EUKARYOTIC ACIDIC PHOSPHOPROTEINS INTERACT WITH THE RIBOSOME THROUGH THEIR AMINO-TERMINAL DOMAIN

Variable-size fragments of the four yeast acidic ribosomal protein gen es rpYP1 alpha, rpYP1 beta, rpYP2 alpha and rpYP2 beta were fused to t he LacZ gene in the vector series YEp356-358. The constructs were used to transform wild-type Saccharomyces cerevisiae and several gene-disr upted strains lacking different acidic ribosomal protein genes. The di stribution of the chimeric proteins between the cytoplasm and the ribo somes, tested as beta-galactosidase activity, was estimated. Hybrid pr oteins containing around a minimum of 65-75 amino acids from their ami no-terminal domain are able to bind to the ribosomes in the presence o f the complete native proteins. Hybrid proteins containing no more tha n 36 amino terminal amino acids bind to the ribosomes in the absence o f a competing native protein. The fused YP1-beta-galactosidase protein s are also able to form a complex with the native YP2 type proteins, p romoting their binding to the ribosome. The stability of the hybrid po lypeptides seems to be inversely proportional to the size of their P p rotein fragment. These results indicate that only the amino-terminal d omain of the eukaryotic P proteins is needed for the P1-P2 complex for mation required for interaction with the ribosome. The highly conserve d P protein carboxyl end is not implicated in the binding to the parti cles and is exposed to the medium.