76
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Mortensen KI, Churchman LS, Spudich JA, Flyvbjerg H. Optimized localization analysis for single-molecule tracking and super-resolution microscopy. Nat Methods 2010; 7:377-81. [PMID: 20364147 PMCID: PMC3127582 DOI: 10.1038/nmeth.1447] [Citation(s) in RCA: 539] [Impact Index Per Article: 38.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/14/2009] [Accepted: 02/22/2010] [Indexed: 11/09/2022]
Abstract
We optimally localize isolated fluorescent beads and molecules imaged as diffraction-limited spots, determine the orientation of molecules, and present reliable formulae for the precisions of various localization methods. For beads, theory and experimental data both show that unweighted least-squares fitting of a Gaussian squanders one third of the available information, a popular formula for its precision exaggerates beyond Fisher's information limit, and weighted least-squares may do worse, while maximum likelihood fitting is practically optimal.
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77
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Spudich JA, Sivaramakrishnan S. Myosin VI: an innovative motor that challenged the swinging lever arm hypothesis. Nat Rev Mol Cell Biol 2010; 11:128-37. [PMID: 20094053 DOI: 10.1038/nrm2833] [Citation(s) in RCA: 69] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
The swinging crossbridge hypothesis states that energy from ATP hydrolysis is transduced to mechanical movement of the myosin head while bound to actin. The light chain-binding region of myosin is thought to act as a lever arm that amplifies movements near the catalytic site. This model has been challenged by findings that myosin VI takes larger steps along actin filaments than early interpretations of its structure seem to allow. We now know that myosin VI does indeed operate by an unusual approximately 180 degrees lever arm swing and achieves its large step size using special structural features in its tail domain.
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78
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Sung J, Sivaramakrishnan S, Dunn AR, Spudich JA. Single-molecule dual-beam optical trap analysis of protein structure and function. Methods Enzymol 2010; 475:321-75. [PMID: 20627164 DOI: 10.1016/s0076-6879(10)75014-x] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
Optical trapping is one of the most powerful single-molecule techniques. We provide a practical guide to set up and use an optical trap, applied to the molecular motor myosin as an example. We focus primarily on studies of myosin function using a dual-beam optical trap, a protocol to build such a trap, and the experimental and data analysis protocols to utilize it.
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79
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Mortensen KI, Churchman LS, Spudich JA, Flyvbjerg H. Optimizing Super Resolution Microscopy. Biophys J 2010. [DOI: 10.1016/j.bpj.2009.12.972] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
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80
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Sivaramakrishnan S, Sommese R, Sung JM, Ali M, Doniach S, Flyvbjerg H, Spudich JA. Combining Single Molecule Optical Trapping and Small Angle X-Ray Scattering Measurements to Compute the Persistence Length of a Protein Alpha-Helix. Biophys J 2010. [DOI: 10.1016/j.bpj.2009.12.145] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022] Open
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81
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Sivaramakrishnan S, Ashley E, Leinwand L, Spudich JA. Insights into human beta-cardiac myosin function from single molecule and single cell studies. J Cardiovasc Transl Res 2009; 2:426-40. [PMID: 20560001 DOI: 10.1007/s12265-009-9129-2] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 07/22/2009] [Accepted: 09/10/2009] [Indexed: 12/13/2022]
Abstract
beta-Cardiac myosin is a mechanoenzyme that converts the energy from ATP hydrolysis into a mechanical force that drives contractility in muscle. Thirty percent of the point mutations that result in hypertrophic cardiomyopathy are localized to MYH7, the gene encoding human beta-cardiac myosin heavy chain (beta-MyHC). Force generation by myosins requires a tight and highly conserved allosteric coupling between its different protein domains. Hence, the effects of single point mutations on the force generation and kinetics of beta-cardiac myosin molecules cannot be predicted directly from their location within the protein structure. Great insight would be gained from understanding the link between the functional defect in the myosin protein and the clinical phenotypes of patients expressing them. Over the last decade, several single molecule techniques have been developed to understand in detail the chemomechanical cycle of different myosins. In this review, we highlight the single molecule techniques that can be used to assess the effect of point mutations on beta-cardiac myosin function. Recent bioengineering advances have enabled the micromanipulation of single cardiomyocyte cells to characterize their force-length dynamics. Here, we briefly review single cell micromanipulation as an approach to determine the effect of beta-MyHC mutations on cardiomyocyte function. Finally, we examine the technical challenges specific to studying beta-cardiac myosin function both using single molecule and single cell approaches.
