Differential roles of regulatory light chain and myosin binding protein-C phosphorylations in the modulation of cardiac force development

Authors

  • Brett A. Colson,

    1. Department of Physiology, University of Wisconsin School of Medicine and Public Health, Madison, WI 53711, USA
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  • Matthew R. Locher,

    1. Department of Physiology, University of Wisconsin School of Medicine and Public Health, Madison, WI 53711, USA
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  • Tanya Bekyarova,

    1. Center for Synchrotron Radiation Research and Instrumentation and Department of Biological, Chemical, and Physical Sciences, Illinois Institute of Technology, Chicago, IL 60616, USA
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  • Jitandrakumar R. Patel,

    1. Department of Physiology, University of Wisconsin School of Medicine and Public Health, Madison, WI 53711, USA
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  • Daniel P. Fitzsimons,

    1. Department of Physiology, University of Wisconsin School of Medicine and Public Health, Madison, WI 53711, USA
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  • Thomas C. Irving,

    1. Center for Synchrotron Radiation Research and Instrumentation and Department of Biological, Chemical, and Physical Sciences, Illinois Institute of Technology, Chicago, IL 60616, USA
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  • Richard L. Moss

    1. Department of Physiology, University of Wisconsin School of Medicine and Public Health, Madison, WI 53711, USA
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Corresponding author B. A. Colson: 601 Science Dr., Madison, WI 53711, USA.  Email: brett@physiology.wisc.edu

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 Ca2+ sensitivity of force (pCa50), PKA treatment has been shown to decrease pCa50, presumably due to cardiac troponin I phosphorylation. Further, MLCK treatment increases Ca2+-independent force and maximum Ca2+-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 (I1,1/I1,0) to assess the proximity of myosin cross-bridge mass relative to actin and to compare lattice spacings (d1,0) 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 I1,1/I1,0 and, as hypothesized, treatment with MLCK also increased I1,1/I1,0, which can explain the accelerated rates of force development during activation. Importantly, interfilament spacing was reduced by ∼2 nm (Δ 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 Ca2+ sensitivity of force is mediated by a distinct mechanism, most probably involving changes in interfilament spacing.

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