1The longitudinal muscle-myenteric plexus strip preparation of the guinea-pig ileum was used to study the binding of

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In vivo and in vitro phosphorylation of synaptic membrane proteins were compared. In vivo phosphorylation was carried out by injecting rats intraventricularly with 1 mCi of :3ZPi-orthophosphate. After a 40-min isotope incorporation period, rats were decapitated and synaptic membranes isolated. In ilitro phosphorylation was accomplished by incubating unlabeled synaptic membranes, isolated from noninjected rats, with 5 ~M-[~-"'P]ATP. In viva and in vitro :32P-labeled synaptic membranes were then fractionated electrophoretically on an SDS-polyacrylamide (7.5%) slab gel. The Coomassie blue protein patterns for in vivo and in vifro "P-labeled synaptic membranes were identical. In contrast, autoradiographs of these gels showed striking differences in the pattern of phosphate incorporation. Cyclic-AMP (10 p~) maximally stimulated the in vitro phosphate labeling of two protein bands (80,000 and 55,000 apparent molecular weight) which were not substantially labeled by the in viva p r o c e d u r e . K e y w o r d s : S y n a p t i c m e m b r a n e s -B r a i n p r o t e i n phosphorylation-Protein phosphorylation in viw-Membrane protein phosphorylation.

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