Coiled-coil Nanomechanics and Uncoiling and Unfolding
of the Superhelix and 
-Helices of Myosin
Douglas D. Root 1, Vamsi K. Yadavalli 2, Jeffrey G. Forbes 2 and Kuan Wang 3*
1 University of North Texas
2 NIAMS, NIH
3 NIH
* To whom correspondence should be addressed. E-mail: wangk{at}exchange.nih.gov.
Submitted on July 28, 2005
Revised on October 11, 2005
Accepted on 4 January 2006
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Abstract |
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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 (AFM) 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 (WLC) 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 reveals 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 agrees well with the experimental value. We suggest that the flexible and elastic S2 region, particularly the hinge region, might undergo force-induced unfolding and extend reversibly during actomyosin powerstroke.
Key Words:
atomic force microscopy, elasticity, force spectroscopy, molecular mechanics, muscle, worm-like chain model
