title: Calmodulin binding to platelet plasma membranes.
abstract: Calmodulin copurifies with platelet plasma membranes isolated by glycerol-induced lysis and density gradient centrifugation. These membranes also bind 125I-labeled calmodulin in vitro in the presence of Ca2+. Binding is largely reduced by replacing Ca2+ by Mg2+ or by addition of an excess unlabeled calmodulin. The specific component of binding is saturable, with an apparent Kd of 27 nM and a maximum of 15.9 pmol binding sites per mg of membrane protein. This is equivalent to approx. 4100 binding sites per platelet. Binding was inhibited by addition of phenothiazines, a group of calmodulin antagonists. Half-maximal inhibition was attained with approx. 20 microM trifluoperazine or 50 microM chlorpromazine. In contrast, chlorpromazine-sulfoxide which is inactive towards calmodulin, did not affect the binding. Calmodulin binding polypeptides of the plasma membrane were identified by a gel-overlay technique. A major calmodulin-binding component of molecular weight 149000 was detected. Binding to this band was Ca2+-dependent and inhibited by chlorpromazine. The molecular weight of this polypeptide is similar to that of glycoprotein I and also that of the red cell (Ca2+ + Mg2+)-stimulated ATPase, which is known to bind calmodulin. The possible role of calmodulin in platelet activation is analysed.
mesh_terms: D001792:Blood Platelets / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000097:blood*; D002147:Calmodulin / Q000097:blood* / Q000302:isolation & purification; D002148:Calmodulin-Binding Proteins; D002352:Carrier Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002746:Chlorpromazine / Q000494:pharmacology; D004912:Erythrocytes / Q000032:analysis; D006801:Humans; D007700:Kinetics; D014268:Trifluoperazine / Q000494:pharmacology
chemical_list: D002135:Calcium-Binding Proteins; D002147:Calmodulin; D002148:Calmodulin-Binding Proteins; D002352:Carrier Proteins; D014268:Trifluoperazine; D002746:Chlorpromazine