A Single Point Mutation in the V3 Region Affects Protein Kinase Cα Targeting and Accumulation at Cell-Cell Contacts

Given the importance of intercellular adhesion for many regulatory processes, we have investigated the control of protein kinase Cα (PKCα) targeting to the cell-cell contacts. We have previously shown that, upon treatment of the pituitary cell line GH3B6 with thyrotropin-releasing hormone (TRH) or phorbol 12-myristate 13-acetate (PMA), human PKCα (hPKCα) is selectively targeted to the cell-cell contacts (42). Here we show that the D294G mutation of hPKCα, previously identified in a subpopulation of human tumors, induces the loss of this selective targeting. The D294G mutant is instead targeted to the entire plasma membrane, including the cell-cell contacts, and the duration of the first rapid and transient translocation induced by TRH (42) is longer than that of the wild-type enzyme (93.3 versus 22.5 s), coinciding with the duration of the [Ca(2+)](i) increase. We found that in the presence or absence of PMA, RACK1 is never localized at the cell-cell contacts nor was it coimmunoprecipitated with hPKCα wild type or the D294G mutant. In contrast, PMA treatment or long-term TRH stimulation resulted in the presence of F-actin and β-catenin at the cell-cell contacts and their exclusion from the rest of the plasma membrane. Upon disruption of the F-actin network with phalloidin or cytochalasin D, wild-type hPKCα translocates but did not accumulate at the plasma membrane and β-catenin did not accumulate at the cell-cell contacts. In contrast, the disruption of the F-actin network affected neither translocation nor accumulation of the D294G mutant. These results show that the presence of PKCα at the cell-cell contacts is a regulated process which depends upon the integrity of both PKCα and the actin microfilament network.