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

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