The purification and properties of two low-molecular-weight proteins required for the initiation of translation in ascites tumour cells.

A fractionated cell-free protein-synthesising system from Krebs II ascites tumour cells is described. It is dependent on the addition of exogenous mRNA. In this system the translation of natural mRNAs requires the presence of four initiation factors: the high-molecular-weight complex IF-M3, IFEMC and two low-molecular-weight proteins IF-Malpha and IF-Mbeta. None of these factors are required for the translation of the synthetic messenger poly(A,U,G). The purification of IF-Malpha and IF-Mbeta was achieved by the use of gel and ion-exchange chromatography. Analysis of the purified proteins by sodium dodecyl sulphate/polyacrylamide gel electrophoresis showed that the molecular weight of IF-Malpha is 19,000 and that of IF-Mbeta is 15,000. The initiation factor IF-E1 isolated from rabbit reticulocytes can substitute for IF-Mbeta in the translation of encephalomyocarditis virus RNA. IF-Malpha does not correspond to any previously recognised eukaryotic initiation factor.