An endogenous substrate for cGMP-dependent protein kinase in mammalian cerebellum

Abstract

CYCLIC guanosine monophosphate (cGMP)-dependent protein kinases, a class of protein kinases which are activated by low concentrations of cGMP, but not of cyclic adenosine monophosphate (cAMP), have been demonstrated in a number of invertebrate1,2 and vertebrate3–11 tissues. In contrast to cAMP-dependent protein kinases, for which numerous substrates have been found, it has been difficult to demonstrate the existence of endogenous substrates for cGMP-dependent protein kinases. Until the present investigation, endogenous substrates for cGMP-dependent protein kinases had been identified in only two types of tissue, smooth muscle5 and intestinal brush border epithelium10; in both tissues the substrates were found only in membrane fractions. Experimental evidence suggests that cGMP may play a part in neuronal function12–14. Although a cGMP-dependent protein kinase has been found in the cytosol of mammalian cerebellum3,4,7, no endogenous substrate has previously been found in this enzyme. We report here the presence in the cerebellum of a soluble protein specifically phosphorylated by the cGMP-dependent protein kinase of that tissue.