Phosphorylation of the yeast ribosomal stalk. Functional effects and enzymes involved in the process

The ribosomal stalk is directly involved in the interaction of the elongati on factors with the ribosome during protein synthesis. The stalk is formed by a complex of five proteins, four small acidic polypeptides and a larger protein which directly interacts with the rRNA at the GTPase center. In euk aryotes the acidic components correspond to the 12-kDa P1 and P2 proteins, and the RNA binding component is the PO protein. All these proteins are fou nd phosphorylated in eukaryotic organisms, and previous in vitro data sugge sted this modification was involved in the activity of this structure. Resu lts from mutational studies have shown that phosphorylation takes place at a serine residue close to the carboxy end of the P proteins. Modification o f this serine residue does not affect the formation of the stalk and the ac tivity of the ribosome in standard conditions but induces an osmoregulation -related phenotype at 37 degrees C. The phosphorylatable serine is part of a consensus casein kinase II phosphorylation site. However, although CKII s eems to be responsible for part of the stalk phosphorylation in vivo, it is probably not the only enzyme in the cell able to perform this modification . Five protein kinases, RAPI, RAPII and RAPIII, in addition to the previous ly reported CKII and PK60 kinases, are able to phosphorylate the stalk prot eins. A comparison of the five enzymes shows differences among them that su ggest some specificity regarding the phosphorylation of the four yeast acid ic proteins. It has been found that some typical effecters of the PKC kinas e stimulate the in vitro phosphorylation of the stalk proteins. All the dat a suggest that although phosphorylation is not involved in the interaction of the acidic P proteins with the ribosome, it can affect the ribosome acti vity and might participate in a possible ribosome regulatory mechanism. (C) 1999 Federation of European Microbiological Societies. Published by Elsevi er Science B.V. All rights reserved.