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82
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Sivaramakrishnan S, Spudich JA. Coupled myosin VI motors facilitate unidirectional movement on an F-actin network. ACTA ACUST UNITED AC 2009; 187:53-60. [PMID: 19786577 PMCID: PMC2762089 DOI: 10.1083/jcb.200906133] [Citation(s) in RCA: 49] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/02/2022]
Abstract
A combination of experimentation and modeling reveal that multiple myosin VI molecules coordinately transport cargo over the actin filament network. Unconventional myosins interact with the dense cortical actin network during processes such as membrane trafficking, cell migration, and mechanotransduction. Our understanding of unconventional myosin function is derived largely from assays that examine the interaction of a single myosin with a single actin filament. In this study, we have developed a model system to study the interaction between multiple tethered unconventional myosins and a model F-actin cortex, namely the lamellipodium of a migrating fish epidermal keratocyte. Using myosin VI, which moves toward the pointed end of actin filaments, we directly determine the polarity of the extracted keratocyte lamellipodium from the cell periphery to the cell nucleus. We use a combination of experimentation and simulation to demonstrate that multiple myosin VI molecules can coordinate to efficiently transport vesicle-size cargo over 10 µm of the dense interlaced actin network. Furthermore, several molecules of monomeric myosin VI, which are nonprocessive in single molecule assays, can coordinate to transport cargo with similar speeds as dimers.
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83
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Vale RD, Spudich JA, Griffis ER. Dynamics of myosin, microtubules, and Kinesin-6 at the cortex during cytokinesis in Drosophila S2 cells. J Cell Biol 2009; 186:727-38. [PMID: 19720876 PMCID: PMC2742194 DOI: 10.1083/jcb.200902083] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2009] [Accepted: 07/27/2009] [Indexed: 12/22/2022] Open
Abstract
Signals from the mitotic spindle during anaphase specify the location of the actomyosin contractile ring during cytokinesis, but the detailed mechanism remains unresolved. Here, we have imaged the dynamics of green fluorescent protein-tagged myosin filaments, microtubules, and Kinesin-6 (which carries activators of Rho guanosine triphosphatase) at the cell cortex using total internal reflection fluorescence microscopy in flattened Drosophila S2 cells. At anaphase onset, Kinesin-6 relocalizes to microtubule plus ends that grow toward the cortex, but refines its localization over time so that it concentrates on a subset of stable microtubules and along a diffuse cortical band at the equator. The pattern of Kinesin-6 localization closely resembles where new myosin filaments appear at the cortex by de novo assembly. While accumulating at the equator, myosin filaments disappear from the poles of the cell, a process that also requires Kinesin-6 as well as possibly other signals that emanate from the elongating spindle. These results suggest models for how Kinesin-6 might define the position of cortical myosin during cytokinesis.
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84
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Pierobon P, Achouri S, Courty S, Dunn AR, Spudich JA, Dahan M, Cappello G. Velocity, processivity, and individual steps of single myosin V molecules in live cells. Biophys J 2009; 96:4268-75. [PMID: 19450497 DOI: 10.1016/j.bpj.2009.02.045] [Citation(s) in RCA: 88] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2008] [Revised: 01/20/2009] [Accepted: 02/05/2009] [Indexed: 11/29/2022] Open
Abstract
We report the tracking of single myosin V molecules in their natural environment, the cell. Myosin V molecules, labeled with quantum dots, are introduced into the cytoplasm of living HeLa cells and their motion is recorded at the single molecule level with high spatial and temporal resolution. We perform an intracellular measurement of key parameters of this molecular transporter: velocity, processivity, step size, and dwell time. Our experiments bridge the gap between in vitro single molecule assays and the indirect measurements of the motor features deduced from the tracking of organelles in live cells.
