Suppression of synaptotagmin II restrains phorbolester-induced downregulation of protein kinase Cα by diverting the kinase from a degradative pathway to the recycling endocytic compartment

Downregulation of protein kinase Cα (PKCα) following long-term exposure to phorbol esters such as TPA is traffic dependent and involves delivery of the active, membrane-associated PKCα to endosomes. In this study, we show that synaptotagmin II (Syt II), a member of the Syt family of proteins, is required for TPA-induced degradation of PKCα. Thus, whereas the kinase half-life in TPA-treated cultured mast cells (the mast cell line rat basophilic leukemia RBL-2H3) is 2 hours, it is doubled in RBL-Syt II^- cells, in which the cellular level of Syt II is reduced by >95% by transfection with Syt II antisense cDNA. We demonstrate that in TPA-treated RBL cells, PKCα travels from the cytosol to the plasma membrane, where it is delivered to early endosomes on its route to degradation. By contrast, in TPA-treated RBL-Syt II^- cells, PKCα is diverted to recycling endosomes and remains distributed between the plasma membrane and the perinuclear recycling endocytic compartment. Notably, in both RBL and RBL-Syt II^- cells, a fraction of PKCα is delivered and maintained in the secretory granules (SG). These results implicate Syt II as a critical factor for the delivery of internalized cargo for degradation. As shown here, one consequence of Syt II suppression is a delay in PKCα downregulation, resulting in its prolonged signaling.