Identification and characterization of alpha-protein kinase C binding proteins in normal and transformed REF52 cells

Immunocytofluorescence studies demonstrated that alpha-PKC is concentrated in focal contacts of REF52 cells but not in their SV40-transformed derivatives [Jaken et al. (1989) J. Cell Biol. 109, 697-704; Hyatt & Jaken (1990) Mol. Carcinog. 3, 45-53]. Discrete localizations imply that PKC is targeted to these areas possibly via protein-protein interactions. We have used an overlay assay to detect alpha-PKC binding proteins. The molecular interactions between alpha-PKC and the binding proteins depended on phospholipid and either calcium or phorbol esters. Unlike the kinase activity, binding activity was detected in the absence of added calcium, indicating that calcium, which is necessary for phosphorylation of most substrates, is not required for binding. Vinculin and talin, two focal contact proteins, bound alpha-PKC. REF52 cells express several annexins (I, II, and VI) which bind PKC. Both annexin I expression and vinculin expression were decreased in SV40-REF52 cells. The two major REF52 cell binding proteins (p71 and p > 200 kDa) were also down-regulated in the transformed cells, indicating transformation-sensitive regulation of PKC binding protein activity.