and later methylate the Lys-9 residue of H3 in the nearby nucleosome(s) (Fig. 4B). It is noteworthy that database mining has detected no Arabidopsis MBD proteins containing the Pre-SET/SET modules and that only one of them contains a motif in connection with histone modification (Table 2), which reflects a fundamental difference of regulatory scenarios between animals and plants.
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FIG. 4. Two scenarios of regulation of chromatin methylation. It is proposed that, unlike plants and fungi (A), DNA methylation could occur upstream of histone methylation through the stepwise functions by a single protein containing both MBD motif and a preSET/SET domain (B).
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This research was supported by the National Research Council, National Health Research Institute, and Academia Sinica, Taipei, Taiwan, Republic of China.
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