Initiation of protein synthesis in eukaryotes. Binding to Sepharose-heparin and partial purification of initiation factors from Krebs II ascites cells

By means of affinity chromatography of lysates from Krebs II ascites cells and rabbit reticulocytes on Sepharose-heparin an active fraction of initiation factors has been obtained. The fraction is eluted from the column at 350 mM KCl using a linear gradient and displays a number of activities, i.e. binding of Met-tRNAfMet to form a ternary complex with GTP; transferring this complex to 40-S subunits in an A-U-G-independent step and finally coupling of the 40-S initiation complex to the 60-S subunit, a reaction which is completely A-U-G-dependent. Moreover, MettRNA is bound into the P-site as is indicated by its puromycin sensitivity. The method is very suitable for large-scale preparation. Further purification and characterization of the factors have been carried out on DEAE-cellulose and phosphocellulose columns. Evidence is presented that the polysomes present in a lysate that has been passed through the Sepharose-heparin column can only complete their nascent chains, initiation of new polypeptides is completely dependent on addition of initiation factors.