Differential phosphorylation of ribosomal acidic proteins from yeast cell by two endogenous protein kinases: casein kinase-2 and 60S kinase

The native 80S ribosomes isolated from Saccharomyces cerevisiae (strain W303) cells was phosphorylated by two endogenous protein kinases: multifunctional casein kinase-2 (CK-2) and specific 60S kinase. Three acidic proteins within the 60S ribosomal subunit: YP1(3, YPlp' and YP2a are phosphorylated by both kinases. The other two proteins: YPla and YP2(3 are predominantly phosphorylated by CK-2 but not by 60S kinase. This was confirmed in the experiment with the recombinant protein, YP2P, as a substrate, which is practically not phosphorylated by specific 60S kinase. These results together with the previous data based on the target amino-acid se­quences suggest that, in addition to the multifunctional casein kinase-2 and specific 60S kinase, there exist probably other protein kinase(s) which phosphorylate the ribosomal acidic proteins in the cell.