Platelet calcium-dependent proteins: identification and localization of the calcium-dependent regulator, calmodulin, in platelets.

The calcium-dependent regulator protein, calmodulin, is a 17,000 molecular weight polypeptide which binds calcium and has been shown to confer calcium sensitivity on contractile and other proteins. In the present study, we have examined the presence and subcellular distribution of this protein in preparations of human platelets. Calmodulin was quantified using a two-stage phosphodiesterase assay. Whole platelets contained 1.33 +/- 0.06 units calmodulin per 10(6) platelets or 26.5 +/- 3.4 fg calmodulin per platelet. The distribution of calmodulin in the platelet was predominantly soluble with over 80 percent of calmodulin activity in the soluble fraction of the cell. There was no apparent difference in the distribution of calmodulin between soluble and particulate compartments in recalcified platelet homogenates compared to homogenates in EDTA. Indirect immunofluorescent studies with monospecific antisera to dinitrophenylated calmodulin showed intense staining of platelets in a diffuse pattern. The identification of calmodulin in platelets raises the possibility that this protein may participate in calcium-dependent reactions important in platelet aggregation and release.