Ribosome-bound EF-1 alpha-like protein of yeast Saccharomyces cerevisiae

The SUP2 (SUP35) omnipotent suppressor gene encodes the EF-1 alpha-like polypeptide, intimately involved in the control of translational ambiguity in the yeast Saccharomyces cerevisiae. The present study is devoted to the immunological characterization of the Sup2 protein. The SUP2 gene was fused to the Escherichia coli lacZ gene and a polyclonal antibody against the corresponding Sup2--beta-galactosidase hybrid protein was obtained. This antibody identified a 79-kDa protein that was absent in those cells where the SUP2 gene was disrupted, and an abundance of this protein was observed in cells overexpressing the SUP2 gene. The localization of this protein was studied in subcellular fractionation experiments. The SUP2 gene product proved to be uniformly distributed throughout ribosome-enriched samples, i.e. free polysomes, crude microsomes and rough endoplasmic reticulum. It was not found in the cytoplasm and smooth endoplasmic reticulum. The SUP2-encoded protein was fully ribosome associated and less abundant than the ribosomal protein L3. Also, in a sucrose gradient, Sup2 preferentially cosedimented with the 40S ribosomal subunit, but not with the 60S subunit. The functional significance of this association is discussed.