© 2000 American Heart Association, Inc.
Editorials |
Myosin Binding Protein C, a Potential Regulator of Cardiac Contractility
From Department of Physiology, School of Medicine, University of Pennsylvania, Philadelphia.
Correspondence to Saul Winegrad, Department of Physiology, School of Medicine, University of Pennsylvania, Philadelphia, PA 19104-6085. E-mail BSG{at}Mail.Med.Upenn.Edu
Key Words: myosin binding protein C • myosin • cardiac contractility • cardiac muscle • cardiac regulation
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Introduction |
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 Top Introduction References |
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The role of myosin binding protein C (MyBP-C) in the formation and function of striated muscle is unclear, even though the demonstration of its presence in the thick filaments of striated muscle was made by Offer et al1 more than 25 years ago.2 Because of the inability of myosin to form normal thick filaments in the absence of MyBP-C and the temporal correlation between the appearance of MyBP-C, thick filaments, and sarcomeres in developing striated muscle,3 4 5 MyBP-C is assumed to be essential for the formation and normal structure of thick filaments. The cardiac isoform of MyBP-C has an additional feature that is absent from the skeletal isoforms. There are 3 phosphorylation sites located in the cardiac motif portion of the molecule in the N-terminal region.6 In addition to these phosphorylatable sites, a Ca2+/calmodulin-regulated kinase specific for cardiac MyBP-C is bound to the thick filament.7 8 The combination of these features suggests that the function of MyBP-C in cardiac muscle can be modified posttranslationally by phosphorylation. Such a phosphorylation could be an important regulator of cardiac contractility. At least 5 transmitter-based mechanisms for the regulation of cardiac contractility are associated with changes in the degree of phosphorylation of MyBP-C: increased or decreased phosphorylation with increased or decreased contractility.
MyBP-C is located in a series of 7 to 9 sites, each separated by 43 nm in the C zone of the sarcomere and most likely oriented perpendicular to the axis of the filament.9 Within the C zone, there are approximately 9 crossbridges per 3 MyBP-C molecules,9 but in the 50% of the thick filament outside the C zone there is no MyBP-C.
The C-terminal region of MyBP-C contains a binding site for the rod (LMM) portion of myosin.10 11 Although the binding site itself is located in the C-terminal module (C10, cardiac MyBP-C contains 11 modules, C0 to C10, N- to C-terminus), the affinity of the binding is increased by the presence of C8 and C9.11 More recently, Gruen and Gautel12 have confirmed that a second binding site for myosin exists in the N-terminal region of MyBP-C. This site binds to subfragment 2 of myosin (S2), near the hinge of the crossbridge, the same region in which certain mutations of myosin cause familial hypertrophic cardiomyopathy.12 13 The binding occurs even with a fragment of MyBP-C containing only the C1 and C2 modules. Of particular importance is their demonstration that phosphorylation of the 3 sites in the MyBP-C motif within the C1C2 fragment inhibits the binding of the fragment to the S2 region of myosin in either skeletal myofibrils or cardiac myocytes.14 In isolated cardiac thick filaments containing the whole MyBP-C molecule, there is a change in crossbridge structure from phosphorylation of MyBP-C.15 16 The change in structure is of a type that could alter crossbridge cycling.
In the most recent work from Gautel’s group,14 published in this issue of Circulation Research, the effects of the binding of the C1C2 fragment to myosin on the mechanical properties of skinned skeletal muscles fiber have been studied. The fragment was allowed to diffuse into the skinned fibers in sufficient concentration to produce full or near full saturation of S2 both in and out of the C zone.
Kunst et al14 measured 6 parameters of contractility and found that 5 were changed by the unphosphorylated C1C2 fragment. None of these changes in mechanical performance occurred when the C1C2 fragment was fully phosphorylated before it diffused into the fiber.
Maximum Ca2+-activated force was decreased, but the stiffness during maximum Ca2+ activation was unchanged. Changes in stiffness are generally accepted to indicate an alteration in the number of crossbridges strongly attached to the thin filament. These results indicate that the number of crossbridges attached is unchanged by the addition of unphosphorylated C1C2, but the amount of force is decreased. Either each crossbridge develops less force or the duty cycle (fraction of crossbridge cycle during which force is generated) has been reduced by the binding of the C1C2 fragment.
