Disease relevance of TIF5

• The yeast gene that encodes eIF-5, designated TIF5, has been isolated and expressed in Escherichia coli to yield a catalytically active eIF-5 protein [1].

High impact information on TIF5

• The yeast eIF3 subunits TIF32/a, NIP1/c, and eIF5 make critical connections with the 40S ribosome in vivo [2].
• Initiation factor 3 (eIF3) forms a multifactor complex (MFC) with eIF1, eIF2, and eIF5 that stimulates Met-tRNA(i)(Met) binding to 40S ribosomes and promotes scanning or AUG recognition [2].
• The eIF5 stimulates GTP hydrolysis by the eIF2/GTP/Met-tRNA(i)(Met) ternary complex on base-pairing between Met-tRNA(i)(Met) and the start codon [3].
• Biochemical characterization of SUI3 suppressor alleles that encode mutant forms of the beta subunit of eIF-2 revealed that these mutant eIF-2 complexes have a higher intrinsic rate of GTP hydrolysis, which is eIF-5 independent [4].
• The SUI5 suppressor gene is identical to the TIF5 gene that encodes eIF-5, a translation initiation factor known to stimulate the hydrolysis of GTP bound to eIF-2 as part of the 43S preinitiation complex [4].

Biological context of TIF5

• In Saccharomyces cerevisiae, eIF5 is encoded by a single copy essential gene, TIF5, that encodes a protein of 45,346 daltons [5].
• First, the presence of two in-frame translational start sites located 51 base pairs apart suggests the possibility that two proteins, differing by an amino-terminal extension of 17 amino acids, could be generated from the TIF5 gene via differential translational starts [1].
• TIF5 is a single-copy gene that maps on yeast chromosome XVI and is essential for cell viability [1].
• Surprisingly, substitution of the TIF5 gene mutated at these sites for the wild-type gene had no obvious effect on cell growth under normal growth conditions [6].
• Among the three, complete open reading frames: YPR041w, YPR042c and YPR043w contained within this fragment, the gene YPR041w was shown to complement the tsa1 mutation and to correspond to the TIF5 gene encoding an essential protein synthesis initiation translation factor [7].

Anatomical context of TIF5

• This is the first demonstration that the TIF5 gene encodes a protein involved in initiation of translation in eukaryotic cells [5].
• Analysis of the polysome profiles of eIF5-depleted cells showed greatly diminished polysomes with simultaneous increase in free ribosomes [5].
• The effect of added deacylase on the labeling of 48 S complexes with [35S]Met-tRNAf can be overcome by adding eIF-5 or a soluble reticulocyte protein that has been termed the reversing factor, but not by the addition of eIF-2 [8].

Associations of TIF5 with chemical compounds

• In agreement with this, archaea appear to lack eIF5, eIF2B and the lysine-rich binding domain for these factors in their eIF2beta homolog [9].
• Finally, like typical GAPs, eIF5 also contains an arginine-finger motif consisting of an invariant arginine residue at its N-terminus that is also essential for its function [10].
• This interaction, which occurs between the lysine-rich N-terminal region of the beta subunit of eIF2 and the glutamic acid-rich C-terminal region of eIF5, is essential for eIF5 function both in vitro and in vivo in yeast cells [10].
• When crude eIF-5 preparations, as well as yeast cells that were lysed directly into a denaturing buffer containing 3% sodium dodecyl sulfate, were analyzed by Western blots probed with affinity-purified anti-eIF-5 antibodies, a major immunoreactive polypeptide (apparent M(r) = 54,000) and a minor band (apparent M(r) = 56,000) were observed [11].
• In this work, we show that eIF5 immunoprecipitated from cell-free extracts of (32)P-labelled yeast cells is phosphorylated on multiple serine residues [6].

Physical interactions of TIF5

• The N-terminal domain (NTD) of NIP1/eIF3c interacts directly with eIF1 and eIF5 and indirectly through eIF5 with the eIF2-GTP-Met-tRNA(i)(Met) ternary complex (TC) to form the multifactor complex (MFC) [12].

Regulatory relationships of TIF5

• Eukaryotic translation initiation factor 5 is critical for integrity of the scanning preinitiation complex and accurate control of GCN4 translation [13].

Other interactions of TIF5

• Overexpressing a CTD-less form of TIF32 exacerbated the initiation defect of an eIF5 mutation that weakens the NIP1-eIF5-eIF2 connection [14].
• To define the factor requirements for these reactions in vivo, we examined the effects of depleting eIF2, eIF3, eIF5, or eIF4G in Saccharomyces cerevisiae cells on binding of the ternary complex, other initiation factors, and RPL41A mRNA to native 43S and 48S preinitiation complexes [15].
• We show that either the absence of Fun12p (eIF5B), or a defect in eIF5, proteins involved in 60S ribosomal subunit joining, specifically reduces the translation of poly(A)(+) mRNA, suggesting that poly(A) may have a role in promoting the joining step [16].
• Finally, at the very C-termini of GCD6, eIF-2B epsilon, and two other eukaryotic translation initiation factors, eIF-4 gamma and eIF-5, there is a previously undetected, conserved domain [17].

Analytical, diagnostic and therapeutic context of TIF5

• In less pure preparations of yeast eIF-5, however, a significant proportion of eIF-5 activity eluted from gel filtration columns as a protein of M(r) > 140,000 [11].
• Here we provide a comparative sequence analysis of the beta subunit of eIF2 and its archaeal counterpart (aIF2beta). aIF2beta differs from eIF2beta in not possessing an N-terminal extension implicated in binding RNA, eIF5 and eIF2B [18].

References