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

Our study identifies tyrosine phosphorylation as a novel protein kinase C delta(PKC delta) activation mechanism that modifies PKC delta-dependent phosphorylation of cardiac troponin I (cTnI), a myofilament regulatory protein. PKC delta phosphorylates cTnI at Ser(23)/Ser(24) when activated by lipid cofactors; Src phosphorylates PKC delta at Tyr(311) and Tyr(332) leading to enhanced PKC delta autophosphorylation at Thr(505) (its activation loop) and PKC delta-dependent cTnI phosphorylation at both Ser(23)/Ser(24) and Thr(144). The Src-dependent acquisition of cTnI-Thr(144) kinase activity is abrogated by Y311F or T505A substitutions. Treatment of detergent-extracted single cardiomyocytes with lipid-activated PKC delta induces depressed tension at submaximum but not maximum [Ca2+] as expected for cTnI-Ser(23)/ Ser(24) phosphorylation. Treatment of myocytes with Src-activated PKC delta leads to depressed maximum tension and cross-bridge kinetics, attributable to a dominant effect of cTnI-Thr144 phosphorylation. Our data implicate PKC delta-Tyr311/ Thr505 phosphorylation as dynamically regulated modifications that alter PKC delta enzymology and allow for stimulus-specific control of cardiac mechanics during growth factor stimulation and oxidative stress.