Beschreibung:
IF-M 2 , one of three initiation factors isolated by DEAE-cellulose chromatography from the 0.5 M KCl-wash fraction of rabbit reticulocyte ribosomes, has been separated by Sephadex G-200 chromatography into two components: IF-M 2A and IF-M 2B . IF-M 2A elutes near the void-volume, while IF-M 2B , which is much smaller in molecular weight than IF-M 2A , elutes slightly after a hemoglobin marker. In the presence of the other appropriate factors, both IF-M 2A and IF-M 2B are required to stimulate poly(U)-directed polyphenylalanine synthesis at low Mg ++ concentration, ApUpG-directed Met-tRNA F binding to washed reticulocyte ribosomes, and initiation of globin synthesis from endogenous mRNA. IF-M 2A stimulates ribosome-dependent GTP hydrolysis, while IF-M 2B does not; IF-M 2B stimulates ApUpG-directed fMet-tRNA F binding in the presence of IF-M 1 , while IF-M 2A does not. Although IF-M 2A and IF-M 2B can be distinguished from each other by size and by activity, a distinct function for IF-M 2B has not yet been found. Therefore, its precise role in the initiation process remains unclear.