Hypusine-containing protein eIF5A promotes translation elongation

Various factors associate with the ribosome to assist in initiation, elongation and termination. Textbook accounts of protein synthesis describe just two universally conserved translation elongation factors — EF-Tu/eEF1A and EF-G/eEF2. Now a study of protein synthesis in the yeast Saccharomyces cerevisiae repositions a factor previously thought to be associated with the initiation process, eIF5A, as a central player in elongation. eIF5A is unusual in that it contains a rare amino acid, hypusine, that is required for its ability to stimulate elongation. Based on the defects observed in the absence of eIF5A, it is proposed that the factor may function with eEF2 in the translocation step. Various factors associate with the ribosome to assist in initiation, elongation and termination of translation. Only two universal factors for elongation have previously been identified; here, a factor previously thought to be associated with the initiation process, eIF5A, and which contains a rare amino acid, hypusine, is found to have a central role in elongation. Translation elongation factors facilitate protein synthesis by the ribosome. Previous studies identified two universally conserved translation elongation factors, EF-Tu in bacteria (known as eEF1A in eukaryotes) and EF-G (eEF2), which deliver aminoacyl-tRNAs to the ribosome and promote ribosomal translocation, respectively1. The factor eIF5A (encoded by HYP2 and ANB1 in Saccharomyces cerevisiae), the sole protein in eukaryotes and archaea to contain the unusual amino acid hypusine (Ne-(4-amino-2-hydroxybutyl)lysine)2, 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 initiation3,4. Hypusine is required for eIF5A to associate with ribosomes5,6 and to stimulate methionyl-puromycin synthesis7. Because eIF5A did not stimulate earlier steps of translation initiation8, and depletion of eIF5A in yeast only modestly impaired protein synthesis9, 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 messenger RNAs. However, the precise cellular role of eIF5A is unknown, and the protein has also been linked to mRNA decay, including the nonsense-mediated mRNA decay pathway10,11, and to nucleocytoplasmic transport12,13. Here we use molecular genetic and biochemical studies to show that eIF5A promotes translation elongation. Depletion or inactivation of eIF5A in the yeast S. cerevisiae resulted in the accumulation of polysomes and an increase in ribosomal transit times. Addition of recombinant eIF5A from yeast, but not a derivative lacking hypusine, enhanced the rate of tripeptide synthesis in vitro. Moreover, inactivation of eIF5A mimicked the effects of the eEF2 inhibitor sordarin, indicating that eIF5A might function together with eEF2 to promote ribosomal translocation. Because eIF5A is a structural homologue of the bacterial protein EF-P14,15, we propose that eIF5A/EF-P is a universally conserved translation elongation factor.