Cleavage Alters the Molecular Determinants of Protein Kinase C-δ Catalytic Activity

Protein kinase C-delta (PKC delta) is an allosterically activated enzyme that acts much like other PKC isoforms to transduce growth factor-dependent signaling responses. However, PKC delta is unique in that activation loop (Thr(507)) phosphorylation is not required for catalytic activity. Since PKC delta can be proteolytically cleaved by caspase-3 during apoptosis, the prevailing assumption has been that the kinase domain fragment (delta KD) freed from autoinhibitory constraints imposed by the regulatory domain is catalytically competent and that Thr(50)7 phosphorylation is not required for delta KD activity. This study provides a counternarrative showing that delta KD activity is regulated through Thr(507) phosphorylation. We show that Thr(507)-phosphorylated delta KD is catalytically active and not phosphorylated at Ser(359) in its ATP-positioning G-loop. In contrast, a delta KD fragment that is not phosphorylated at Thr(507) (which accumulates in doxorubicin-treated cardiomyocytes) displays decreased C-terminal tail priming-site phosphorylation, increased G-loop Ser(359) phosphorylation, and defective kinase activity. delta KD is not a substrate for Src, but Src phosphorylates delta KD-T507A at Tyr(334) (in the newly exposed delta KD N terminus), and this (or an S(359)A substitution) rescues delta KD-T507A catalytic activity. These results expose a unique role for delta KD-Thr(507) phosphorylation (that does not apply to full-length PKC delta) in structurally organizing diverse elements within the enzyme that critically regulate catalytic activity.