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Coiled-Coil Nanomechanics and Uncoiling and Unfolding of the Superhelix and -Helices of Myosin

Douglas D. Root ^*, Vamsi K. Yadavalli , Jeffrey G. Forbes  and Kuan Wang 

^* Department of Biological Sciences, University of North Texas, Denton, Texas 76203-5220; and Muscle Proteomics and Nanotechnology Section, Laboratory of Muscle Biology, National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health, Department of Health and Human Services, Bethesda, Maryland 20892

Correspondence: Address reprint requests to Dr. Kuan Wang, B50/Rm1140, LMB, NIAMS, NIH, Bethesda, MD 29892. Tel.: 301-496-4097; Fax: 301-402-8566; E-mail: wangk{at}exchange.nih.gov.

The nanomechanical properties of the coiled-coils of myosin are fundamentally important in understanding muscle assembly and contraction. Force spectra of single molecules of double-headed myosin, single-headed myosin, and coiled-coil tail fragments were acquired with an atomic force microscope and displayed characteristic triphasic force-distance responses to stretch: a rise phase (R) and a plateau phase (P) and an exponential phase (E). The R and P phases arise mainly from the stretching of the coiled-coils, with the hinge region being the main contributor to the rise phase at low force. Only the E phase was analyzable by the worm-like chain model of polymer elasticity. Restrained molecular mechanics simulations on an existing x-ray structure of scallop S2 yielded force spectra with either two or three phases, depending on the mode of stretch. It revealed that coiled-coil chains separate completely near the end of the P phase and the stretching of the unfolded chains gives rise to the E phase. Extensive conformational searching yielded a P phase force near 40 pN that agreed well with the experimental value. We suggest that the flexible and elastic S2 region, particularly the hinge region, may undergo force-induced unfolding and extend reversibly during actomyosin powerstroke.

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