Eukaryotic eIF-5A was initially thought to function as a translation initiation factor, based on its ability to stimulate methionyl-puromycin synthesis. However, subsequent work revealed a role for eIF5A in translation elongation [, ]. Depletion or inactivation of eIF-5A in the yeast Saccharomyces cerevisiae (Baker's yeast) resulted in the accumulation of polysomes and an increase in ribosomal transit times. Addition of recombinant eIF-5A from yeast, but not a derivative lacking hypusine, enhanced the rate of tripeptide synthesis in vitro. Moreover, inactivation of eIF-5A mimicked the effects of the eEF2 inhibitor sordarin, indicating that eIF-5A might function together with eEF2 to promote ribosomal translocation. Finally, it was shown that eIF5A is specifically required to promote peptide-bond formation between consecutive proline residues. It has been proposed to stimulate the peptidyl-transferase activity of the ribosome and facilitate the reactivity of poor substrates like proline [].eIF-5A is a cofactor for the Rev and Rex transactivator proteins of human immunodeficiency virus-1 and T-cell leukaemia virus I, respectively [, , ]. IF-5A is the sole protein in eukaryotes and archaea to contain the unusual amino acid hypusine (Ne-(4-amino-2-hydroxybutyl)lysine) that is an absolute functional requirement. The first step in the post-translational modification of lysine to hypusine is catalyzed by the enzyme deoxyhypusine synthase, the structure of which has been reported []. The archaeal IF-5A proteins have not been studied as comprehensively as their eukaryotic homologues, though the crystal structure of the Pyrobaculum aerophilum protein has been determined. Unmodified P. aerophilum IF-5A is found to be a beta structure with two domains and three separate hydrophobic cores. The lysine (Lys42) that is post-translationally modified by deoxyhypusine synthase is found at one end of the IF-5A molecule in a turn between beta strands beta4 and beta5; this lysine residue is freely solvent accessible. The C-terminal domain is found to be homologous to the cold-shock protein CspA of E. coli, which has a well characterised RNA-binding fold, suggesting that IF-5A is involved in RNA binding [].This entry represents the archaeal IF-5A proteins.
Eukaryotic eIF-5A was initially thought to function as a translation initiation factor, based on its ability to stimulate methionyl-puromycin synthesis. However, subsequent work revealed a role for eIF5A in translation elongation [, ]. Depletion or inactivation of eIF-5A in the yeast Saccharomyces cerevisiae (Baker's yeast) resulted in the accumulation of polysomes and an increase in ribosomal transit times. Addition of recombinant eIF-5A from yeast, but not a derivative lacking hypusine, enhanced the rate of tripeptide synthesis in vitro. Moreover, inactivation of eIF-5A mimicked the effects of the eEF2 inhibitor sordarin, indicating that eIF-5A might function together with eEF2 to promote ribosomal translocation. Finally, it was shown that eIF5A is specifically required to promote peptide-bond formation between consecutive proline residues. It has been proposed to stimulate the peptidyl-transferase activity of the ribosome and facilitate the reactivity of poor substrates like proline [].eIF-5A is a cofactor for the Rev and Rex transactivator proteins of human immunodeficiency virus-1 and T-cell leukaemia virus I, respectively [, , ]. IF-5A is the sole protein in eukaryotes and archaea to contain the unusual amino acid hypusine (Ne-(4-amino-2-hydroxybutyl)lysine) that is an absolute functional requirement. The first step in the post-translational modification of lysine to hypusine is catalyzed by the enzyme deoxyhypusine synthase, the structure of which has been reported []. The archaeal IF-5A proteins have not been studied as comprehensively as their eukaryotic homologues, though the crystal structure of the Pyrobaculum aerophilum protein has been determined. Unmodified P. aerophilum IF-5A is found to be a beta structure with two domains and three separate hydrophobic cores. The lysine (Lys42) that is post-translationally modified by deoxyhypusine synthase is found at one end of the IF-5A molecule in a turn between beta strands beta4 and beta5; this lysine residue is freely solvent accessible.The C-terminal domain is found to be homologous to the cold-shock protein CspA of E. coli, which has a well characterised RNA-binding fold, suggesting that IF-5A is involved in RNA binding [].This family also includes the Woronin body major protein Hex1, whose sequence and structure are similar to eukaryotic initiation factor 5A (eIF5A), suggesting they share a common ancestor during evolution []. Woronin bodies are important for stress resistance and virulence [].
Co-chaperones are helper interacting proteins that modulate the chaperone cycle, being involved in substrate specificity and stimulation of chaperone activity of HSP90/70 and include other heat shock proteins, TPR containing proteins, cyclophilins and others. The TPR containing proteins possess an N-terminal TPR domain, which are more closely related to each other than to TPR domains from other proteins with different functionality [, ], which is involved in HSP90/70 direct interaction. The first N-terminal residues prior to the TRP domain and the C-terminal domain are involved and important for domain interplay and stabilisation of its interactions []. The Hsp90 chaperone machinery in eukaryotes comprises a number of distinct accessory factors, among them TTC4 from human and its homologues Cns1 from yeast and Dpit47 from Drosophila, structurally and functionally conserved from yeast to human. Cns1 is one of the few essential co-chaperones in yeast, important for maintaining translation elongation, specifically chaperoning the elongation factor eEF2. Cns1 interacts with Hgh1 and forms a quaternary complex together with eEF2 and Hsp90 mediating the proper folding and solubility of eEF2. Recently, the C-terminal structure has been solved and is called the "wheel"domain according to its 2D projection. It shows an overall fold consisting of a twisted five-stranded beta sheet surrounded by several alpha helices [].This entry represents the wheel domain found at the C terminus of yeast Cns1, human TTC4 and Drosophila Dpit47 proteins.
