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de Jonge JJ, Graw A, Kargas V, Batters C, Montanarella AF, O'Loughlin T, Johnson C, Arden SD, Warren AJ, Geeves MA, Kendrick-Jones J, Zaccai NR, Kröss M, Veigel C, Buss F. Motor domain phosphorylation increases nucleotide exchange and turns MYO6 into a faster and stronger motor. Nat Commun 2024; 15:6716. [PMID: 39112473 PMCID: PMC11306250 DOI: 10.1038/s41467-024-49898-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2023] [Accepted: 06/20/2024] [Indexed: 08/10/2024] Open
Abstract
Myosin motors perform many fundamental functions in eukaryotic cells by providing force generation, transport or tethering capacity. Motor activity control within the cell involves on/off switches, however, few examples are known of how myosins regulate speed or processivity and fine-tune their activity to a specific cellular task. Here, we describe a phosphorylation event for myosins of class VI (MYO6) in the motor domain, which accelerates its ATPase activity leading to a 4-fold increase in motor speed determined by actin-gliding assays, single molecule mechanics and stopped flow kinetics. We demonstrate that the serine/threonine kinase DYRK2 phosphorylates MYO6 at S267 in vitro. Single-molecule optical-tweezers studies at low load reveal that S267-phosphorylation results in faster nucleotide-exchange kinetics without change in the working stroke of the motor. The selective increase in stiffness of the acto-MYO6 complex when proceeding load-dependently into the nucleotide-free rigor state demonstrates that S267-phosphorylation turns MYO6 into a stronger motor. Finally, molecular dynamic simulations of the nucleotide-free motor reveal an alternative interaction network within insert-1 upon phosphorylation, suggesting a molecular mechanism, which regulates insert-1 positioning, turning the S267-phosphorylated MYO6 into a faster motor.
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Affiliation(s)
- Janeska J de Jonge
- Cambridge Institute for Medical Research, Department of Clinical Biochemistry, University of Cambridge, Cambridge Biomedical Campus, The Keith Peters Building, Hills Road, Cambridge, CB2 0XY, UK
| | - Andreas Graw
- Department of Cellular Physiology, Biomedical Centre (BMC), Ludwig-Maximilians-Universität München, Grosshadernerstrasse 9, 82152, Planegg-Martinsried, Germany
- Centre for NanoScience (CeNS), Ludwig-Maximilians-Universität München, Schellingstrasse 4, 80799, München, Germany
| | - Vasileios Kargas
- Cambridge Institute for Medical Research, Department of Clinical Biochemistry, University of Cambridge, Cambridge Biomedical Campus, The Keith Peters Building, Hills Road, Cambridge, CB2 0XY, UK
- Wellcome MRC Cambridge Stem Cell Institute, University of Cambridge, Cambridge, UK
- Department of Haematology, University of Cambridge, Cambridge, UK
| | - Christopher Batters
- Cambridge Institute for Medical Research, Department of Clinical Biochemistry, University of Cambridge, Cambridge Biomedical Campus, The Keith Peters Building, Hills Road, Cambridge, CB2 0XY, UK
- Department of Cellular Physiology, Biomedical Centre (BMC), Ludwig-Maximilians-Universität München, Grosshadernerstrasse 9, 82152, Planegg-Martinsried, Germany
- Centre for NanoScience (CeNS), Ludwig-Maximilians-Universität München, Schellingstrasse 4, 80799, München, Germany
| | - Antonino F Montanarella
- Department of Cellular Physiology, Biomedical Centre (BMC), Ludwig-Maximilians-Universität München, Grosshadernerstrasse 9, 82152, Planegg-Martinsried, Germany
- Centre for NanoScience (CeNS), Ludwig-Maximilians-Universität München, Schellingstrasse 4, 80799, München, Germany
| | - Tom O'Loughlin
- Cambridge Institute for Medical Research, Department of Clinical Biochemistry, University of Cambridge, Cambridge Biomedical Campus, The Keith Peters Building, Hills Road, Cambridge, CB2 0XY, UK
| | - Chloe Johnson
- Cambridge Institute for Medical Research, Department of Clinical Biochemistry, University of Cambridge, Cambridge Biomedical Campus, The Keith Peters Building, Hills Road, Cambridge, CB2 0XY, UK
| | - Susan D Arden
- Cambridge Institute for Medical Research, Department of Clinical Biochemistry, University of Cambridge, Cambridge Biomedical Campus, The Keith Peters Building, Hills Road, Cambridge, CB2 0XY, UK
| | - Alan J Warren
- Cambridge Institute for Medical Research, Department of Clinical Biochemistry, University of Cambridge, Cambridge Biomedical Campus, The Keith Peters Building, Hills Road, Cambridge, CB2 0XY, UK
- Wellcome MRC Cambridge Stem Cell Institute, University of Cambridge, Cambridge, UK