The addition of C1C2 fragments to myofibrils increased rigor force and to a smaller extent rigor stiffness, indicating an increase in the number of rigor links. Because all crossbridges are believed to be attached to thin filaments during rigor,17 the experiments of Kunst et al14 show that the properties of the crossbridge in rigor are modified by C1C2 binding. The changes in Ca2+-activated force and rigor force could be explained by a decrease in the number and an increase in the stiffness of attached crossbridges from binding of C1C2, a not unreasonable effect from a protein fragment binding to the hinge region of the crossbridge.
The increased rate of formation of rigor links produced by C1C2 when ATP and creatine phosphate were withdrawn was attributed to a change in the kinetics of formation of rigor links.14 Even in the presence of C1C2 bound to S2, however, the rigor links formed over many seconds in the ATP-free solution. This slow time course is more consistent with time for diffusion of ATP from the skinned fibers rather than crossbridge cycling. It is probably a reflection of shortened diffusion time for ATP from the fiber, although an increased rate of ATP hydrolysis could also contribute to the more rapid development of rigor tension.
The increased Ca2+ sensitivity and the decreased cooperativity that are produced by the addition of C1C2 are not readily explained, particularly because extraction of a major portion of MyBP-C produces the same effect.18 Kunst et al14 address this apparent discrepancy by pointing out that the extraction procedure may alter other myofibrillar proteins, especially titin.
All of these effects produced by the unphosphorylated C1C2 fragment of cardiac MyBP-C and inhibited by the phosphorylation of the MyBP-C motif within the fragment are provocative and intriguing, because they raise the possibility of regulation of contractility of cardiac muscle by a variable interaction of the N-terminus of MyBP-C with the hinge region of the crossbridge. In interpreting these results, it is important to consider the stoichiometry of MyBP-C to myosin. Because there are about 3 myosin molecules per molecule of MyBP-C within the C zone only one third of myosin molecules there can interact with the N-terminus of MyBP-C. Given that almost half of the crossbridges lie outside the C zone, the vast majority of crossbridges cannot be regulated by direct interaction with MyBP-C. However, as a result of mechanical coupling of crossbridges within a thick filament, a change in the kinetics of cycling of as few as 5% to 10% of crossbridges can affect the cycling of all the crossbridges in the filament and lead to a change in the kinetics of the contraction of the fiber.19 20 Another possibility to be considered is that the high concentration of added cardiac C1C2 could contribute to modification of crossbridge function by displacing some or even all of the native MyBP-C. This interpretation would explain the similarity of effects from the addition of unphosphorylated C1C2 and extraction of MyBP-C.
It is possible that the results of Kunst et al14 may be particularly relevant to familial hypertrophic cardiomyopathy caused by truncation of MyBP-C,21 22 in which a high concentration of the N-terminal fragment may be present. A high concentration of C1C2 could produce a decrease in the generation of force. The mechanical properties of the transgenic hearts of Yang et al,23 in which there is still normal as well as truncated MyBP-C, resemble those of the skinned fibers exposed to a high concentration of C1C2. This similarity raises the intriguing possibility that phosphorylation of the N-terminus of MyBP-C in myopathic hearts with a high concentration of N-terminal fragment might improve cardiac performance.
The results of Kunst et al14 have added important new information and opened new ways of looking at the function of MyBP-C in cardiac muscle. Novel roles for this cryptic, but no doubt important, myofibrillar protein must be considered.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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References |
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TopIntroductionReferences |
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1. Offer G, Moos C, Starr R. A new protein of the thick filaments of vertebrate skeletal myofibrils. Extraction. purification and characterization. J Mol Biol. 1973;74:653–676.[Medline] [Order article via Infotrieve]
2.
Winegrad S. Cardiac myosin binding protein C.
Circ Res. 1999;84:1117–1126.
3. Koretz JF. Effects of C-protein on synthetic myosin filament structure. Biophys J. 1979;27:433–446.[Medline] [Order article via Infotrieve]
4. Rhee D, Sanger JM, Sanger JW. The premyofibril: evidence for its role in myofibrillogenesis. Cell Motil Cytoskeleton. 1994;28:1–24.[Medline] [Order article via Infotrieve]
5.
Schultheiss T, Lin ZX, Lu MH, Fischman DA, Weber K,
Masaki T, Imamura M, Holtzer H. Differential distribution of subsets of
myofibrillar proteins in cardiac nonstriated and striated myofibrils.
J Cell Biol. 1990;110:1159–1172.
6. Gautel M, Zuffardi O, Freiburg A, Labeit S. Phosphorylation switches specific for the cardiac isoform of myosin binding protein-C: a modulator of cardiac contraction. EMBO J. 1995;14:1952–1960.[Medline] [Order article via Infotrieve]
7.
