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

• Published in: Biophysical journal 2006-04, Vol.90 (8), p.2852-2866
• Main Authors: Root, Douglas D., Yadavalli, Vamsi K., Forbes, Jeffrey G., Wang, Kuan
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; Biomechanical Phenomena; Hydrogen Bonding; Microscopy, Atomic Force; Models, Molecular; Molecular Sequence Data; Muscles and Contractility; Myosins - chemistry; Myosins - ultrastructure; Protein Folding; Protein Structure, Secondary; Rabbits
• ISSN: 0006-3495; 1542-0086
• DOI: 10.1529/biophysj.105.071597

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.