Disease relevance of SUI2

• We report the preparation of recombinant yeast eIF-2 alpha from Escherichia coli and its use in in vitro phosphorylation studies [1].

High impact information on SUI2

• In vivo, phosphorylation of eIF-2 alpha increases in response to amino acid starvation, which is dependent on GCN2 [2].
• Substitution of Ser-51 with alanine eliminates phosphorylation of eIF-2 alpha by GCN2 in vivo and in vitro and abolishes increased expression of GCN4 and amino acid biosynthetic genes under its control in amino acid-starved cells [2].
• The latter result indicates that the gamma subunit of eIF-2 participates in recognition of the start site for protein synthesis, a role previously demonstrated in yeast for eIF-2 alpha and eIF-2 beta [3].
• These mutations may identify a region in eIF-2 alpha that participates directly in a physical interaction with the GCN3 subunit of eIF-2B [4].
• We have cloned the cDNA of the heme-regulated eIF-2 alpha kinase (HRI) of rabbit reticulocytes [5].

Biological context of SUI2

• Although substitution of SUI2 genes mutated at these sites for the wild-type gene have no obvious effect on cell growth, one test that we have used appears to demonstrate that the inability to phosphorylate these sites has a physiological consequence on eIF-2 function in S. cerevisiae [6].
• In mammalian cells, phosphorylation of eIF-2 alpha inhibits the activity of eIF-2B, the GDP-GTP exchange factor for eIF-2 [7].
• In this report, molecular and biochemical characterizations show that the sui2 suppressor gene encodes the alpha subunit of eIF-2 [8].
• The amino acid sequence of sui2 is 58% homologous to that encoded by the cDNA of the human eIF-2 alpha [8].
• Because the mutant tRNA(Val) exacerbated the slow-growth phenotype associated with eIF-2 alpha hyperphosphorylation by an activated GCN2c kinase, we suggest that the GCN2-independent derepression mechanism involves down-regulation of eIF-2 activity [9].

Anatomical context of SUI2

• This would explain why mutations in eIF-3 mimic eIF-2 alpha phosphorylation in allowing ribosomes to bypass the uORFs and reinitiate at GCN4 [10].

Associations of SUI2 with chemical compounds

• Phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF-2 alpha) impairs translation initiation by inhibiting the guanine nucleotide exchange factor for eIF-2, known as eIF-2B [11].
• Biochemical experiments show that eIF-2 alpha is phosphorylated in response to purine starvation and that this reaction is completely dependent on GCN2 [12].
• Met-tRNA(iMet) ternary complex is the principal component limiting translation in cells when eIF-2 alpha is phosphorylated on serine 51 [13].
• Only when cells were starved for a carbon source for 2 h in 1 M sorbitol was eIF-2 alpha isolated in the nonphosphorylated state [14].

Enzymatic interactions of SUI2

• Here we describe that yeast eIF-2 alpha is a constitutively phosphorylated protein species that is multiply phosphorylated by a GCN2-independent mechanism [6].
• The protein kinase GCN2 stimulates expression of the yeast transcriptional activator GCN4 at the translational level by phosphorylating the alpha subunit of translation initiation factor 2 (eIF-2 alpha) in amino acid-starved cells [15].

Regulatory relationships of SUI2

• We isolated mutations in several unlinked genes that suppress the growth-inhibitory effect of eIF-2 alpha phosphorylation catalyzed by mutationally activated forms of GCN2 [11].
• This double GCD7 mutation also completely suppressed the lethal effect of expressing the mammalian eIF-2 alpha kinase dsRNA-PK in yeast cells, showing that the translational machinery had been rendered completely insensitive to phosphorylated eIF-2 [11].
• Consistent with eIF-2 functioning in translation as part of a ternary complex composed of eIF-2, GTP, and Met-tRNA(iMet), reduced gene dosage of initiator tRNA(Met) mimicked phosphorylation of eIF-2 alpha and stimulated GCN4 translation [13].

Other interactions of SUI2

• This finding suggests that mutations in GCD-encoded subunits of the complex derepress GCN4 translation because they mimic eIF-2 alpha phosphorylation in decreasing eIF-2B activity [7].
• The SUI2 and SUI3 genes of Saccharomyces cerevisiae encode the alpha and beta subunits, respectively, of translation initiation factor eIF-2 (eukaryotic initiation factor 2) [16].
• 32Pi labeling and isoelectric focusing analysis of a SUI2+ delta gcn2 strain identifies eIF-2 alpha as radiolabeled and a single isoelectric protein species [6].
• Both sui1 and sui2 are recessive and cause temperature-sensitive growth on enriched medium [17].
• Protein sequence analysis shows that a sui2 mutant yeast strain allows initiation at a UUG codon in the absence of an AUG codon at HIS4 [8].

Analytical, diagnostic and therapeutic context of SUI2

• Cell fractionation studies further demonstrated that the synthesised human eIF-2 alpha protein, though present in the cytoplasm, was largely associated with the yeast ribosomes, but could be removed from these by washing with 0.3 M KCl [18].
• Isoelectric focusing coupled with Western-blot analysis demonstrated that the human eIF-2 alpha subunit synthesized in yeast under a variety of growth conditions was detected as two bands which co-migrated with the phosphorylated and unphosphorylated forms of rabbit eIF-2 alpha, suggesting covalent modification in vivo [18].
• The phosphorylation state of the alpha subunit of initiation factor 2 (eIF-2 alpha) in Saccharomyces cerevisiae has been determined by two-dimensional gel electrophoresis and autoradiography of lysates from cultures grown under a variety of conditions [14].
• We now describe the identification of S. mansoni eukaryotic translation initiation factor 2 alpha subunit (eIF2 alpha), through its interaction with SmRK1 in a yeast two-hybrid assay [19].

References