Abstract
Abstract The 0.5 m KCl wash fraction of rabbit liver microsomes has been fractionated by DEAE-cellulose chromatography into the protein synthesis initiation factors IF-M1, IF-M2(a+b), and IF-M3. IF-M2(a+b) was also separated into two components, IF-M2a and IF-M2b, by Sephadex G-200 chromatography. Further purification of the factors was obtained by ammonium sulfate fractionation, ion exchange chromatography, and/or gel filtration chromatography. The chromatographic behavior of the liver initiation factors on ion exchange resins and Sephadex G-200 is very similar to that of the reticulocyte factors at comparable stages of purification. Furthermore, there is a high degree of interchangeability of liver and reticulocyte IF-M1, IF-M2a, and IF-M2b in polyphenylalanine synthesis and also in the Met-tRNAfmet binding assay. Liver IF-M3 substitutes for reticulocyte IF-M3 in endogenous and exogenous mRNA-directed protein synthesis and in single peptide bond formation assays. These findings suggest that liver initiation factors IF-M1, IF-M2a, IF-M2b, and IF-M3 are identical, or at least very similar, to the corresponding reticulocyte factors.
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