How Myosin Generates Force on Actin Filaments

• Published in: Trends in biochemical sciences (Amsterdam. Regular ed.) Vol. 41; no. 12; pp. 989 - 997
• Main Authors: Houdusse, Anne, Sweeney, H. Lee
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.12.2016
• Subjects: actin; Actin Cytoskeleton - metabolism; Actin Cytoskeleton - ultrastructure; Actins - chemistry; Actins - metabolism; Adenosine Diphosphate - chemistry; Adenosine Diphosphate - metabolism; Adenosine Triphosphate - chemistry; Adenosine Triphosphate - metabolism; allostery; Amino Acid Motifs; Binding Sites; Biomechanical Phenomena; Catalytic Domain; Cellular Biology; chemo-mechanical transduction; cryo-electron microscopy; energy transfer; Fluorescence Resonance Energy Transfer; force generation; Humans; Life Sciences; Mechanotransduction, Cellular; microfilaments; molecular motors; mutation; myosin; Myosins - chemistry; Myosins - metabolism; phosphates; Phosphates - chemistry; Phosphates - metabolism; Protein Binding; Protein Conformation, alpha-Helical; Protein Conformation, beta-Strand; Protein Interaction Domains and Motifs; force generation; molecular motors; chemo-mechanical transduction; allostery; Allostery; Chemomechanical transduction; Molecular motors; Force generation
• ISSN: 0968-0004; 1362-4326; 0968-0004
• DOI: 10.1016/j.tibs.2016.09.006

How myosin interacts with actin to generate force is a subject of considerable controversy. The major debate centers on understanding at what point in force generation the inorganic phosphate is released with respect to the lever arm swing, or powerstroke. Resolving the controversy is essential for understanding how force is produced as well as the mechanisms underlying disease-causing mutations in myosin. Recent structural insights into the powerstroke have come from a high-resolution structure of myosin in a previously unseen state and from an electron cryomicroscopy (cryo-EM) 3D reconstruction of the actin–myosin–MgADP complex. Here, we argue that seemingly contradictory data from time-resolved fluorescence resonance energy transfer (FRET) studies can be reconciled, and we put forward a model for myosin force generation on actin. A new high-resolution structure reveals rearrangements in the myosin motor that promote release of inorganic phosphate (following ATP hydrolysis). FRET experiments reveal that, following binding to actin, the movement of the myosin lever arm (powerstroke) is extremely rapid and faster than the rate of release of inorganic phosphate into solution. Mutagenesis experiments suggest that phosphate release occurs before closure of the actin-binding cleft. Whether the myosin lever arm is coupled to closure of the actin-binding cleft and precedes or follows the release of inorganic phosphate is controversial. Resolving this controversy is central to understanding chemomechanical force transduction by the myosin motor on actin. We present a new model in which phosphate must move out of the active site before closure of the actin-binding cleft coupled to the lever arm swing. However, the phosphate can transiently remain bound to a second binding site at the mouth of the release tunnel, thus delaying its detection in solution.