Phosphorylation of chick synaptosomal proteins

Abstract

Chick synaptic plasma membranes (SPM) were phosphorylated when incubated with magnesium adenosine triphosphate. Addition of 3', 5'-cyclic adenosine monophosphate (cAMP) stimulated overall protein phosphorylation by 25%. Separation of SPM polypeptides by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) indicated that at least ten individual polypeptides were kinase substrates. cAMP stimulated phosphate incorporation into several polypeptides while the phosphorylation of other substrates appeared cAMP-independent. Chick synaptosomes incorporated phosphate into proteins when incubated in physiological buffer containing energy sources. Separation of polypeptides by SDS-PAGE indicated that only two polypeptides were significantly phosphorylated after 15 seconds incubation while at least fifteen polypeptides were active kinase substrates after 15 minutes incubation. Labeled synaptosomes were hypotonically lysed and separated by centrifugation into soluble, membrane, and mitochondrial fractions. Every fraction exhibited significant phosphate incorporation. Electrophoresis revealed that each fraction had several unique phosphorylated polypeptides and a distinctive phosphorylation pattern. The same polypeptides appeared to be labeled whether MgATP was added to SPM, or SPM were isolated after synaptosomal autophosphorylation. To further characterize neuronal polypeptides, an isoelectric focusing gel system was developed which produced a pH gradient spanning the range 4-9. When chick brain mitochondrial polypeptides were focused on such a gel, extra polypeptide spots were observed in the basic region which were not seen in a gel prepared by a previously published method. When this system was employed to separate phosphorylated synaptosomal polypeptides, more polypeptides and radioactive regions were observed than with one-dimensional SDS-PAGE.