Phosphorylation of PKC activation loop plays an important role in receptor-mediated translocation of PKC.

Protein kinase C (PKC) is translocated to various cellular regions in a subtype and stimulation-dependent manner. Thereafter, the activated PKC phosphorylates its substrate and causes subsequent cellular responses (PKC targeting). The 3-phosphoinositide-dependent protein kinase-1 (PDK1) has an essential role in the maturation of PKC by phosphorylating a threonine residue in the PKC activation loop. To elucidate the role of PDK1 in PKC targeting, we expressed mutant gamma- or delta-PKC fused with GFP (gamma- or delta-PKC-ALM (activation loop mutant)-GFP), whose threonine residue in the activation loop was replaced with alanine, and compared their P2Y receptor-mediated translocation with wild-type PKC-GFP in CHO cells. ATP (1 mm) induced the transient translocation of wild-type gamma- or delta-PKC-GFP from cytoplasm to plasma membrane and following retranslocation from membrane to the cytoplasm. gamma- or delta-PKC-ALM-GFP was also translocated to plasma membrane, which was, however, retained at the membrane for a longer period than wild type. Similar results were observed in kinase-negative PKC mutants, indicating that the phosphorylation by PDK1 affects the retranslocation step of PKC by regulating the kinase activity. The simultaneous monitoring of [Ca2+]i and diacylglycerol (DG) levels with the translocation of PKC demonstrated that PKC-ALM induced the prolonged accumulation of DG, resulting in the prolonged retention of PKC-ALM at the plasma membrane. It is possible that PKC-ALM with decreased kinase activity could delay the conversion of DG at the plasma membrane. Our present study suggests that the activation loop phosphorylation plays an important role in receptor-mediated PKC targeting.