eIF5A
The translation factor eIF5A, the sole protein containing the unusual amino acid hypusine [Nε-(4-amino-2-hydroxybutyl)lysine], was originally identified based on its ability to stimulate the yield (endpoint) of methionyl-puromycin synthesis, a model assay for first peptide bond synthesis thought to report on certain aspects of translation initiation. Hypusine is required for eIF5A to associate with ribosomes, and to stimulate methionyl-puromycin synthesis. As eIF5A did not stimulate earlier steps of translation initiation, and depletion of eIF5A in yeast only modestly impaired protein synthesis, it was proposed that eIF5A function was limited to stimulating synthesis of the first peptide bond or that eIF5A functioned on only a subset of cellular mRNAs. However, the precise cellular role of eIF5A is unknown, and the protein has also been linked to mRNA decay and to nucleocytoplasmic transport. Using molecular genetic and biochemical studies, we recently showed that eIF5A promotes translation elongation. Depletion of eIF5A or shifting a temperature-sensitive (ts-) eIF5A mutant to the non-permissive temperature resulted in the accumulation of polysomes, mimicking the affect of the translation elongation inhibitor cycloheximide. Moreover, inactivation of eIF5A increased the ribosomal transit time, the amount of time required for a ribosome following initiation to synthesize and release a completed protein. The translation elongation defect in extracts from the eIF5A ts- mutant strain was suppressed by addition of recombinant eIF5A from yeast, but not by a derivative lacking hypusine. Moreover, togther with Rachel Green (Johns Hopkins) we showed that eIF5A enhanced the rate of tripeptide synthesis in reconstituted translation elongation assays. Finally, inactivation of eIF5A mimicked the effects of the eEF2 inhibitor sordarin and impaired programmed ribosomal frameshifting. These results indicate that eIF5A might function together with eEF2 to promote ribosomal translocation. As eIF5A is a structural homolog of the bacterial protein EF-P, we propose that eIF5A/EF-P is a universally conserved translation elongation factor (Saini et al. Nature 2009).
