Protein kinase C ε subcellular localization domains and proteolytic degradation sites: A model for protein kinase C conformational changes

Protein kinase C (PKC) ε has been found to have unique properties among the PKC isozymes in terms of its membrane association, oncogenic potential, and substrate specificity. Recently we have demonstrated that PKCε localizes to the Golgi network via its zinc finger domain and that both the holoenzyme and its zinc finger region modulate Golgi function. To further characterize the relationship between the domain organization and the subcellular localization of PKCε, a series of NIH 3T3 cell lines were created, each overexpressing a different truncated version of PKCε. The overexpressed proteins each were designed to contain an ε-epitope tag peptide at the COOH terminus to allow ready detection with an antibody specific for the tag. The subcellular localization of the recombinant proteins was analyzed by in vivo phorbol ester binding, immunocytochemistry, and cell fractionation followed by immunoblotting. Results revealed several regions of PKCε that contain putative subcellular localization signals. The presence either of the hinge region or of a 33-amino-acid region including the pseudosubstrate sequence in the recombinant proteins resulted in association with the plasma membrane and cytoskeletal components. The catalytic domain was found predominantly in the cytosolic fraction. The accessibility and thus the dominance of these localization signals is likely to be affected by the overall conformation of the recombinant proteins. Regions with putative proteolytic degradation sites also were identified. The susceptibility of the overexpressed proteins to proteolytic degradation was dependent on the protein conformation. Based on these observations, a model depicting the interaction and hierarchy of the suspected localization signals and proteolytic degradation sites is presented.