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Eukaryotic Cell, May 2005, p. 948-959, Vol. 4, No. 5
1535-9778/05/$08.00+0     doi:10.1128/EC.4.5.948-959.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Identification in the Ancient Protist Giardia lamblia of Two Eukaryotic Translation Initiation Factor 4E Homologues with Distinctive Functions{dagger}

Lei Li and Ching C. Wang*

Department of Pharmaceutical Chemistry, University of California, San Francisco, California 94143-2280

Received 2 March 2005/ Accepted 16 March 2005

Eukaryotic translation initiation factor 4E (eIF4E) binds to the m7GTP of capped mRNAs and is an essential component of the translational machinery that recruits the 40S small ribosomal subunit. We describe here the identification and characterization of two eIF4E homologues in an ancient protist, Giardia lamblia. Using m7GTP-Sepharose affinity column chromatography, a specific binding protein was isolated and identified as Giardia eIF4E2. The other homologue, Giardia eIF4E1, bound only to the m2,2,7GpppN structure. Although neither homologue can rescue the function of yeast eIF4E, a knockdown of eIF4E2 mRNA in Giardia by a virus-based antisense ribozyme decreased translation, which was shown to use m7GpppN-capped mRNA as a template. Thus, eIF4E2 is likely the cap-binding protein in a translation initiation complex. The same knockdown approach indicated that eIF4E1 is not required for translation in Giardia. Immunofluorescence assays showed wide distribution of both homologues in the cytoplasm. But eIF4E1 was also found concentrated and colocalized with the m2,2,7GpppN cap, 16S-like rRNA, and fibrillarin in the nucleolus-like structure in the nucleus. eIF4E1 depletion from Giardia did not affect mRNA splicing, but the protein was bound to Giardia small nuclear RNAs D and H known to have an m2,2,7GpppN cap, thus suggesting a novel function not yet observed among other eIF4Es in eukaryotes.

* Corresponding author. Mailing address: Department of Pharmaceutical Chemistry, University of California San Francisco, N572C, Mission Bay Genentech Hall, 600 16th St., San Francisco, CA 94122-2280. Phone: (415) 476-1321. Fax: (415) 476-3382. E-mail: ccwang{at}cgl.ucsf.edu.

{dagger} Supplemental material for this article may be found at http://ec.asm.org/.

Eukaryotic Cell, May 2005, p. 948-959, Vol. 4, No. 5
1535-9778/05/$08.00+0     doi:10.1128/EC.4.5.948-959.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Simoes-Barbosa, A., Louly, C., Franco, O. L., Rubio, M. A., Alfonzo, J. D., Johnson, P. J. (2008). The divergent eukaryote Trichomonas vaginalis has an m7G cap methyltransferase capable of a single N2 methylation. Nucleic Acids Res 36: 6848-6858 [Abstract] [Full Text]  
  • Hausmann, S., Shuman, S. (2005). Giardia lamblia RNA Cap Guanine-N2 Methyltransferase (Tgs2). J. Biol. Chem. 280: 32101-32106 [Abstract] [Full Text]  


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