Structure and function of the stalk, a putative regulatory element of the yeast ribosome. Role of stalk protein phosphorylation

The ribosomal stalk is involved directly 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 protein P0. All these proteins are found to be phosphorylated in eukaryotic organisms. Previous in vitro data sugges ted this modification was involved in the activity of this structure. To co nfirm this possibility a mutational study has shown that phosphorylation ta kes place at a serine residue close to the carboxyl end of proteins P1, P2 and P0. This serine is part of a consensus casein kinase II phosphorylation site. However, by using a yeast strain carrying a temperature sensitive mu tant, it has been shown that CKII is probably not the only enzyme responsib le for this modification. Three new protein kinases, RAPI, RAPII and RAPIII , have been purified and compared with CKII and PK60, a previously reported enzyme that phosphorylates the stalk proteins. Differences among the five enzymes have been studied. It has also been found that some typical effecte rs of the PKC kinase stimulate the in vitro phosphorylation of the stalk pr oteins. All the data available suggest that phosphorylation, although it is not involved in the interaction of the acidic proteins with the ribosome, affects ribosome activity and might participate in some ribosome regulatory mechanism.