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

Protein kinase C-δ (PKCδ) is an allosterically activated enzyme that acts much like other PKC isoforms to transduce growth factor-dependent signaling responses. However, PKCδ is unique in that activation loop (Thr507) phosphorylation is not required for catalytic activity. Since PKCδ can be proteolytically cleaved by caspase-3 during apoptosis, the prevailing assumption has been that the kinase domain fragment (δKD) freed from autoinhibitory constraints imposed by the regulatory domain is catalytically competent and that Thr507 phosphorylation is not required for δKD activity. This study provides a counternarrative showing that δKD activity is regulated through Thr507 phosphorylation. We show that Thr507-phosphorylated δKD is catalytically active and not phosphorylated at Ser359 in its ATP-positioning G-loop. In contrast, a δKD fragment that is not phosphorylated at Thr507 (which accumulates in doxorubicin-treated cardiomyocytes) displays decreased C-terminal tail priming-site phosphorylation, increased G-loop Ser359 phosphorylation, and defective kinase activity. δKD is not a substrate for Src, but Src phosphorylates δKD-T507A at Tyr334 (in the newly exposed δKD N terminus), and this (or an S359A substitution) rescues δKD-T507A catalytic activity. These results expose a unique role for δKD-Thr507 phosphorylation (that does not apply to full-length PKCδ) in structurally organizing diverse elements within the enzyme that critically regulate catalytic activity.