title: Electron microscopic study of eukaryotic 40S initiation complex in protein synthesis.
abstract: This electron microscopic study demonstrates that formation of a functional eukaryotic 40S initiation complex is accompanied by conformational changes which obscure the characteristic structural features of the 40S ribosomal subunits and of the initiation factor eIF-3, the only macromolecular components of the complex individually resolvable by conventional high resolution electron microscopy. The complex, characterized by a sedimentation coefficient of 46S, appears as a globular particle with a diameter of about 280 A and several characteristic protrusions and incisions. Similar structures were obtained with [40S X eIF-3] initiation complexes formed by interaction of eIF-3 from rabbit reticulocytes with 40S ribosomal subunits from either A. salina cysts or mouse liver. Incubation of eIF-3 with prokaryotic 30S subunits from E. coli produced no [30S X eIF-3] structures. The binding of eIF-3 to 40S subunits is weak, and both the [40S X eIF-3] and the complete 40S initiation complexes have to be stabilized by glutaraldehyde fixation. The extensive conformational changes associated with the complex formation preclude direct electron microscopic localization of eIF-3, a globular protein approximately 100 A in diameter, in the initiation domain of the 40S subunit.
mesh_terms: D000818:Animals; D005057:Eukaryotic Cells / Q000378:metabolism / Q000648:ultrastructure; D039621:Eukaryotic Initiation Factor-3; D066298:In Vitro Techniques; D008099:Liver / Q000378:metabolism* / Q000648:ultrastructure; D051379:Mice; D010448:Peptide Initiation Factors / Q000378:metabolism*; D014176:Protein Biosynthesis; D012269:Ribosomal Proteins / Q000096:biosynthesis; D012270:Ribosomes / Q000378:metabolism* / Q000648:ultrastructure
chemical_list: D039621:Eukaryotic Initiation Factor-3; D010448:Peptide Initiation Factors; D012269:Ribosomal Proteins