The highly conserved protein eIF5A found in Archaea and all eukaryotes uniquely contains the posttranslationally formed amino acid hypusine. Despite being essential the functions of this protein and its modification remain unclear. To gain more insight into these functions temperature-sensitive mutants of the human EIF5A1 were characterized in the yeast Saccharomyces cerevisiae. Expression of the point mutated form V81G in a ⌬eIF5A strain of yeast led to a strongly temperature-sensitive phenotype and to a significantly reduced protein level at restrictive temperature. The mutant showed accumulation of a subset of mRNAs that was also observed in nonsense-mediated decay (NMD)-deficient yeast strains. After short incubation at restrictive temperature the mutant exhibited increased half-lives of the intron containing CYH2 pre-mRNA and mature transcripts of NMD-dependent genes. Reduced telomere silencing and shortening was detected in the V81G mutant further supporting similarities to NMD-deficient strains. Our data suggest that eIF5A mediates important cellular processes like cell viability and senescence through its effects on the stability of certain mRNAs.The cellular physiology of mRNA processing, transport, localization, and turnover is central to the process of gene expression at the posttranscriptional level. Increasing evidence has been found for a close connection between mRNA degradation processes and the steps of translation. The highly conserved hypusine-containing protein eIF5A has been implicated in both of these aspects of RNA metabolism; however, its precise cellular function is not yet fully understood.Hypusine formation is a two-step enzymatic reaction catalyzed by deoxyhypusine synthase and then deoxyhypusine hydroxylase (1). The disruption of genes encoding either eIF5A or deoxyhypusine synthase in yeast leads to a lethal phenotype (2) demonstrating that the deoxyhypusine residue is essential for the function of eIF5A and thus for proliferation and cell survival. The genome of Saccharomyces cerevisiae contains two HYP genes (HYP1, alias TIF51B or ANB1, and HYP2, alias TIF51A) coding for eIF5A. These genes are differentially expressed under aerobic and anaerobic conditions (3) and share an identity of 90%. HYP genes of other higher eukaryotes, e.g. one of the two human HYP genes (encoding EIF5A1), can functionally replace these yeast-specific genes (4, 5).The eIF5A protein was first isolated from rabbit reticulocyte lysate ribosomes and classified as a translation initiation factor because in vitro the protein enhanced the building reaction of the first peptide bond measured as the yield of methioninepuromycin (6). However, cell fractionation revealed that only a small fraction of the protein associated with ribosomes (7). Moreover depletion or inactivation of the protein in S. cerevisiae reduced the global protein synthesis rate by only 30% (8, 9). Thus, the role of eIF5A as a translation initiation factor remains to be confirmed.eIF5A is an RNA-binding protein (10), and it was hypothesized to re...

The human glycerol kinase gene family consists of at least six genomic loci, four of which encode expressed sequences. The X-linked gene responsible for GKD maps to Xp21.3. Analysis of cosmid and YAC clones shows that this locus is in excess of 50 kbp, and is comprised of 19 exons. In contrast, the remaining members of the gene family, on chromosomes 1, 4 and Xq, appear to be organized as intronless genes. Northern analysis shows expression of GK transcripts of three sizes in a wide range of adult tissues. Only the smallest hybridizing species is present in testis where it occurs at an elevated level. Two different testis transcripts have been identified and both of these originate from chromosome 4.