Schlender KK, Bean LJ. Phosphorylation
of chicken cardiac C-protein by
calcium/calmodulin-dependent protein kinase II.
J Biol Chem. 1991;266:2811–2817.
8.
Hartzell HC, Glass DB. Phosphorylation
of purified cardiac muscle C-protein by purified cAMP-dependent and
endogenous
Ca2+-calmodulin-dependent protein
kinases. J Biol Chem. 1984;259:15587–15596.
9. Craig R, Offer G. The location of C-protein in rabbit skeletal muscle. Proc R Soc Lond B Biol Sci. 1976;192:451–461.[Medline] [Order article via Infotrieve]
10.
Okagaki T, Weber FE, Fischman DA, Vaughan KT, Mikawa T,
Reinach FC. The major myosin binding domain of skeletal muscle MyBP-C
(C protein) resides in the COOH-terminal, immunoglobulin C2 repeat.
J Cell Biol. 1993;123:619–626.
11.
Alyonycheva TN, Mikawa T, Reinach FC, Fischman DA.
Isoform-specific interaction of the myosin-binding proteins (MyBPs)
with skeletal and cardiac myosin is a property of the C-terminal
immunoglobulin domain. J Biol Chem. 1997;272:20866–20872.
12. Gruen M, Gautel M. Mutations in ß-myosin S2 that cause familial hypertrophic cardiomyopathy (FHC) abolish the interaction with the regulatory domain of myosin binding protein-C. J Mol Biol. 1999;286:933–949.[Medline] [Order article via Infotrieve]
13.
Niimura H, Bachinski LL, Sangwatanaroj S, Watkins H,
Chudley AE, McKenna W, Kristinsonn A, Roberts R, Maron BJ, Seidman JG,
Seidman CE. Mutations in the gene for cardiac myosin-binding protein C
and late-onset familial hypertrophic
cardiomyopathy. N Engl J Med. 1998;338:1248–1257.
14.
Kunst G, Kress KR, Gruen M, Uttenweiler D, Gautel M,
Fink RHA. Myosin Binding Protein C, a
phosphorylation-dependent force regulator in muscle
that controls the attachment of myosin heads by its interaction with
myosin S2. Circ Res. 2000;86:51–58.
15.
Weisberg A, Winegrad S. Alteration of myosin cross
bridges by phosphorylation of myosin-binding protein C
in cardiac muscle. Proc Natl Acad Sci U S A. 1996;93:8999–9003.
16.
Weisberg A, Winegrad S. Relation between crossbridge
structure and actomyosin ATPase activity in rat heart. Circ
Res. 1998;83:60–72.
17.
Cooke R. Actomyosin interaction in striated muscle.
Physiol Rev. 1997;77:671–697.
18.
Hofmann P, Hartzell C, Moss R. Alterations in Ca
sensitive tension due to partial extraction of C protein from rat
skinned cardiac myocytes and rabbit skeletal muscle fibers.
J Gen Physiol. 1991;97:1141–1163.
19. Harris DE, Work SS, Wright RK, Alpert NR, Warshaw DM. Smooth, cardiac and skeletal muscle myosin force and motion generation assessed by cross-bridge mechanical interactions in vitro. J Muscle Res Cell Motil. 1994;15:11–9.[Medline] [Order article via Infotrieve]
20.
VanBuren P, Harris DE, Alpert NR, Warshaw DM. Cardiac
V1 and V3 myosins differ in
their hydrolytic and mechanical activities in vitro. Circ
Res. 1995;77:439–444.
21. Bonne G, Carrier L, Bercovici J, Cruad C, Richard P, Hainque M, Gautel M, Labeit S, Jones M, Beckman J, Vosberg H-P, Komajda M, Schwartz K. Cardiac myosin-binding protein C gene splice is associated with familial hypertrophic cardiomyopathy. Nat Genet. 1995;11:438–440.[Medline] [Order article via Infotrieve]
22. Watkins H, Conner D, Thierfelder L, Jarcho JA, MacCrea C, McKenna WJ, Maron BJ, Seidman JG, Seidman CE. Mutations in the cardiac myosin-binding protein-C on chromosome 11 cause familial hypertrophic cardiomyopathy. Nat Genet. 1995;11:433–438.
23. Yang Q, Sanbe AJ, Osinska H, Hewett TE, Klevitsky R, Robbins J. A mouse model of myosin binding protein C human familial hypertrophic cardiomyopathy. Clin Investig. 1998;102:1292–1300.[Medline] [Order article via Infotrieve]
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