GTP-preferring protein phosphorylation systems in brain membranes: Possible role in adenylate cyclase regulation

Preincubation of brain membranes with GTP under phosphorylating conditions resulted in activation of adenylate cyclase which withstood sedimentation and washing. Investigation into the possible mechanism(s) underlying this activation revealed that these membranes contain endogenous systems which prefer to utilize GTP, rather than ATP, in the phosphorylation of specific protein substrates with apparent M.W. of 54K and 33K. This activity is highly stimulated by Mn++ ions, inhibited by cyclic AMP and independent of Ca++. Triton-X-100 extracts of brain membranes, which contain the catalytic and regulatory subunits of adenylate cyclase, were found to be enriched in endogenous activity which phosphorylated the 54K protein with GTP, but not ATP. These findings provide a means for direct testing of the hypothesis that protein phosphorylation plays a role in adenylate cyclase regulation.