- Department of Haematology, University of Cambridge, Cambridge, UK
| | | | - John Kendrick-Jones
- MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge Biomedical Campus, Cambridge, CB2 0QH, UK
| | - Nathan R Zaccai
- Cambridge Institute for Medical Research, Department of Clinical Biochemistry, University of Cambridge, Cambridge Biomedical Campus, The Keith Peters Building, Hills Road, Cambridge, CB2 0XY, UK
| | - Markus Kröss
- Department of Cellular Physiology, Biomedical Centre (BMC), Ludwig-Maximilians-Universität München, Grosshadernerstrasse 9, 82152, Planegg-Martinsried, Germany
- Centre for NanoScience (CeNS), Ludwig-Maximilians-Universität München, Schellingstrasse 4, 80799, München, Germany
| | - Claudia Veigel
- Department of Cellular Physiology, Biomedical Centre (BMC), Ludwig-Maximilians-Universität München, Grosshadernerstrasse 9, 82152, Planegg-Martinsried, Germany.
- Centre for NanoScience (CeNS), Ludwig-Maximilians-Universität München, Schellingstrasse 4, 80799, München, Germany.
| | - Folma Buss
- Cambridge Institute for Medical Research, Department of Clinical Biochemistry, University of Cambridge, Cambridge Biomedical Campus, The Keith Peters Building, Hills Road, Cambridge, CB2 0XY, UK.
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Allen C, Bureau HR, McGee TD, Quirk S, Hernandez R. Benchmarking Adaptive Steered Molecular Dynamics (ASMD) on CHARMM Force Fields. Chemphyschem 2022; 23:e202200175. [PMID: 35594194 PMCID: PMC9543079 DOI: 10.1002/cphc.202200175] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2022] [Revised: 05/19/2022] [Indexed: 11/07/2022]
Abstract
The potentials of mean force (PMFs) along the end‐to‐end distance of two different helical peptides have been obtained and benchmarked using the adaptive steered molecular dynamics (ASMD) method. The results depend strongly on the choice of force field driving the underlying all‐atom molecular dynamics, and are reported with respect to the three most popular CHARMM force field versions: c22, c27 and c36. Two small peptides, ALA10
and 1PEF, serve as the particular case studies. The comparisons between the versions of the CHARMM force fields provides both a qualitative and quantitative look at their performance in forced unfolding simulations in which peptides undergo large changes in structural conformations. We find that ASMD with the underlying c36 force field provides the most robust results for the selected benchmark peptides.
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Affiliation(s)
- Caley Allen
- Johns Hopkins University Zanvyl Krieger School of Arts and Sciences, Chemistry, UNITED STATES
| | - Hailey R Bureau
- Johns Hopkins University Zanvyl Krieger School of Arts and Sciences, Department of Chemistry, UNITED STATES
| | | | | | - Rigoberto Hernandez
- Johns Hopkins University - Homewood Campus: Johns Hopkins University, Chemistry, 3400 N Charles St, 21218, Baltimore, UNITED STATES
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Abstract
Myosin VI (MVI) is the only known member of the myosin superfamily that, upon dimerization, walks processively toward the pointed end of the actin filament. The leading head of the dimer directs the trailing head forward with a power stroke, a conformational change of the motor domain exaggerated by the lever arm. Using a unique coarse-grained model for the power stroke of a single MVI, we provide the molecular basis for its motility. We show that the power stroke occurs in two major steps. First, the motor domain attains the poststroke conformation without directing the lever arm forward; and second, the lever arm reaches the poststroke orientation by undergoing a rotational diffusion. From the analysis of the trajectories, we discover that the potential that directs the rotating lever arm toward the poststroke conformation is almost flat, implying that the lever arm rotation is mostly uncoupled from the motor domain. Because a backward load comparable to the largest interhead tension in a MVI dimer prevents the rotation of the lever arm, our model suggests that the leading-head lever arm of a MVI dimer is uncoupled, in accord with the inference drawn from polarized total internal reflection fluorescence (polTIRF) experiments. Without any adjustable parameter, our simulations lead to quantitative agreement with polTIRF experiments, which validates the structural insights. Finally, in addition to making testable predictions, we also discuss the implications of our model in explaining the broad step-size distribution of the MVI stepping pattern.
