Ca²⁺ -calmodulin-dependent phosphorylation and platelet secretion

Abstract

Protein phosphorylation may play a critical role in stimuluscoupled secretion of platelets. Some platelet proteins become phosphorylated on exposure to agents such as thrombin and collagen^1–5, and the smallest of these phosphoproteins (molecular weight 20,000), has been identified as a light chain of myosin^6–8. Phosphorylation of myosin light chain increases the activity of actin-activated myosin ATPase⁸ and the resultant contraction of the actomyosin presumably mediates the release reaction^6,9. Platelet myosin light chain kinase has been identified as a calcium-dependent enzyme requiring calmodulin for its activity^10,11. Calmodulin is a Ca²⁺-binding protein with a molecular weight of ∼18,000 which seems to be involved in a wide variety of cellular processes¹². Although a growing body of evidence suggests that non-muscle actomyosin, such as that isolated from platelets, is regulated by Ca²⁺-calmodulin-dependent light chain phosphorylation, the precise relationship between the phosphorylation and the function of platelets is not clearly established. We now present pharmacological evidence that a calmodulin-mediated system, such as Ca²⁺-dependent myosin light chain phosphorylation, also plays an important role in the phenomenon of the release reaction. N-(6-aminohexyl)-5-chloro-1-napthalene-sulphonamide (W-7) (refs 13–15) is shown to bind selectively to calmodulin in vitro and inhibit its biological activity.