Comparison of proteins involved with cyclic AMP metabolism between synaptic membrane and postsynaptic density...

Synaptic membrane and postsynaptic density (PSD) fractions isolated from canine cerebral cortex and cerebellum were assayed for the following proteins: adenylate cyclase and phosphodiesterase (PDE) activities against cyclic AMP and cyclic GMP, the regulatory subunit of the cyclic AMP-dependent protein kinase, and the substrate proteins for this kinase. The results were expressed on the basis of both the protein content of the fractions and the number of synapses in the synaptic membrane fractions. The number of synapses on a constant protein content basis was about three times higher in the cerebral cortex synaptic membrane fraction than in the comparable cerebellar fraction. Adenylate cyclase activity was from 3.4 to 5.6 times higher in the cerebral cortex membrane fraction than in the cerebellar membrane fraction based on protein content but only slightly higher based on synapse counts. PSD fractions had no adenylate cyclase activity. The cyclic AMP-PDE activity was from 17 to 27 times higher in the cerebral cortex membrane fraction than in the cerebellar membrane fraction based on protein content, and about five times higher based on synapse counts. By doing PDE histochemistry at the electron microscopy level it was found that all the cerebral cortex PSDs in the isolated fraction contained PDE activity, none being found associated with the broken-up material in the fraction. The amount of the regulatory subunit of the cyclic AMP-dependent protein kinase was about equal in the two fractions based on protein, but about one-third lower in cerebral cortex fraction than in cerebellar fractions. In the cerebral cortex membrane fraction the primary substrate for the cyclic AMP-dependent protein kinase is synapsin I, with much lower amounts in the cerebellar membrane fraction. The PSD fraction from the two sources also showed these differences in synapsin I content. In the cerebellar membrane fraction, the primary substrate for the enzyme is a approximately 245,000 Mr protein not found in the cerebral cortex membrane fraction. The findings that the turnover of cyclic AMP is much higher in cerebral cortex synapses than in cerebellar synapses, and that differences are found between the cerebral cortex and cerebellum with regard to the substrate proteins for the cyclic AMP-dependent protein kinase indicate a divergence in the effect of cyclic AMP between cerebral cortex and cerebellar synapses.