title: Protein kinase A-mediated phosphorylation of cMyBP-C increases proximity of myosin heads to actin in resting myocardium.
abstract: Protein kinase A-mediated (PKA) phosphorylation of cardiac myosin binding protein C (cMyBP-C) accelerates the kinetics of cross-bridge cycling and may relieve the tether-like constraint of myosin heads imposed by cMyBP-C. We favor a mechanism in which cMyBP-C modulates cross-bridge cycling kinetics by regulating the proximity and interaction of myosin and actin. To test this idea, we used synchrotron low-angle x-ray diffraction to measure interthick filament lattice spacing and the equatorial intensity ratio, I(11)/I(10), in skinned trabeculae isolated from wild-type and cMyBP-C null (cMyBP-C(-/-)) mice. In wild-type myocardium, PKA treatment appeared to result in radial or azimuthal displacement of cross-bridges away from the thick filaments as indicated by an increase (approximately 50%) in I(11)/I(10) (0.22+/-0.03 versus 0.33+/-0.03). Conversely, PKA treatment did not affect cross-bridge disposition in mice lacking cMyBP-C, because there was no difference in I(11)/I(10) between untreated and PKA-treated cMyBP-C(-/-) myocardium (0.40+/-0.06 versus 0.42+/-0.05). Although lattice spacing did not change after treatment in wild-type (45.68+/-0.84 nm versus 45.64+/-0.64 nm), treatment of cMyBP-C(-/-) myocardium increased lattice spacing (46.80+/-0.92 nm versus 49.61+/-0.59 nm). This result is consistent with the idea that the myofilament lattice expands after PKA phosphorylation of cardiac troponin I, and when present, cMyBP-C, may stabilize the lattice. These data support our hypothesis that tethering of cross-bridges by cMyBP-C is relieved by phosphorylation of PKA sites in cMyBP-C, thereby increasing the proximity of cross-bridges to actin and increasing the probability of interaction with actin on contraction.
authors.affiliation: Department of Physiology, University of Wisconsin Medical School, Madison, WI, USA. bacolson@wisc.edu
authors.forename: Brett A
authors.initials: BA
authors.lastname: Colson
authors.forename: Tanya
authors.initials: T
authors.lastname: Bekyarova
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Locher
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: D000199:Actins / Q000378:metabolism*; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism* / Q000502:physiology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism*; D006321:Heart / Q000502:physiology; D007700:Kinetics; D051379:Mice; D018345:Mice, Knockout; D008840:Microfilament Proteins / Q000378:metabolism; D009200:Myocardial Contraction; D009206:Myocardium / Q000378:metabolism*; D009218:Myosins / Q000378:metabolism*; D010766:Phosphorylation; D053838:Scattering, Small Angle; D014961:X-Ray Diffraction; D000099163:Myosin Binding Protein C
chemical_list: D000199:Actins; D002352:Carrier Proteins; D017868:Cyclic AMP-Dependent Protein Kinases; D008840:Microfilament Proteins; D009218:Myosins; D000099163:Myosin Binding Protein C