Phosphorylation is required for PMA- and cell-cycle-induced degradation of protein kinase Cδ

Classical and novel protein kinase C (PKC) isoforms are down-regulated as a result of chronic activation by certain tumour promoters and physiological stimuli; however, the mechanisms leading to down-regulation are not fully understood. In the present study, we have studied the PMA ('TPA')-induced degradation of PKCΔ in NIH 3T3 cells under culture conditions where PKCΔ displays cell-cycle-dependent down-regulation. In contrast with previous studies, a hyperphosphorylated form of this PKC isoform, promoted by calyculin A, was rapidly degraded in PMA-treated cells. Similarly, the presence of calyculin A enhanced the down-regulation of PKCΔ observed on G₁/S-phase progression through the cell cycle. Analysis of phosphorylation-site mutants indicated that the T-loop Thr⁵⁰⁵ phosphorylation site was critical for induced degradation.