title: Phosphorylation of synapsin I at a novel site by proline-directed protein kinase.
abstract: Previous studies identified synapsin I as a potential substrate for a newly discovered growth factor-sensitive, proline-directed protein kinase originally isolated from rat pheochromocytoma. The present study describes the site-specific phosphorylation of synapsin I by highly purified preparations of proline-directed protein kinase. The incorporation of [32P]phosphate into bovine brain synapsin I was dependent upon both the amount of kinase present and the time of incubation. The maximum stoichiometry of phosphorylation approached 1 mol of phosphate/mol of synapsin I protein. When analyzed by sodium dodecyl sulfate-gel electrophoresis and autoradiography, [32P]phosphate was found to be incorporated into both synapsin Ia and Ib. Phosphoamino acid analysis demonstrated that serine residues were phosphorylated exclusively. Digestion of phosphorylated synapsin I with trypsin followed by high performance liquid chromatography (HPLC) phosphopeptide analysis indicated that the tryptic peptide containing the major phosphorylation site eluted as a single peak at approximately 17% acetonitrile. The primary structure of this phosphopeptide, determined by gas-phase sequencing, was found to be Gln-Ser-Arg-Pro-Val-Ala-Gly-Gly-Pro-Gly-Ala-Pro-Pro-Ala-Thr-Arg-Pro-Pro- Ala-Ser-Pro-Ser-Pro-Gln-Arg. Sequential Edman degradation of this HPLC-purified tryptic phosphopeptide revealed that serine 20 of this peptide was the major phosphorylated residue. This phosphoacceptor site is immediately flanked by a carboxyl-terminal proline residue, an observation that further verifies the proline-directed nature of this protein kinase. The tryptic phosphopeptide corresponds exactly to a sequence in the collagenase-sensitive, proline-rich "tail" region of bovine synapsin I. This novel phosphorylation site is close to but distinct from phosphorylation sites 2 and 3, which are known to be phosphorylated by calcium/calmodulin-dependent protein kinase II and are considered to be of regulatory importance.
authors.affiliation: Division of Orthopaedic Surgery, University of Southern California School of Medicine, Children's Hospital 95616.
authors.forename: F L
authors.initials: FL
authors.lastname: Hall
authors.forename: J P
authors.initials: JP
authors.lastname: Mitchell
authors.forename: P R
authors.initials: PR
authors.lastname: Vulliet
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001921:Brain / Q000378:metabolism*; D002417:Cattle; D002460:Cell Line; D002851:Chromatography, High Pressure Liquid; D005260:Female; D007700:Kinetics; D008325:Mammary Neoplasms, Experimental / Q000201:enzymology; D051379:Mice; D008969:Molecular Sequence Data; D008970:Molecular Weight; D009419:Nerve Tissue Proteins / Q000302:isolation & purification / Q000378:metabolism*; D010449:Peptide Mapping; D010710:Phosphates / Q000378:metabolism; D010748:Phosphopeptides / Q000302:isolation & purification; D010766:Phosphorylation; D038461:Proline-Directed Protein Kinases; D011494:Protein Kinases / Q000302:isolation & purification / Q000378:metabolism*; D013379:Substrate Specificity; D016704:Synapsins; D014407:Tumor Cells, Cultured / Q000201:enzymology
chemical_list: D009419:Nerve Tissue Proteins; D010710:Phosphates; D010748:Phosphopeptides; D016704:Synapsins; D011494:Protein Kinases; D038461:Proline-Directed Protein Kinases