title: C0 and C1 N-terminal Ig domains of myosin binding protein C exert different effects on thin filament activation.
abstract: Mutations in genes encoding myosin, the molecular motor that powers cardiac muscle contraction, and its accessory protein, cardiac myosin binding protein C (cMyBP-C), are the two most common causes of hypertrophic cardiomyopathy (HCM). Recent studies established that the N-terminal domains (NTDs) of cMyBP-C (e.g., C0, C1, M, and C2) can bind to and activate or inhibit the thin filament (TF). However, the molecular mechanism(s) by which NTDs modulate interaction of myosin with the TF remains unknown and the contribution of each individual NTD to TF activation/inhibition is unclear. Here we used an integrated structure-function approach using cryoelectron microscopy, biochemical kinetics, and force measurements to reveal how the first two Ig-like domains of cMyPB-C (C0 and C1) interact with the TF. Results demonstrate that despite being structural homologs, C0 and C1 exhibit different patterns of binding on the surface of F-actin. Importantly, C1 but not C0 binds in a position to activate the TF by shifting tropomyosin (Tm) to the "open" structural state. We further show that C1 directly interacts with Tm and traps Tm in the open position on the surface of F-actin. Both C0 and C1 compete with myosin subfragment 1 for binding to F-actin and effectively inhibit actomyosin interactions when present at high ratios of NTDs to F-actin. Finally, we show that in contracting sarcomeres, the activating effect of C1 is apparent only once low levels of Ca(2+) have been achieved. We suggest that Ca(2+) modulates the interaction of cMyBP-C with the TF in the sarcomere.
authors.affiliation: Department of Cellular and Molecular Medicine, University of Arizona, Tucson, AZ 85724;
authors.forename: Samantha P
authors.initials: SP
authors.lastname: Harris
authors.affiliation: Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507.
authors.forename: Betty
authors.initials: B
authors.lastname: Belknap
authors.affiliation: Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507.
authors.forename: Robert E
authors.initials: RE
authors.lastname: Van Sciver
authors.affiliation: Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507.
authors.forename: Howard D
authors.initials: HD
authors.lastname: White
authors.affiliation: Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23507 galkinve@evms.edu.
authors.forename: Vitold E
authors.initials: VE
authors.lastname: Galkin
mesh_terms: D008841:Actin Cytoskeleton / Q000187:drug effects / Q000378:metabolism*; D000199:Actins / Q000378:metabolism; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism*; D002463:Cell Membrane Permeability / Q000187:drug effects; D006801:Humans; D006868:Hydrolysis; D008958:Models, Molecular; D032342:Muscle Cells / Q000187:drug effects / Q000378:metabolism; D009119:Muscle Contraction / Q000187:drug effects; D015879:Myosin Subfragments / Q000378:metabolism; D011485:Protein Binding / Q000187:drug effects; D017434:Protein Structure, Tertiary; D011817:Rabbits; D012518:Sarcomeres / Q000187:drug effects / Q000378:metabolism; D013329:Structure-Activity Relationship; D034421:Sus scrofa; D000099163:Myosin Binding Protein C
chemical_list: D000199:Actins; D000251:Adenosine Triphosphatases; D000255:Adenosine Triphosphate; D002118:Calcium; D002352:Carrier Proteins; D015879:Myosin Subfragments; D000099163:Myosin Binding Protein C
keywords: actin; cryo-EM; muscle regulation; myosin binding protein C; tropomyosin
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