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85
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Liao JC, Elting MW, Delp SL, Spudich JA, Bryant Z. Engineered myosin VI motors reveal minimal structural determinants of directionality and processivity. J Mol Biol 2009; 392:862-7. [PMID: 19631216 DOI: 10.1016/j.jmb.2009.07.046] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/17/2009] [Revised: 07/11/2009] [Accepted: 07/16/2009] [Indexed: 11/17/2022]
Abstract
Myosins have diverse mechanical properties reflecting a range of cellular roles. A major challenge is to understand the structural basis for generating novel functions from a common motor core. Myosin VI (M6) is specialized for processive motion toward the (-) end of actin filaments. We have used engineered M6 motors to test and refine the "redirected power stroke" model for (-) end directionality and to explore poorly understood structural requirements for processive stepping. Guided by crystal structures and molecular modeling, we fused artificial lever arms to the catalytic head of M6 at several positions, retaining varying amounts of native structure. We found that an 18-residue alpha-helical insert is sufficient to reverse the directionality of the motor, with no requirement for any calmodulin light chains. Further, we observed robust processive stepping of motors with artificial lever arms, demonstrating that processivity can arise without optimizing lever arm composition or mechanics.
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86
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Chuan P, Dunn AR, Bryant Z, Spudich JA. Structural and Mechanistic Determinants of Myosin VI Processivity and Anchoring. Biophys J 2009. [DOI: 10.1016/j.bpj.2008.12.3883] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
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87
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Dietz H, Derr ND, Dunn A, Spudich JA, Shih W. Towards Custom-topology Tracks For Probing Myosin Motor Dynamics. Biophys J 2009. [DOI: 10.1016/j.bpj.2008.12.588] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022] Open
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88
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Spink BJ, Sivaramakrishnan S, Lipfert J, Doniach S, Spudich JA. Long single alpha-helical tail domains bridge the gap between structure and function of myosin VI. Nat Struct Mol Biol 2008; 15:591-7. [PMID: 18511944 PMCID: PMC2441774 DOI: 10.1038/nsmb.1429] [Citation(s) in RCA: 98] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2007] [Accepted: 04/10/2008] [Indexed: 11/08/2022]
Abstract
Myosin VI has challenged the lever arm hypothesis of myosin movement because of its ability to take approximately 36-nm steps along actin with a canonical lever arm that seems to be too short to allow such large steps. Here we demonstrate that the large step of dimeric myosin VI is primarily made possible by a medial tail in each monomer that forms a rare single alpha-helix of approximately 10 nm, which is anchored to the calmodulin-bound IQ domain by a globular proximal tail. With the medial tail contributing to the approximately 36-nm step, rather than dimerizing as previously proposed, we show that the cargo binding domain is the dimerization interface. Furthermore, the cargo binding domain seems to be folded back in the presence of the catalytic head, constituting a potential regulatory mechanism that inhibits dimerization.
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89
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Abstract
A recent study has revealed an unexpected change in conformation of the myosin VI converter domain, essential for twisting the lever arm through a approximately 180 degrees rotation to achieve a large step along actin.
