Characterization of Protein Kinase C θ Activation Loop Autophosphorylation and the Kinase Domain Catalytic Mechanism

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Abstract
Protein kinase C θ (PKCθ), a member of the Ca2+-independent novel subfamily of PKCs, is required for T-cell receptor (TCR) signaling and IL2 production. PKCθ-deficient mice have impaired Th2 responses in a murine ova-induced asthma model, while Th1 responses are normal. As an essential component of the TCR signaling complex, PKCθ is a unique T-cell therapeutic target in the specific treatment of T-cell-mediated diseases. We report here the PKCθ autophosphorylation characteristics and elucidation of the catalytic mechanism of the PKCθ kinase domain using steady-state kinetics. Key phosphorylated residues of the active PKCθ kinase domain expressed in Escherichia coli were characterized, and mutational analysis of the kinase domain was performed to establish the autophosphorylation and kinase activity relationships. Initial velocity, product inhibition, and dead-end inhibition studies provided assignments of the kinetic mechanism of PCKθ362-706 as ordered, wherein ATP binds kinase first and ADP is released last. Effects of solvent viscosity and ATPγS on PKCθ catalysis demonstrated product release is partially rate limiting. Our studies provide important mechanistic insights into kinase activity and phosphorylation-mediated regulation of the novel PKC isoform, PKCθ. These results should aid the design and discovery of PKCθ antagonists as therapeutics for modulating T-cell-mediated immune and respiratory diseases.