title: Differential roles of regulatory light chain and myosin binding protein-C phosphorylations in the modulation of cardiac force development.
abstract: Phosphorylation of myosin regulatory light chain (RLC) by myosin light chain kinase (MLCK) and myosin binding protein-C (cMyBP-C) by protein kinase A (PKA) independently accelerate the kinetics of force development in ventricular myocardium. However, while MLCK treatment has been shown to increase the Ca(2+) sensitivity of force (pCa(50)), PKA treatment has been shown to decrease pCa(50), presumably due to cardiac troponin I phosphorylation. Further, MLCK treatment increases Ca(2+)-independent force and maximum Ca(2+)-activated force, whereas PKA treatment has no effect on either force. To investigate the structural basis underlying the kinase-specific differential effects on steady-state force, we used synchrotron low-angle X-ray diffraction to compare equatorial intensity ratios (I(1,1)/I(1,0)) to assess the proximity of myosin cross-bridge mass relative to actin and to compare lattice spacings (d(1,0)) to assess the inter-thick filament spacing in skinned myocardium following treatment with either MLCK or PKA. As we showed previously, PKA phosphorylation of cMyBP-C increases I(1,1)/I(1,0) and, as hypothesized, treatment with MLCK also increased I(1,1)/I(1,0), which can explain the accelerated rates of force development during activation. Importantly, interfilament spacing was reduced by 2 nm (3.5%) with MLCK treatment, but did not change with PKA treatment. Thus, RLC or cMyBP-C phosphorylation increases the proximity of cross-bridges to actin, but only RLC phosphorylation affects lattice spacing, which suggests that RLC and cMyBP-C modulate the kinetics of force development by similar structural mechanisms; however, the effect of RLC phosphorylation to increase the Ca(2+) sensitivity of force is mediated by a distinct mechanism, most probably involving changes in interfilament spacing.
authors.affiliation: Department of Physiology, University of Wisconsin School of Medicine and Public Health, Madison, WI 53711, USA. brett@physiology.wisc.edu
authors.forename: Brett A
authors.initials: BA
authors.lastname: Colson
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Locher
authors.forename: Tanya
authors.initials: T
authors.lastname: Bekyarova
authors.forename: Jitandrakumar R
authors.initials: JR
authors.lastname: Patel
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Fitzsimons
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Irving
authors.forename: Richard L
authors.initials: RL
authors.lastname: Moss
mesh_terms: D000818:Animals; D002118:Calcium / Q000502:physiology; D002352:Carrier Proteins / Q000502:physiology*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000494:pharmacology; D005260:Female; D008297:Male; D051379:Mice; D008815:Mice, Inbred Strains; D023421:Models, Animal; D009200:Myocardial Contraction / Q000187:drug effects / Q000502:physiology*; D018994:Myosin Light Chains / Q000502:physiology*; D009219:Myosin-Light-Chain Kinase / Q000494:pharmacology; D010766:Phosphorylation / Q000187:drug effects / Q000502:physiology; D016278:Ventricular Function, Right / Q000187:drug effects / Q000502:physiology*; D000099163:Myosin Binding Protein C
chemical_list: D002118:Calcium; D002352:Carrier Proteins; D017868:Cyclic AMP-Dependent Protein Kinases; D018994:Myosin Light Chains; D009219:Myosin-Light-Chain Kinase; D000099163:Myosin Binding Protein C