INVIVO AND INVITRO PHOSPHORYLATION OF RIBOSOMAL-PROTEINS BY PROTEIN-KINASES FROM SACCHAROMYCES-CEREVISIAE

From the high salt wash of the ribosomes of the yeast S. cerevisiae, 3 protein kinases were isolated and separated by DEAE-cellulose chromatography. The 3 kinases differed in their abilities to phosphorylate substrates such as histones (calf thymus), casein and S. cerevisiae ribosomes; 2 of the kinases showed increased activity in the presence of cyclic AMP when histones and 40S ribosomal subunits were used as substrates. The protein kinases catalyzed phosphorylation of certain proteins of the 40S and 60S ribosomal subunits and 80S ribosomes in vitro. Nine proteins of the 80S ribosome, 7 proteins of the 40S subunit and 11 of the 60S subunit were phosphorylated; different proteins were modified to various extents when different kinases were used. Several proteins of 40S and 60S ribosomal subunits were identified which are not available to the kinases in the 80S particles. Ribosomes isolated from S. cerevisiae cells growing in logarithmic phase of growth contained a number of phosphorylated proteins. Studies by 2-dimensional polyacrylamide gel electrophoresis indicated that the ribosomal proteins phosphorylated in vivo corresponded with those phosphorylated in vitro. The relationship of in vivo phosphorylation of ribosomes to the growth and physiology of S. cerevisiae is not known.