How are the sites of phosphorylation on PKC rendered inaccessible to phosphatases? We proposed a "protected site" model, whereby the conformation of PKC regulates accessibility of the phosphates to phos-phatase. In this model, in the unstimulated conformation, sites of PKC phosphorylation are protected through steric hindrance by the surrounding areas of the protein. Upon binding of activating ligands, a conforma-tional change occurs, making the phos-phorylation sites accessible at the surface of the molecule. This model has the interesting implication that the dephosphory-lation of PKC is regulatable. Only when PKC is in the appropriate conformation (e.g., in the presence of activating factors), will the protein be susceptible to dephos-phorylation.
In order to gain preliminary insights into the structural correlates of protected sites of PKC phosphorylation, we undertook computer modeling studies of the catalytic core and carboxy-terminal autophosphorylation domain of PKC, based upon the known crystal structure of the catalytic subunit of PKA. The lower right-hand panel of the
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