Translation initiation factor eIF-5A (previously named eIF-4D) is a highly conserved protein that promotes formation of the first peptide bond. One of its lysine residues is modified by spermidine to form hypusine, a posttranslational modification unique to eIF-5A. To elucidate the function of eIF-5A and determine the role of its hypusine modification, the cDNA encoding human eIF-5A was used as a probe to identify and clone the corresponding genes from the yeast Saccharomyces cerevisiae. Two genes named TIFS1A and TIF51B were cloned and sequenced. The two yeast proteins are closely related, sharing 90% sequence identity, and each is ca. 63% identical to the human protein. The purified protein expressed from the TIF51A gene substitutes for HeLa eIF-5A in the mammalian methionyl-puromycin synthesis assay. Strains lacking the A form of eIF-5A, constructed by disruption of TIFS1A with LEU2, grow slowly, whereas strains lacking the B form, in which HIS3 was used to disrupt TIF51B, show no growth rate phenotype. However, strains with both TIFS5A and TIFS1B disrupted are not viable, indicating that eIF-5A is essential for cell growth in yeast cells. Northern (RNA) blot analysis shows two mRNA species, a larger mRNA (0.9 kb) transcribed from TIF51A and a smaller mRNA (0.8 kb) encoded by TIF51B. Under the aerobic growth conditions of this study, the 0.8-kb TIF51B transcript is not detected in the wild-type strain and is expressed only when TIF51A is disrupted. The TIF51A gene was altered by site-directed mutagenesis at the site of hypusination by changing the Lys codon to that for Arg, thereby producing a stable protein that retains the positive charge but is not modified to the hypusine derivative. The plasmid shuffle technique was used to replace the wild-type gene with the mutant form, resulting in failure of the yeast cells to grow. This result indicates that hypusine very likely is required for the vital in vivo function of eIF-5A and suggests a precise, essential role for the polyamine spermidine in cell metabolism.Eukaryotic initiation factor 5A (eIF-5A; previously named eIF-4D) (initiation factors are named according to the revised nomenclature recommended by the International Union of Biochemistry [37]) is one of a number of protein factors that stimulate the initiation phase of protein synthesis (29). The purified protein from mammalian cells is small (16 to 18 kDa) and acidic (pI = 5.4) and is one of the most abundant of the initiation factors (2, 21). eIF-5A is distinguished by possession of a unique residue, hypusine [N--(4-amino-2-hydroxybutyl)-lysine], formed posttranslationally by transfer of a butylamino group from spermidine to a specific lysine followed by a hydroxylation reaction (9, 33). The eIF-5A protein and its hypusine modification are highly conserved from yeasts to humans (12), suggesting an important role in protein synthesis, cellular metabolism, or both. eIF-5A appears to function in protein synthesis by promoting formation of the first peptide bond, a reaction usually studied in vitro by a ...

In eukaryotic cells, the unfolded protein response (UPR) pathway plays a crucial role in cellular homeostasis of the endoplasmic reticulum (ER) during exposure to diverse environmental conditions that cause ER stress. Here we report that the human fungal pathogen Cryptococcus neoformans has evolved a unique UPR pathway composed of an evolutionarily conserved Ire1 protein kinase and a novel bZIP transcription factor encoded by HXL1 (HAC1 and XBP1-Like gene 1). C. neoformans HXL1 encodes a protein lacking sequence homology to any known fungal or mammalian Hac1/Xbp1 protein yet undergoes the UPR-induced unconventional splicing in an Ire1-dependent manner upon exposure to various stresses. The structural organization of HXL1 and its unconventional splicing is widely conserved in C. neoformans strains of divergent serotypes. Notably, both C. neoformans ire1 and hxl1 mutants exhibited extreme growth defects at 37°C and hypersensitivity to ER stress and cell wall destabilization. All of the growth defects of the ire1 mutant were suppressed by the spliced active form of Hxl1, supporting that HXL1 mRNA is a downstream target of Ire1. Interestingly, however, the ire1 and hxl1 mutants showed differences in thermosensitivity, expression patterns for a subset of genes, and capsule synthesis, indicating that Ire1 has both Hxl1-dependent and -independent functions in C. neoformans. Finally, Ire1 and Hxl1 were shown to be critical for virulence of C. neoformans, suggesting UPR signaling as a novel antifungal therapeutic target.

