RAP KINASE, A NEW ENZYME PHOSPHORYLATING THE ACIDIC P-PROTEIN FROM SACCHAROMYCES-CEREVISIAE

A new protein kinase, showing a high specificity for the ribosomal aci dic P proteins (RAP kinase) has been purified and characterized from S accharomyces cerevisiae extracts. Purification was carried out by four chromatographic steps, including DEAE-cellulose, phosphocellulose, he parin-Sepharose and P protein-Sepharose. The purified enzyme preparati on contains only one polypeptide of around 55 kDa as determined by SDS gel electrophoresis and gradient centrifugation. RAP kinase is differ ent from all previous well-characterized kinases and does not show cro ss-reaction with antibodies to the 71 kDa 60S ribosomal subunit-specif ic kinase PK60 previously reported. The enzyme uses ATP as a better ph osphate donor and is less sensitive to heparin than casein kinase LI b ut is moderately affected by salt. Among the different substrates test ed, ribosomal acidic proteins are preferentially modified by RAP kinas e, which phosphorylates only serine residues in the four P proteins as well as the related ribosomal protein P0. Casein is phosphorylated at a much lower level. All the data indicate that RAP kinase might be th e enzyme responsible for the phosphorylation of the P proteins, and in this way may also participate in a possible translational regulatory mechanism.