Tyrosine phosphorylation modifies protein kinase C δ-dependent phosphorylation of cardiac troponin I

Our study identifies tyrosine phosphorylation as a novel protein kinase Cδ (PKCδ) activation mechanism that modifies PKCδ-dependent phosphorylation of cardiac troponin I (cTnI), a myofilament regulatory protein. PKCδ phosphorylates cTnI at Ser²³/Ser²⁴ when activated by lipid cofactors; Src phosphorylates PKCδ at Tyr³¹¹ and Tyr³³² leading to enhanced PKCδ autophosphorylation at Thr⁵⁰⁵ (its activation loop) and PKCδ-dependent cTnI phosphorylation at both Ser²³/Ser²⁴ and Thr¹⁴⁴. The Src-dependent acquisition of cTnI-Thr¹⁴⁴ kinase activity is abrogated by Y311F or T505A substitutions. Treatment of detergent-extracted single cardiomyocytes with lipid-activated PKCδ induces depressed tension at submaximum but not maximum [Ca²⁺] as expected for cTnI-Ser ²³/Ser²⁴ phosphorylation. Treatment of myocytes with Src-activated PKCδ leads to depressed maximum tension and cross-bridge kinetics, attributable to a dominant effect of cTnI-Thr¹⁴⁴ phosphorylation. Our data implicate PKCδ-Tyr³¹¹/Thr ⁵⁰⁵ phosphorylation as dynamically regulated modifications that alter PKCδ enzymology and allow for stimulus-specific control of cardiac mechanics during growth factor stimulation and oxidative stress.