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90
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Altman D, Goswami D, Hasson T, Spudich JA, Mayor S. Precise positioning of myosin VI on endocytic vesicles in vivo. PLoS Biol 2008; 5:e210. [PMID: 17683200 PMCID: PMC1939883 DOI: 10.1371/journal.pbio.0050210] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/12/2006] [Accepted: 06/07/2007] [Indexed: 11/19/2022] Open
Abstract
Myosin VI has been studied in both a monomeric and a dimeric form in vitro. Because the functional characteristics of the motor are dramatically different for these two forms, it is important to understand whether myosin VI heavy chains are brought together on endocytic vesicles. We have used fluorescence anisotropy measurements to detect fluorescence resonance energy transfer between identical fluorophores (homoFRET) resulting from myosin VI heavy chains being brought into close proximity. We observed that, when associated with clathrin-mediated endocytic vesicles, myosin VI heavy chains are precisely positioned to bring their tail domains in close proximity. Our data show that on endocytic vesicles, myosin VI heavy chains are brought together in an orientation that previous in vitro studies have shown causes dimerization of the motor. Our results are therefore consistent with vesicle-associated myosin VI existing as a processive dimer, capable of its known trafficking function.
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91
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Tang S, Liao JC, Dunn AR, Altman RB, Spudich JA, Schmidt JP. Predicting allosteric communication in myosin via a pathway of conserved residues. J Mol Biol 2007; 373:1361-73. [PMID: 17900617 PMCID: PMC2128046 DOI: 10.1016/j.jmb.2007.08.059] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2007] [Revised: 08/21/2007] [Accepted: 08/27/2007] [Indexed: 11/27/2022]
Abstract
We present a computational method that predicts a pathway of residues that mediate protein allosteric communication. The pathway is predicted using only a combination of distance constraints between contiguous residues and evolutionary data. We applied this analysis to find pathways of conserved residues connecting the myosin ATP binding site to the lever arm. These pathway residues may mediate the allosteric communication that couples ATP hydrolysis to the lever arm recovery stroke. Having examined pre-stroke conformations of Dictyostelium, scallop, and chicken myosin II as well as Dictyostelium myosin I, we observed a conserved pathway traversing switch II and the relay helix, which is consistent with the understood need for allosteric communication in this conformation. We also examined post-rigor and rigor conformations across several myosin species. Although initial residues of these paths are more heterogeneous, all but one of these paths traverse a consistent set of relay helix residues to reach the beginning of the lever arm. We discuss our results in the context of structural elements and reported mutational experiments, which substantiate the significance of the pre-stroke pathways. Our method provides a simple, computationally efficient means of predicting a set of residues that mediate allosteric communication. We provide a refined, downloadable application and source code (on https://simtk.org) to share this tool with the wider community (https://simtk.org/home/allopathfinder).
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92
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Chen Q, Lakshmikanth GS, Spudich JA, De Lozanne A. The localization of inner centromeric protein (INCENP) at the cleavage furrow is dependent on Kif12 and involves interactions of the N terminus of INCENP with the actin cytoskeleton. Mol Biol Cell 2007; 18:3366-74. [PMID: 17567958 PMCID: PMC1951774 DOI: 10.1091/mbc.e06-10-0895] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
The inner centromeric protein (INCENP) and other chromosomal passenger proteins are known to localize on the cleavage furrow and to play a role in cytokinesis. However, it is not known how INCENP localizes on the furrow or whether this localization is separable from that at the midbody. Here, we show that the association of Dictyostelium INCENP (DdINCENP) with the cortex of the cleavage furrow involves interactions with the actin cytoskeleton and depends on the presence of the kinesin-6-related protein Kif12. We found that Kif12 is found on the central spindle and the cleavage furrow during cytokinesis. Kif12 is not required for the redistribution of DdINCENP from centromeres to the central spindle. However, in the absence of Kif12, DdINCENP fails to localize on the cleavage furrow. Domain analysis indicates that the N terminus of DdINCENP is necessary and sufficient for furrow localization and that it binds directly to the actin cytoskeleton. Our data suggest that INCENP moves from the central spindle to the furrow of a dividing cell by a Kif12-dependent pathway. Once INCENP reaches the equatorial cortex, it associates with the actin cytoskeleton where it then concentrates toward the end of cytokinesis.
