ABSTRACT
Delivery of the initiator methionine transfer RNA (Met-tRNAiMet) to the ribosome is a key step in the initiation of protein synthesis. Previous results have indicated that this step is catalyzed by the structurally dissimilar translation factors in prokaryotes and eukaryotes-initiation factor 2 (IF2) and eukaryotic initiation factor 2 (eIF2), respectively. A bacterial IF2 homolog has been identified in both eukaryotes and archaea. By using a combination of molecular genetic and biochemical studies, the Saccharomyces cerevisiae IF2 homolog is shown to function in general translation initiation by promoting Met-tRNAiMet binding to ribosomes. Thus, the mechanism of protein synthesis in eukaryotes and prokaryotes is more similar than was previously realized.
Subject(s)
DNA-Binding Proteins , Peptide Initiation Factors/metabolism , Protein Biosynthesis , RNA, Transfer, Met/metabolism , Ribosomes/metabolism , Saccharomyces cerevisiae Proteins , Saccharomyces cerevisiae/genetics , Codon, Initiator , Cytoplasm/chemistry , Eukaryotic Initiation Factor-2/metabolism , Eukaryotic Initiation Factor-2/pharmacology , Fungal Proteins/biosynthesis , Fungal Proteins/genetics , Fungal Proteins/metabolism , Peptide Chain Initiation, Translational , Peptide Initiation Factors/analysis , Peptide Initiation Factors/genetics , Prokaryotic Initiation Factor-2 , Protein Kinases/genetics , Protein Kinases/metabolism , Recombinant Fusion Proteins/metabolism , Saccharomyces cerevisiae/metabolismABSTRACT
Initiation of translation in eukaryotes is mediated by a set of initiation factors. Mammalian initiation factor 3 is composed of at least 8 subunits, with the largest being about 180 kDa in size. Here we report the cloning of the p180 subunit of human eukaryotic translation initiation factor (eIF) 3. The amino acid sequence deduced from the cDNA agrees with the sequences of CNBr fragments of eIF-3, confirming the identity of the clone. The 1382 amino acid open reading frame contains a high percentage of charged residues (48%) and an unusual repetitive domain near the carboxyl terminus composed of 25 repeats of 10 amino acids each. Data base searches identified related sequences found in members of the plant and fungal kingdoms as well as in other mammals and the nematode Caenorhabditis elegans. These sequences share significant identity with the human clone and probably represent the homologues of the p180 subunit in these organisms. This is the first report identifying the sequence of the large subunit of eIF-3.
