Stimulation of plasma membrane Ca2+-pump ATPase of vascular smooth muscle by cGMP-dependent protein kinase: Functional reconstitution with purified proteins

A 240-kDa protein isolated from porcine aortic smooth muscle as a substrate for cGMP-dependent protein kinase (cGMP kinase) whose phosphorylation was in a close association with stimulation of partially purified plasma membrane Ca2+-pump ATPase by the kinase was later shown to represent splicing variants of type 1 inositol 1,4,5-trisphosphate (IP3) receptor. To further clarify the role played by this protein in the stimulation of Ca2+-pump ATPase, it was attempted in the present study to specifically remove the protein by immunoprecipitation with an antibody specific to type 1 IP3 receptor. Contrary to expectation, stimulation of the ATPase by cGMP kinase was still observed after removal of the IP3 receptor. Furthermore, cGMP kinase stimulated a highly purified preparation of Ca2+-pump ATPase deprived of IP3 receptor when the concentrations of the ATPase were low enough (10-20 nM) to make it retain a monomeric form, while it did not produce stimulation when the concentration of the enzyme was increased to 40 nM at which the enzyme is known to take an oligomeric, fully activated form insensitive to activation by calmodulin. Heat-inactivated cGMP kinase and cGMP kinase without cGMP failed to stimulate the highly purified Ca2+-pump ATPase. In addition, type I but not type I cGMP kinase was found to stimulate the ATPase. The stimulation of Ca2+-pump ATPase by cGMP kinase occurs without any detectable phosphorylation of the ATPase. In conclusion, cGMP kinase can stimulate the plasma membrane Ca2+-pump ATPase when it is in a monomeric form without phosphorylating the Ca2+-pump ATPase and that of the two cGMP kinase isozymes found in the vascular smooth muscle, only type I cGMP kinase participates in the stimulation.