Caldesmon enhances the binding of myosin to the cytoskeleton during platelet activation

Activation of platelets with physiological agents results in distinct cellular events such as shape change, cell aggregation, granule secretion, and clot retraction. Translocation of soluble cytoplasmic myosin to the actin cytoskeleton occurs during activation and may be involved in some of these physiological responses. Phosphorylation of the 20,000-dalton myosin light chain occurs in parallel with myosin translocation; however, exceptions to this correlation have been reported. The present study tests the hypothesis that the actin- and myosin-binding protein, caldesmon, is required for this enhanced binding of myosin to the actin cytoskeleton. Caldesmon, a putative regulatory protein found in non-muscle and smooth muscle cells, binds actin and myosin simultaneously to form an actin-caldesmon-myosin complex and "tethers" myosin to actin in a manner that promotes, rather than inhibits, translocation of actin filaments relative to myosin. In this study, we demonstrated that a purified myosin-binding fragment of caldesmon competitively blocks caldesmon-dependent tethering in an in vitro motility assay and that this effect is prevented by phosphorylating the fragment. More importantly, we demonstrated that the unphosphorylated, but not the phosphorylated, fragment displaces myosin from the cytoskeleton of activated platelets; this suggests that caldesmon enhances the binding of myosin to the cytoskeleton during platelet activation.