The Mechanism of Action of Eukaryotic Initiation Factor 4C in Protein Synthesis

Binding of eukaryotic initiation factor 4C (eIF-4C) to a 40-S initiation complex could be detected by analysis of assay mixtures for 40-S initiation complex formation on columns of Sepharose 6B and on sucrose gradients. On Sepharose 6B columns an almost stoichiometric binding of [14C]eIF-4C was observed, provided that eIF-1, eIF-2, eIF-3, Met-tRNAf and mRNA were included in the assay mixtures. On sucrose gradients the presence of 0.5 microM edeine was an additional prerequisite for stable binding of eIF-4C. The presence of eIF-4C on the 40-S initiation complex resulted in a pronounced increase of its stability, which might explain the observed stimulatory effect of the factor on the binding of Met-tRNAf into initiation complexes. The interaction between eIF-4C and the small ribosomal subunit presumably occurs at an early stage of the initiation process, since eIF-4C was capable of dissociating 80-S ribosomes into subunits. Protein synthesis in a 'pH-5' assay was almost totally dependent on the addition of eIF-4C, when the assay was performed with purified, nearly homogeneous, initiation factors.