Guanosine 3′:5′-Cyclic Monophosphate-Dependent Phosphorylation of Endogenous Substrate Proteins in Membranes of Mammalian Smooth Muscle

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Abstract
Guanosine 3':5'-cyclic monophosphate (cyclic GMP) stimulated the endogenous phosphorylation of two proteins in isolated membrane fractions from mammalian organs rich in smooth muscle, including ductus deferens, uterus, and small intestine. The apparent molecular weights of the substrate proteins were 130,000 and 100,000. In the presence of 10 mM MnCl(2), a half-maximal increase in phosphorylation of these proteins was achieved with 20-30 nM cyclic GMP. Approximately 10-fold higher concentrations of adenosine 3':5'-cyclic monophosphate (cyclic AMP) were required to produce the same increase in phosphorylation of these two proteins. Cyclic AMP, but not cyclic GMP, regulated the phosphorylation of a third protein present in these same membrane fractions; the apparent molecular weight of this protein was 50,000. Cyclic GMP-dependent phosphorylation of endogenous protein was not observed in the cell sap of any of the three preparations of smooth muscle studied. The finding of endogenous cyclic GMP-dependent protein kinase activity and associated substrate proteins in membrane fractions from several mammalian organs containing smooth muscle raises the possibility that physiological actions of cyclic GMP in smooth muscle may be mediated by the phosphorylation of membrane proteins.

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