Purification and characterization of the 47,000-dalton protein phosphorylated during degranulation of human platelets

Secretion of platelet granule constituents is closely associated with the phosphorylation of a cytosol polypeptide of Mr = 47,000 that we have called P47 (Haslam, R. J., Lynham, J. A., and Fox, J. E. B. (1979) Biochem. J. 178, 397-406). This polypeptide is a substrate of Ca2+-activated phospholipid-dependent protein kinase (Kawahara, Y., Takai, Y., Minakuchi, R., Sano, K., and Nishizuka, Y. (1980) Biochem. Biophys. Res. Commun. 97, 309-317). Two-dimensional gel electrophoresis of protein from human platelets that had been preincubated with 32Pi demonstrated the presence under control conditions of 2-3 major forms of P47 that contained very little 32P (pI values, 6.6-6.8) and, after induction of secretion with thrombin, their replacement by 7-9 highly labeled phosphorylated forms of P47 (pI values, 6.1-6.5). Native phosphorylated P47 was purified from thrombin-stimulated 32P-labeled platelets by ammonium sulfate fractionation and column chromatography on DEAE-cellulose, phenyl-Sepharose, and hydroxylapatite. The final 32P-labeled product was obtained in a yield of 20-25% and was purified about 400-fold relative to platelet lysate. This material was homogeneous on sodium dodecyl sulfate-polyacrylamide gel electrophoresis but, like the starting material, contained 7-9 separable phosphorylated components with different pI values. Purified phosphorylated P47 had a sedimentation coefficient (s20,w) of 3.57 S and a Stokes radius of 3.33 nm from which an Mr = 49,000 and a frictional ratio (f/f0) of 1.4 were calculated. These findings and failure to detect multimers after treatment of the protein with dimethyl suberimidate indicate that P47 normally exists as a monomer. The 32P-labeled phosphate present in purified P47 had the chemical stability of serine or threonine phosphoesters and analysis indicated the presence of 83% phosphoserine and 17% phosphothreonine. Limited proteolysis of purified 32P-labeled P47 by Staphylococcus aureus V8 protease generated a major unlabeled fragment (Mr = 23,500) and up to six labeled fragments (Mr = 24,700-14,800), the relative amounts of the latter depending on the extent of proteolysis. The same labeled fragments were obtained after proteolysis of each of the major phosphorylated components of P47, suggesting that these represent different phosphorylation states of variants of the same protein and that most or all of the phosphorylation sites are on a single 14,800-Da segment of the protein. The availability of pure native phosphorylated P47 should facilitate investigation of the physiological role of this protein in platelets.