Protein Kinase Cθ C2 Domain Is a Phosphotyrosine Binding Module That Plays a Key Role in Its Activation

Protein kinase C theta (PKC theta) is a novel PKC that plays a key role in T lymphocyte activation. To understand how PKC theta is regulated in T cells, we investigated the properties of its N-terminal C2 domain that functions as an autoinhibitory domain. Our measurements show that a Tyr(P)-containing peptide derived from CDCP1 binds the C2 domain of PKC theta with high affinity and activates the enzyme activity of the intact protein. The Tyr(P) peptide also binds the C2 domain of PKC delta tightly, but no enzyme activation was observed with PKC delta. Mutations of PKC theta-C2 residues involved in Tyr(P) binding abrogated the enzyme activation and association of PKC theta with Tyr-phosphorylated full-length CDCP1 and severely inhibited the T cell receptor/CD28-mediated activation of a PKC theta-dependent reporter gene in T cells. Collectively, these studies establish the C2 domain of PKC theta as a Tyr(P)-binding domain and suggest that the domain may play a major role in PKC theta activation via its Tyr(P) binding.