Separation of Reticulocyte Initiation Factor M2 Activity into Two Components

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RESUMO

IF-M2, 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-M2A and IF-M2B. IF-M2A elutes near the void-volume, while IF-M2B, which is much smaller in molecular weight than IF-M2A, elutes slightly after a hemoglobin marker. In the presence of the other appropriate factors, both IF-M2A and IF-M2B are required to stimulate poly(U)-directed polyphenylalanine synthesis at low Mg++ concentration, ApUpG-directed Met-tRNAF binding to washed reticulocyte ribosomes, and initiation of globin synthesis from endogenous mRNA. IF-M2A stimulates ribosome-dependent GTP hydrolysis, while IF-M2B does not; IF-M2B stimulates ApUpG-directed fMet-tRNAF binding in the presence of IF-M1, while IF-M2A does not. Although IF-M2A and IF-M2B can be distinguished from each other by size and by activity, a distinct function for IF-M2B has not yet been found. Therefore, its precise role in the initiation process remains unclear.

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