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Affiliation(s)
- Mauro L Mugnai
- Biophysics Program, Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742;
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742
| | - D Thirumalai
- Biophysics Program, Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742;
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742
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Abstract
The myosin holoenzyme is a multimeric protein complex consisting of heavy chains and light chains. Myosin light chains are calmodulin family members which are crucially involved in the mechanoenzymatic function of the myosin holoenzyme. This review examines the diversity of light chains within the myosin superfamily, discusses interactions between the light chain and the myosin heavy chain as well as regulatory and structural functions of the light chain as a subunit of the myosin holoenzyme. It covers aspects of the myosin light chain in the localization of the myosin holoenzyme, protein-protein interactions and light chain binding to non-myosin binding partners. Finally, this review challenges the dogma that myosin regulatory and essential light chain exclusively associate with conventional myosin heavy chains while unconventional myosin heavy chains usually associate with calmodulin.
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Affiliation(s)
- Sarah M Heissler
- a Laboratory of Molecular Physiology; National Heart, Lung, and Blood Institute; National Institutes of Health ; Bethesda , MD USA
| | - James R Sellers
- a Laboratory of Molecular Physiology; National Heart, Lung, and Blood Institute; National Institutes of Health ; Bethesda , MD USA
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6
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Li J, Lu Q, Zhang M. Structural Basis of Cargo Recognition by Unconventional Myosins in Cellular Trafficking. Traffic 2016; 17:822-38. [PMID: 26842936 DOI: 10.1111/tra.12383] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/02/2015] [Revised: 01/29/2016] [Accepted: 01/29/2016] [Indexed: 01/17/2023]
Abstract
Unconventional myosins are a superfamily of actin-based molecular motors playing diverse roles including cellular trafficking, mechanical supports, force sensing and transmission, etc. The variable neck and tail domains of unconventional myosins function to bind to specific cargoes including proteins and lipid vesicles and thus are largely responsible for the diverse cellular functions of myosins in vivo. In addition, the tail regions, together with their cognate cargoes, can regulate activities of the motor heads. This review outlines the advances made in recent years on cargo recognition and cargo binding-induced regulation of the activity of several unconventional myosins including myosin-I, V, VI and X in cellular trafficking. We approach this topic by describing a series of high-resolution structures of the neck and tail domains of these unconventional myosins either alone or in complex with their specific cargoes, and by discussing potential implications of these structural studies on cellular trafficking of these myosin motors.
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Affiliation(s)
- Jianchao Li
- Division of Life Science, State Key Laboratory of Molecular Neuroscience, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
| | - Qing Lu
- Division of Life Science, State Key Laboratory of Molecular Neuroscience, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
| | - Mingjie Zhang
- Division of Life Science, State Key Laboratory of Molecular Neuroscience, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.,Center of Systems Biology and Human Health, School of Science and Institute for Advanced Study, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
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7
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GC JB, Gerstman BS, Chapagain PP. The Role of the Interdomain Interactions on RfaH Dynamics and Conformational Transformation. J Phys Chem B 2015; 119:12750-9. [DOI: 10.1021/acs.jpcb.5b05681] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Jeevan B. GC
- Department of Physics, Florida International University, Miami, Florida 33199, United States
| | - Bernard S. Gerstman
- Department of Physics, Florida International University, Miami, Florida 33199, United States
| | - Prem P. Chapagain
- Department of Physics, Florida International University, Miami, Florida 33199, United States
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Santamaria R, Jones K, Arroyo M, González-García T. Polymer folding via external potentials in ab-initio calculations. COMPUT THEOR CHEM 2015. [DOI: 10.1016/j.comptc.2015.06.012] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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Mukherjea M, Ali MY, Kikuti C, Safer D, Yang Z, Sirkia H, Ropars V, Houdusse A, Warshaw DM, Sweeney HL. Myosin VI must dimerize and deploy its unusual lever arm in order to perform its cellular roles. Cell Rep 2014; 8:1522-32. [PMID: 25159143 DOI: 10.1016/j.celrep.2014.07.041] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/19/2014] [Revised: 06/13/2014] [Accepted: 07/22/2014] [Indexed: 12/21/2022] Open
Abstract
It is unclear whether the reverse-direction myosin (myosin VI) functions as a monomer or dimer in cells and how it generates large movements on actin. We deleted a stable, single-α-helix (SAH) domain that has been proposed to function as part of a lever arm to amplify movements without impact on in vitro movement or in vivo functions. A myosin VI construct that used this SAH domain as part of its lever arm was able to take large steps in vitro but did not rescue in vivo functions. It was necessary for myosin VI to internally dimerize, triggering unfolding of a three-helix bundle and calmodulin binding in order to step normally in vitro and rescue endocytosis and Golgi morphology in myosin VI-null fibroblasts. A model for myosin VI emerges in which cargo binding triggers dimerization and unfolds the three-helix bundle to create a lever arm essential for in vivo functions.
