Function of Platelet 47k Phosphoprotein

Abstract

Secretion of platelet granule constituents is closely associated with the phosphorylation of a cytosol polypeptide that we called P47 of Mr 47,000 (Imaoka, T. and Haslam, R.J., J. Biol. Chem. 258, 11404, 1983), by protein kinase C. Since the identity and function remains to be known, we purified protein kinase C, unphosphorylated and phosphorylated P47 to homogeneity from human platelets. Then precise phosphorylation reaction of P47 in vitro and a biological function of P47 were studied. Protein kinase C catalysed the phosphorylation reaction of P47 protein, platelet myosin light chain, histone III-S with Km of 0.8±0.2, 4.2±0.5, 4.7±0.7μM, and Vmax of 0.312, 0.189, 0.874 nmole/min/mg, respectively. Some data previously obtained in this laboratory and others utilizing histone III-S as substrate are consistent with the synergistic effect by diacylglycerol (DG) in the presence of Ca++ , phosphatidylserine (PS). Using P47 as substrate, the enzyme required both Ca++ and PS, but not DG for activity. 125I labelled unphosphorylated P47 had an ability to bind with platelet membrane fraction in the presense of phosphatidylserine. Effect of diacylglycerol was inhibitory in this PS dependent P47 binding with membrane. Unphosphorylated P47 had a inhibitory activity in platelet actin polymerization. Molar ratio to inhibit actin polymerization was 1:8(P47:actin). These activities were Ca++ independent. Purified 32P-labelled P47 lost the binding ability with membrane, also the inhibitory activity in actin polymerization.Therefore, we propose the hypothesis that unphosphorylated P47 may loosely bind with the inside of plasma membrane of platelet and inhibit actin polymerization as a modulator, when stimulated, protein Kinase C rapidly phosphorylate P47 and induce the activation of cytoskeletal network and subsequently release reaction. On the other hand, whether DG in fact can act as a second messenger remain uncertain, (supported by MESC of Japan)

Comparisons between the inorganic content of healthy and hypertensive rat tissues by inductively coupled plasma-mass spectrometry

“…Reductions of Ca 2+ concentration in erythrocyte membranes, platelet cells and synapsis membranes of neuronal cells have been recorded (Imaoka 1987). Decreases of 33.7, 17.2 and 18.5% of calcium, associated with rises of 25.0, 7.9 and 38.6% of sodium in hearts, erythrocytes and plasmas of spontaneously hypertensive rats also agree with those recent results.…”