RAM (regulation of Ace2p transcription factor and polarized morphogenesis) is a conserved signaling network that regulates polarized morphogenesis in yeast, worms, flies, and humans. To investigate the role of the RAM network in cell polarity and hyphal morphogenesis of Candida albicans, each of the C. albicans RAM genes (CaCBK1, CaMOB2, CaKIC1, CaPAG1, CaHYM1, and CaSOG2) was deleted. All C. albicans RAM mutants exhibited hypersensitivity to cell-wall- or membrane-perturbing agents, exhibiting cell-separation defects, a multinucleate phenotype and loss of cell polarity. Yeast two-hybrid and in vivo functional analyses of CaCbk1p and its activator, CaMob2p, the key factors in the RAM network, demonstrated that the direct interaction between the SMA domain of CaCbk1p and the Mob1/phocein domain of CaMob2p was necessary for hyphal growth of C. albicans. Genome-wide transcription profiling of a Camob2 mutant suggested that the RAM network played a role in serum- and antifungal azoles-induced activation of ergosterol biosynthesis genes, especially those involved in the late steps of ergosterol biosynthesis, and might be associated, at least indirectly, with the Tup1p-Nrg1p pathway. Collectively, these results demonstrate that the RAM network is critically required for hyphal growth as well as normal vegetative growth in C. albicans.

The production of recombinant therapeutic proteins is one of the fast-growing areas of molecular medicine and currently plays an important role in treatment of several diseases. Yeasts are unicellular eukaryotic microbial host cells that offer unique advantages in producing biopharmaceutical proteins. Yeasts are capable of robust growth on simple media, readily accommodate genetic modifications, and incorporate typical eukaryotic post-translational modifications. Saccharomyces cerevisiae is a traditional baker's yeast that has been used as a major host for the production of biopharmaceuticals; however, several nonconventional yeast species including Hansenula polymorpha, Pichia pastoris, and Yarrowia lipolytica have gained increasing attention as alternative hosts for the industrial production of recombinant proteins. In this review, we address the established and emerging genetic tools and host strains suitable for recombinant protein production in various yeast expression systems, particularly focusing on current efforts toward synthetic biology approaches in developing yeast cell factories for the production of therapeutic recombinant proteins.

Cancer is a very complicated process, characterized by the uncontrolled, unbalanced overgrowth of malignant cells. The complexity of oncogenic processes and cancer progressions has demanded the discovery of biomarkers with a high sensitivity and specificity for diagnosis, prognosis, diseases monitoring, and therapeutic response prediction. Unfortunately, a discrete biomarker for colon cancer has yet to be discovered, although nearly 800,000 new colorectal cancer cases are thought to globally occur each year, which account for ϳ10% of all incident cancers, and the mortality from colorectal cancer is estimated at nearly 450,000 per year (1). MLI1 and MSH genes are associated with hereditary non-polyposis colon cancer (2), and the APC gene is associated with familial adenomatous polyposis (3), but those factors fail to account for an occurrence of wide range of colon cancer. Moreover, colon cancer is one of the epithelium-derived cancers in which the circumstantial factors govern over hereditary genetic factors. These require a clear marker that serves as tracer molecule for the efficacious treatment of colon cancer.Recent proteomics have focused on a dynamic alteration of post-translational modification of proteins, and many lines of evidence indicate that changes in post-translational modification of proteins are closely associated with the pathogenic processes of cells. An aberrant glycosylation induced by Nacetylglucosaminyltransferase V (GnT-V), 1 is a representative example of such protein modification as is implicated in tumor progression. An increase in ␤1,6-branching on N-linked glycans is associated with metastatic potential of cancer cells (4). Several target molecules for GnT-V were proposed to be involved in cancer progressions, including matriptase (5), ␤ 1 integrin (6), and N-cadherin (7). However, those proteins are membrane-bound proteins and were not demonstrated to be aberrantly glycosylated in sera or tissues of cancer patients. Recent work stresses the discrete roles of the microenviron-