Distribution of endogenously phosphorylated proteins in subcellular fractions of rat cerebral cortex

The cerebral cortex from adult rats was separated into several subcellular fractions by using established methods of differential and sucrose density gradient centrifugation. Aliquots from each fraction were incubated with γ-(32)P-ATP, in the presence and absence of adenosine 3',5'-monophosphate (cyclic AMP), and its protein constituents were separated by means of SDS-slab gel electrophoresis. Fractions containing nuclei, synaptosomes, myelin, microsomes, and soluble proteins each showed a characteristic pattern of protein staining and of endogenously phosphorylated proteins detected by autoradiography of the gels. Cyclic AMP-stimulated phosphorylation of proteins with MW 78K and 84K can serve as markers for membranes of synaptic origin, while cyclic AMP-independent phosphorylation of low-molecular-weight proteins (15K-20K) is characteristic of myelin. The finding of different phosphoproteins in various subcellular fractions may be related to the diversity of cellular functions known to be regulated by phosphorylative activity.