title: The human translation initiation multi-factor complex promotes methionyl-tRNAi binding to the 40S ribosomal subunit.
abstract: The delivery of Met-tRNA(i) to the 40S ribosomal subunit is thought to occur by way of a ternary complex (TC) comprising eIF2, GTP and Met-tRNA(i). We have generated from purified human proteins a stable multifactor complex (MFC) comprising eIF1, eIF2, eIF3 and eIF5, similar to the MFC reported in yeast and plants. A human MFC free of the ribosome also is detected in HeLa cells and rabbit reticulocytes, indicating that it exists in vivo. In vitro, the MFC-GTP binds Met-tRNA(i) and delivers the tRNA to the ribosome at the same rate as the TC. However, MFC-GDP shows a greatly reduced affinity to Met-tRNA(i) compared to that for eIF2-GDP, suggesting that MFC components may play a role in the release of eIF2-GDP from the ribosome following AUG recognition. Since an MFC-Met-tRNA(i) complex is detected in cell lysates, it may be responsible for Met-tRNA(i)-40S ribosome binding in vivo, possibly together with the TC. However, the MFC protein components also bind individually to 40S ribosomes, creating the possibility that Met-tRNA(i) might bind directly to such 40S-factor complexes. Thus, three distinct pathways for Met-tRNA(i) delivery to the 40S ribosomal subunit are identified, but which one predominates in vivo remains to be elucidated.
authors.affiliation: Department of Biochemistry and Molecular Medicine, University of California, Davis, CA 95616, USA.
authors.forename: Masaaki
authors.initials: M
authors.lastname: Sokabe
authors.forename: Christopher S
authors.initials: CS
authors.lastname: Fraser
authors.forename: John W B
authors.initials: JW
authors.lastname: Hershey
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000818:Animals; D015852:Eukaryotic Initiation Factor-2 / Q000378:metabolism; D039642:Eukaryotic Initiation Factors / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D010442:Peptide Chain Initiation, Translational; D012346:RNA, Transfer, Amino Acyl / Q000378:metabolism*; D011817:Rabbits; D012156:Reticulocytes / Q000378:metabolism; D054682:Ribosome Subunits, Small, Eukaryotic / Q000378:metabolism*
chemical_list: D015852:Eukaryotic Initiation Factor-2; D039642:Eukaryotic Initiation Factors; D012346:RNA, Transfer, Amino Acyl; C026464:tRNA(m)(Met), methionine-; D000244:Adenosine Diphosphate