Phosphorylation of endogenous protein of rat brain by cyclic adenosine 3',5'-monophosphate-dependent protein kinase.

The ability of proteins in various subcellular fractions of the rat cerebrum to act as substrates for a partially purified cyclic adenosine 3’,5’-monophosphate (cyclic AMP)-dependent protein kinase has been studied in vitro. Distribution of substrates generally paralleled the distribution of activity of protein kinase as well as of adenylate cyclase and cyclic nucleotide phosphodiesterase. Although all particulate subfractions were capable of becoming phosphorylated, highest substrate activities were found in the synaptic plasma membrane, microsomal, and synaptic vesicle fractions. Substrates in the microsomal fraction included both membrane and ribosomal proteins. The synaptic plasma membrane and the microsomal fractions exhibited a high endogenous rate of incorporation of radioactive phosphate from [Y-~~P]ATP into phosphoprotein, compared to other subcellular fractions, and this rate was increased by addition of protein kinase and cyclic AMP. Phosphorylation of the synaptic plasma membrane fractions indicated that some membrane proteins were at least comparable to and possibly better than the histones in effectiveness as substrate for the enzyme. The results support other evidence implicating the cyclic AMP system in the molecular events underlying synaptic transmission in the nervous system.