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Affiliation(s)
- Monalisa Mukherjea
- Department of Physiology, University of Pennsylvania Perelman School of Medicine, 415 Curie Boulevard, 700 CRB, Philadelphia, PA 19104-6085, USA
| | - M Yusuf Ali
- Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, VT 05405, USA
| | - Carlos Kikuti
- Structural Motility, Centre de Recherche, Institut Curie, 75248 Paris, France; CNRS, UMR144, 26 rue d'Ulm, 75248 Paris Cedex 05, France
| | - Daniel Safer
- Department of Physiology, University of Pennsylvania Perelman School of Medicine, 415 Curie Boulevard, 700 CRB, Philadelphia, PA 19104-6085, USA
| | - Zhaohui Yang
- Department of Physiology, University of Pennsylvania Perelman School of Medicine, 415 Curie Boulevard, 700 CRB, Philadelphia, PA 19104-6085, USA
| | - Helena Sirkia
- Structural Motility, Centre de Recherche, Institut Curie, 75248 Paris, France; CNRS, UMR144, 26 rue d'Ulm, 75248 Paris Cedex 05, France
| | - Virginie Ropars
- Structural Motility, Centre de Recherche, Institut Curie, 75248 Paris, France; CNRS, UMR144, 26 rue d'Ulm, 75248 Paris Cedex 05, France
| | - Anne Houdusse
- Structural Motility, Centre de Recherche, Institut Curie, 75248 Paris, France; CNRS, UMR144, 26 rue d'Ulm, 75248 Paris Cedex 05, France
| | - David M Warshaw
- Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, VT 05405, USA
| | - H Lee Sweeney
- Department of Physiology, University of Pennsylvania Perelman School of Medicine, 415 Curie Boulevard, 700 CRB, Philadelphia, PA 19104-6085, USA.
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10
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Zheng W. All-atom and coarse-grained simulations of the forced unfolding pathways of the SNARE complex. Proteins 2014; 82:1376-86. [DOI: 10.1002/prot.24505] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/16/2013] [Revised: 12/11/2013] [Accepted: 01/06/2014] [Indexed: 01/03/2023]
Affiliation(s)
- Wenjun Zheng
- Department of Physics; University at Buffalo, State University of New York; New York
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11
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Karagiannis P, Ishii Y, Yanagida T. Molecular machines like myosin use randomness to behave predictably. Chem Rev 2014; 114:3318-34. [PMID: 24484383 DOI: 10.1021/cr400344n] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Peter Karagiannis
- Quantitative Biology Center, Riken (QBiC) , Furuedai 6-2-3, Suita, Osaka 565-0874, Japan
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12
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Preller M, Manstein D. Myosin Structure, Allostery, and Mechano-Chemistry. Structure 2013; 21:1911-22. [DOI: 10.1016/j.str.2013.09.015] [Citation(s) in RCA: 46] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2013] [Revised: 09/19/2013] [Accepted: 09/25/2013] [Indexed: 01/10/2023]
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13
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Yu C, Lou J, Wu J, Pan L, Feng W, Zhang M. Membrane-induced lever arm expansion allows myosin VI to walk with large and variable step sizes. J Biol Chem 2012; 287:35021-35035. [PMID: 22936804 DOI: 10.1074/jbc.m111.328781] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Myosin VI, the only known minus-ended actin filament-dependent motor, plays diverse cellular roles both as a processive motor and as a mechanical anchor. Although myosin VI has a short lever arm containing only one "IQ-motif" and a unique insertion for CaM binding, the motor walks with large and variable step sizes of ∼30-36 nm. Here, we show that the previously predicted coiled-coil domain immediately following the IQ-motifs (referred to as the lever arm extension (LAE)) adopts a stable monomeric, three-helix bundle fold in solution. Importantly, the LAE can undergo reversible, lipid membrane-dependent conformational changes. Upon exposure to lipid membranes, the LAE adopts a partially extended rod shape, and the removal of lipids from the LAE converts it back into the compact helix bundle structure. Molecular dynamics simulations indicate that lipid membrane binding may initiate unfolding and thereby trigger the LAE expansion. This reversible, lipid membrane-dependent expansion of the LAE provides a mechanistic base for myosin VI to walk with large and variable step sizes.
