cAMP-dependent protein kinase substrates in platelets. Evidence that thrombolamban, a 22,000 dalton substrate, and the Ca super(++)-ATPase are not associated proteins

Inhibition of platelet function by cAMP is due at least in part to a reduction in the agonist stimulated increase in cytoplasmic calcium during cell activation. This inhibition is also associated with cAMP-dependent phosphorylation of thrombolamban, a 22 kDa phosphoprotein which is present in the same membrane fraction as the calcium-dependent ATPase. Phosphorylation of this protein has been correlated with increased uptake of calcium by microsomal membranes. The present study was undertaken to examine the interaction of thrombolamban with the Ca super(2+)-ATPase in order to assess the possibility that the increased calcium uptake was by a direct effect of thrombolamban on Ca super(++)-ATPase activity or that thrombolamban was a component of the Ca super(++)-ATPase.