The activity-controlling phosphorylation site is not the same in the four acidic ribosomal proteins from...

By using site-directed mutagenesis and chemical analysis of phosphopeptides, a unique phosphorylation site has been shown at serine 73 in the amino acid sequence of the Saccharomyces cerevisiae acidic ribosomal protein YP1 beta (L44'). The mutation in this position prevents in vitro phosphorylation by protein kinases that modify the wild-type polypeptide. The unphosphorylatable mutated protein is unable to bind to the ribosomes and to rescue the growth deficiency of yeast strains in which the corresponding original gene is inactivated by gene disruption. Sequencing of tryptic phosphopeptides has shown that acidic proteins YP1 alpha and YP2 alpha (L44) are also phosphorylated at positions near the carboxyl end. These results contrast with the data indicating that in the highly homologous protein YP2 beta, phosphorylation takes place at serine 19, close to the amino terminus. The results show that phosphorylation is definitely required for the biological activity of these ribosomal proteins. However, the differences in the phosphorylation sites suggest that the effect of this modification is not the same in all of them, confirming the heterologous role of these peculiar ribosomal components. In fact, the different context of the modification sites in the four polypeptides suggests the existence of more than one protein kinase specific for this set of proteins.