Cyclic GMP‐Dependent Protein Kinase and Phosphorylation of the Endogenous Substrate Proteins in the Rabbit Superior Cervical Ganglion

When the homogenate of rabbit superior cervical ganglia (SCG) was incubated in the presence of [γ‐ 32 P]ATP and Mg 2+ , two specific proteins were strongly labeled. Their apparent molecular weights were 90,000 and 54,000, respectively. The phosphorylation of the latter was significantly stimulated by 10–50 n M cyclic GMP but to a lesser extent by cyclic AMP, whereas that of the former was not stimulated significantly by either of the cyclic nucleotides. The purified protein kinase inhibitor from rabbit skeletal muscle did not inhibit the phosphorylation. These results indicated that the observed phosphorylation of 54K protein was dependent on cyclic GMP but not on cyclic AMP. When intact SCG was incubated in the presence of 32 P i , phosphorylation of 90K protein was stimulated by cyclic GMP, dibutyryl cyclic GMP, and 8‐bromo‐cyclic GMP (10 μ M ), whereas phosphorylation of 54K protein was not significantly stimulated by any of these substances. The present demonstration of endogenous cyclic GMP‐dependent protein kinase activity and its endogenous substrate proteins raises a possibility that the physiological actions of cyclic GMP in SCG are mediated by the phosphorylation of these proteins. Key Words: Superior cervical ganglion—Slow synaptic transmission—Cyclic GMP—Protein phosphorylation—G‐kinase—Muscarinic actions. Takahashi S. Y . et al. Cyclic GMP‐dependent protein kinase and phosphorylation of the endogenous substrate proteins in the rabbit superior cervical ganglion.