Involvement of a 50-kDa mRNP Protein fromSaccharomyces cerevisiaein mRNA Binding to Ribosomes

A yeast 50-kDa mRNA-binding protein (50mRNP) is found selectively associated with the 48S and 80S initiation complexes. This protein is structurally related to the translational elongation factor EF-1α. The protein reacts with antibodies directed against EF-1α and, similarly, EF-1α recognizes antibodies against the 50mRNP protein. This is evidence that they share at least one epitope which allows a similar antigenic behavior. In addition, both proteins show similar cleavage patterns upon treatment with the endoproteinase Lys-C. A murine antibody raised against 50mRNP inhibits both 48S and 80S initiation complex formation. The inhibitory effect is relieved by preincubating anti-50mRNP with EF-1α. Antibody to EF-1α manifests a similar inhibitory pattern for the formation of 48S and 80S complexes. These data strongly suggest that 50mRNP is an EF-1α-like polypeptide essential for the formation of the above complexes.