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Affiliation(s)
- Cong Yu
- Division of Life Science, State Key Laboratory of Molecular Neuroscience, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
| | - Jizhong Lou
- National Laboratory of Biomacromolecules, Chinese Academy of Sciences, 15 Datun Road, Beijing 100101, China; Laboratory of Noncoding RNA, Institute of Biophysics, Chinese Academy of Sciences, 15 Datun Road, Beijing 100101, China
| | - Jingjing Wu
- Division of Life Science, State Key Laboratory of Molecular Neuroscience, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
| | - Lifeng Pan
- Division of Life Science, State Key Laboratory of Molecular Neuroscience, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
| | - Wei Feng
- National Laboratory of Biomacromolecules, Chinese Academy of Sciences, 15 Datun Road, Beijing 100101, China.
| | - Mingjie Zhang
- Division of Life Science, State Key Laboratory of Molecular Neuroscience, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China; Center of Systems Biology and Human Health, School of Science and Institute for Advanced Study, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.
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Abstract
Myosin-X (Myo10) is an unconventional myosin with MyTH4-FERM domains that is best known for its striking localization to the tips of filopodia and its ability to induce filopodia. Although the head domain of Myo10 enables it to function as an actin-based motor, its tail contains binding sites for several molecules with central roles in cell biology, including phosphatidylinositol (3,4,5)-trisphosphate, microtubules and integrins. Myo10 also undergoes fascinating long-range movements within filopodia, which appear to represent a newly recognized system of transport. Myo10 is also unusual in that it is a myosin with important roles in the spindle, a microtubule-based structure. Exciting new studies have begun to reveal the structure and single-molecule properties of this intriguing myosin, as well as its mechanisms of regulation and induction of filopodia. At the cellular and organismal level, growing evidence demonstrates that Myo10 has crucial functions in numerous processes ranging from invadopodia formation to cell migration.
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Affiliation(s)
- Michael L Kerber
- Department of Cell and Molecular Physiology, School of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7545, USA
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15
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Ramamurthy B, Cao W, De la Cruz EM, Mooseker MS. Plus-end directed myosins accelerate actin filament sliding by single-headed myosin VI. Cytoskeleton (Hoboken) 2012; 69:59-69. [PMID: 22213699 DOI: 10.1002/cm.21002] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/07/2011] [Revised: 12/19/2011] [Accepted: 12/20/2011] [Indexed: 12/19/2022]
Abstract
Myosin VI (Myo6) is unique among myosins in that it moves toward the minus (pointed) end of the actin filament. Thus to exert tension on, or move cargo along an actin filament, Myo6 is working against potentially multiple plus (barbed)-end myosins. To test the effect of plus-end motors on Myo6, the gliding actin filament assay was used to assess the motility of single-headed Myo6 in the absence and presence of cardiac myosin II (Myo2) and myosin Va (Myo5a). Myo6 alone exhibited a filament gliding velocities of 60.34 ± 13.68 nm/s. Addition of either Myo2 or Myo5a, at densities below that required to promote plus-end movement resulted in an increase in Myo6 velocity (~100-150% increase). Movement in the presence of these plus-end myosins was minus-end directed as determined using polarity tagged filaments. High densities of Myo2 or Myo5a were required to convert to plus-end directed motility indicating that Myo6 is a potent inhibitor of Myo2 and Myo5a. Previous studies have shown that two-headed Myo6 slows and then stalls in an anchored state under load. Consistent with these studies, velocity of a two headed heavy mero myosin form of Myo6 was unaffected by Myo5a at low densities, and was inhibited at high Myo5a densities.
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Affiliation(s)
- Bhagavathi Ramamurthy
- Department of Molecular, Cellular and Developmental Biology, Yale University, New Haven, Connecticut 06520-8103, USA
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