PHOSPHORYLATION SITE-SPECIFICITY OF THE CDC2-RELATED KINASE PITALRE

PITALRE is a human protein kinase belonging to the cell division cycle 2 (CDC2) kinase family, and is the catalytic subunit of a multimeric complex that contains several cellular proteins. PITALRE complexes fro m several cell lines and tissues phosphorylate retinoblastoma protein and myelin basic protein (MBP). In the present work, we have found tha t MBP is phosphorylated by PITALRE complexes on both Ser and Thr resid ues. Two different antibodies raised to PITALRE purified virtually ide ntical kinase activities, as analysed by MBP phosphopeptide mapping an d phosphoamino acid analysis. We have identified the proline-directed residue Ser-162 of MBP as a major phosphorylation site for PITALRE. In addition, our results suggest that one of the two MBP proline-directe d threonine residues, Thr-97, is also selectively phosphorylated by PI TALRE. These data, together with analysis of different peptide substra tes derived from sites on MBP that are phosphorylated by PITALRE, indi cate that PITALRE is a Ser/Thr proline-directed kinase. In addition, o ur results show that PITALRE has a substrate site specificity distingu ishable from those of the CDC2 and cyclin-dependent kinase 2 (CDK2).