PROTECTED-SITE PHOSPHORYLATION OF PROTEIN-KINASE-C IN HIPPOCAMPAL LONG-TERM POTENTIATION

One important aspect of synaptic plasticity is that transient stimulat ion of neuronal cell surface receptors can lead to long-lasting bioche mical and physiological effects in neurons. In long-term potentiation (LTP), generation of autonomously active protein kinase C (PKC) is one biochemical effect persisting beyond the NMDA receptor activation tha t triggers plasticity. We previously observed that the expression of e arly LTP is associated with a phosphatase-reversible alteration in PKC immunoreactivity, suggesting that autophosphorylation of PKC might be elevated in LTP. In the present studies we tested the hypothesis that PKC phosphorylation is persistently increased in the early maintenanc e of LTP, We generated an antiserum that selectively recognizes the ct and pll isoforms of PKC autophosphorylated in the C-terminal domain. Using western blotting with this antiserum we observed an NMDA recepto r-mediated increase in phosphorylation of PKC 1 h after LTP was induce d. How is the increased phosphorylation maintained in the cell in the face of ongoing phosphatase activity? We observed that dephosphorylati on of PKC in vitro requires the presence of cofactors normally serving to activate PKC, i.e., Ca2+, phosphatidylserine, and diacylglycerol. Based on these observations and computer modeling of the three-dimensi onal structure of the PKC catalytic core, we propose a ''protected sit e'' model of PKC autophosphorylation, whereby the conformation of PKC regulates accessibility of the phosphates to phosphatase. Although we have proposed the protected site model based on our studies of PKC pho sphorylation in LTP, phosphorylation of protected sites might be a gen eral biochemical mechanism for the generation of stable, long-lasting physiologic changes.