Dephosphorylation of cofilin in stimulated platelets: roles for a GTP-binding protein and Ca2+.
In human platelets, thrombin not only stimulates the phosphorylation of pleckstrin (P47) and of myosin P-light chains, but also induces the dephosphorylation of an 18-19 kDa phosphoprotein (P18) [Imaoka, Lynham and Haslam (1983) J. Biol. Chem. 258, 11404-11414]. We have now studied this protein in detail. The thrombin-induced dephosphorylation reaction did not begin until the phosphorylation of myosin P-light chains and the secretion of dense-granule 5-hydroxytryptamine were nearly complete, but did parallel the later stages of platelet aggregation. Experiments with ionophore A23187 and phorbol 12-myristate 13-acetate indicated that dephosphorylation of P18 was stimulated by Ca2+, but not by protein kinase C. Two-dimensional analysis of platelet proteins, using non-equilibrium pH gradient electrophoresis followed by SDS/PAGE, showed that thrombin decreased the amount of phosphorylated P18 in platelets by up to 70% and slightly increased the amount of a more basic unlabelled protein that was present in 3-fold excess of P18 in unstimulated platelets. These two proteins were identified as the phosphorylated and non-phosphorylated forms of the pH-sensitive actin-depolymerizing protein, cofilin, by sequencing of peptide fragments and immunoblotting with a monoclonal antibody specific for cofilin. The molar concentration of cofilin in platelets was approx. 10% that of actin. Platelet cofilin was phosphorylated exclusively on serine. Experiments with electropermeabilized platelets showed that dephosphorylation of cofilin could be stimulated by guanosine 5'-[gamma-thio]triphosphate (GTP[S]) in the absence of Ca2+ or by a free Ca2+ concentration of 10 microM. This GTP[S]-induced dephosphorylation reaction was inhibited by 1-naphthyl phosphate, but not by okadaic acid. Our results add cofilin to the actin-binding proteins that may regulate the platelet cytoskeleton, and suggest that platelet cofilin can be activated by dephosphorylation reactions initiated either by a GTP-binding protein or Ca2+.
Department of Pathology, McMaster University, Hamilton, Ontario, Canada.
M M
MM
Davidson
R J
RJ
Haslam
D051339:Actin Depolymerizing Factors; D000595:Amino Acid Sequence; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000097:blood*; D019204:GTP-Binding Proteins / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D008840:Microfilament Proteins; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins / Q000097:blood* / Q000235:genetics; D010766:Phosphorylation; D010974:Platelet Aggregation / Q000187:drug effects / Q000502:physiology; D012701:Serotonin / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013917:Thrombin / Q000494:pharmacology
D051339:Actin Depolymerizing Factors; D008840:Microfilament Proteins; D009419:Nerve Tissue Proteins; D012701:Serotonin; D000001:Calcimycin; D013917:Thrombin; D019204:GTP-Binding Proteins; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium