Muscle contraction is powered by the interaction of the molecular motor myosin with actin. With new techniques for single molecule manipulation and f luorescence detection, it is now possible to correlate, within the same molecule and in real time, conformational states and mechanical function of myosin. A spot-confocal microscope, capable of detecting single f luorophore polarization, was developed to measure orientational states in the smooth muscle myosin light chain domain during the process of motion generation. Fluorescently labeled turkey gizzard smooth muscle myosin was prepared by removal of endogenous regulatory light chain and re-addition of the light chain labeled at cysteine-108 with the 6-isomer of iodoacetamidotetramethylrhodamine (6-IATR). Single myosin molecule f luorescence polarization data, obtained in a motility assay, provide direct evidence that the myosin light chain domain adopts at least two orientational states during the cyclic interaction of myosin with actin, a randomly disordered state, most likely associated with myosin whereas weakly bound to actin, and an ordered state in which the light chain domain adopts a finite angular orientation whereas strongly bound after the powerstroke.At the molecular level, muscular force and motion generation are the result of a cyclic interaction between myosin and actin.
Naturally occurring groups of muscle myosin behave differently from individual myosins or small groups commonly assayed in vitro. Here, we investigate the emergence of myosin group behavior with increasing myosin group size. Assuming the number of myosin binding sites (N) is proportional to actin length (L) (N = L/35.5 nm), we resolve in vitro motility of actin propelled by skeletal muscle myosin for L = 0.2-3 μm. Three distinct regimes were found: L < 0.3 μm, sliding arrest; 0.3 μm ≤ L ≤ 1 μm, alternation between arrest and continuous sliding; L > 1 μm, continuous sliding. We theoretically investigated the myosin group kinetics with mechanical coupling via actin. We find rapid actin sliding steps driven by power-stroke cascades supported by postpower-stroke myosins, and phases without actin sliding caused by prepower-stroke myosin buildup. The three regimes are explained: N = 8, rare cascades; N = 15, cascade bursts; N = 35, continuous cascading. Two saddle-node bifurcations occur for increasing N (mono → bi → mono-stability), with steady states corresponding to arrest and continuous cascading. The experimentally measured dependence of actin sliding statistics on L and myosin concentration is correctly predicted.
On the terminology for describing the length-force relationship and its changes in airway smooth muscle. J Appl Physiol 97: 2029 -2034, 2004; doi:10.1152/japplphysiol.00884.2004.-The observation that the length-force relationship in airway smooth muscle can be shifted along the length axis by accommodating the muscle at different lengths has stimulated great interest. In light of the recent understanding of the dynamic nature of length-force relationship, many of our concepts regarding smooth muscle mechanical properties, including the notion that the muscle possesses a unique optimal length that correlates to maximal force generation, are likely to be incorrect. To facilitate accurate and efficient communication among scientists interested in the function of airway smooth muscle, a revised and collectively accepted nomenclature describing the adaptive and dynamic nature of the lengthforce relationship will be invaluable. Setting aside the issue of underlying mechanism, the purpose of this article is to define terminology that will aid investigators in describing observed phenomena. In particular, we recommend that the term "optimal length" (or any other term implying a unique length that correlates with maximal force generation) for airway smooth muscle be avoided. Instead, the in situ length or an arbitrary but clearly defined reference length should be used. We propose the usage of "length adaptation" to describe the phenomenon whereby the length-force curve of a muscle shifts along the length axis due to accommodation of the muscle at different lengths. We also discuss frequently used terms that do not have commonly accepted definitions that should be used cautiously.smooth muscle contraction; adaptation; plasticity; cytoskeleton; contractile apparatus THE CAPACITIES OF AIRWAY SMOOTH MUSCLE to generate force and to shorten are not a unique function of muscle length. Instead, they change appreciably depending on the histories of muscle loading, length, and activation. These changes can occur over the course of days, hours, and even seconds (9, 11-14, 24, 35, 41, 44, 46). As a result, the length-force relationship of airway smooth muscle is highly mutable, and its characterization is meaningful only when the histories on which the relationship is derived are included. Length-dependent force generation in other smooth muscles is also known to be influenced by various factors (18,29,34,36,39), with the extent of influence varying from one type of smooth muscle to another. The following description of phenomena and terminology is based on and intended for airway smooth muscle, and it may or may not apply to other smooth muscle types. Current terminology that describes the length-force characteristic in airway smooth muscle is borrowed from the physiology of striated muscle but is inadequate, and in some cases ill-suited, to depict the mutable relationship in airway smooth muscle. Thus there is a need to seek a consensual agreement among scientists working in the field of airway smooth muscle biomechanics concern...
Two smooth muscle myosin heavy chain isoforms differ by a 7-amino-acid insert in a flexible surface loop located near the nucleotide binding site. The non-inserted isoform is predominantly found in tonic muscle, while the inserted isoform is mainly found in phasic muscle. The inserted isoform has twice the actin-activated ATPase activity and actin filament velocity in the in vitro motility assay as the non-inserted isoform. We used the laser trap to characterize the molecular mechanics and kinetics of the inserted isoform ((+)insert) and of a mutant lacking the insert ((-)insert), analogous to the isoform found in tonic muscle. The constructs were expressed as heavy meromyosin using the baculovirus/insect cell system. Unitary displacement (d) was similar for both constructs (approximately 10 nm) but the attachment time (t(on) for the (-)insert was twice as long as for the (+)insert regardless of the [MgATP]. Both the relative average isometric force (Favg(-insert)/Favg(+insert) = 1.1 +/- 0.2 (mean +/- SE) using the in vitro motility mixture assay, and the unitary force (F approximately 1 pN) using the laser trap, showed no difference between the two constructs. However, as under unloaded conditions, t(on) under loaded conditions was longer for the (-)insert compared with the (+)insert construct at limiting [MgATP]. These data suggest that the insert in this surface loop does not affect the mechanics but rather the kinetics of the cross-bridge cycle. Through comparisons of t(on) from d measurements to various [MgATP], we conclude that the insert affects two specific steps in the cross-bridge cycle, that is, MgADP release and MgATP binding.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.