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93
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Geething NC, Spudich JA. Identification of a minimal myosin Va binding site within an intrinsically unstructured domain of melanophilin. J Biol Chem 2007; 282:21518-28. [PMID: 17513864 DOI: 10.1074/jbc.m701932200] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Myosin V is a molecular motor that transports a variety of cellular cargo, including organelles, vesicles, and messenger RNA. The proper peripheral distribution of melanosomes, a dense pigment-containing organelle, is dependent on actin and the activity of myosin Va. The recruitment of myosin Va to the melanosome and proper transport of the melanosome requires melanophilin, which directly binds to myosin Va and is tethered to the melanosome membrane via Rab27a. Here we use highly purified proteins to demonstrate that the globular tail domain of myosin Va binds directly to an intrinsically unstructured domain of melanophilin. The myosin Va binding domain of melanophilin lacks stable secondary structure, and (1)H NMR measurements indicate that the protein is unfolded. This domain is extremely sensitive to mild proteolysis and has a hydrodynamic radius that is consistent with a random coil-like polypeptide. We show that myosin Va binding does not induce the global folding of melanophilin. Truncations of melanophilin were utilized to define a short peptide sequence (26 residues) within melanophilin that is critical for myosin Va binding. We demonstrate that a peptide corresponding to these residues binds directly to the globular tail domain with the same affinity as melanophilin. We discuss the possible implications of protein intrinsic disorder in recruitment and maintenance of myosin Va on melanosome membranes.
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94
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Liao JC, Spudich JA, Parker D, Delp SL. Extending the absorbing boundary method to fit dwell-time distributions of molecular motors with complex kinetic pathways. Proc Natl Acad Sci U S A 2007; 104:3171-6. [PMID: 17360624 PMCID: PMC1805548 DOI: 10.1073/pnas.0611519104] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Dwell-time distributions, waiting-time distributions, and distributions of pause durations are widely reported for molecular motors based on single-molecule biophysical experiments. These distributions provide important information concerning the functional mechanisms of enzymes and their underlying kinetic and mechanical processes. We have extended the absorbing boundary method to simulate dwell-time distributions of complex kinetic schemes, which include cyclic, branching, and reverse transitions typically observed in molecular motors. This extended absorbing boundary method allows global fitting of dwell-time distributions for enzymes subject to different experimental conditions. We applied the extended absorbing boundary method to experimental dwell-time distributions of single-headed myosin V, and were able to use a single kinetic scheme to fit dwell-time distributions observed under different ligand concentrations and different directions of optical trap forces. The ability to use a single kinetic scheme to fit dwell-time distributions arising from a variety of experimental conditions is important for identifying a mechanochemical model of a molecular motor. This efficient method can be used to study dwell-time distributions for a broad class of molecular motors, including kinesin, RNA polymerase, helicase, F(1) ATPase, and to examine conformational dynamics of other enzymes such as ion channels.
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95
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Dunn AR, Spudich JA. Dynamics of the unbound head during myosin V processive translocation. Nat Struct Mol Biol 2007; 14:246-8. [PMID: 17293871 DOI: 10.1038/nsmb1206] [Citation(s) in RCA: 126] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/17/2006] [Accepted: 01/22/2007] [Indexed: 11/08/2022]
Abstract
Myosin V moves cargoes along actin filaments by walking hand over hand. Although numerous studies support the basic hand-over-hand model, little is known about the fleeting intermediate that occurs when the rear head detaches from the filament. Here we use submillisecond dark-field imaging of gold nanoparticle-labeled myosin V to directly observe the free head as it releases from the actin filament, diffuses forward and rebinds. We find that the unbound head rotates freely about the lever-arm junction, a trait that likely facilitates travel through crowded actin meshworks.
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96
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Dean SO, Spudich JA. Rho kinase's role in myosin recruitment to the equatorial cortex of mitotic Drosophila S2 cells is for myosin regulatory light chain phosphorylation. PLoS One 2006; 1:e131. [PMID: 17205135 PMCID: PMC1762308 DOI: 10.1371/journal.pone.0000131] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/16/2006] [Accepted: 11/28/2006] [Indexed: 01/30/2023] Open
Abstract
Background Myosin II recruitment to the equatorial cortex is one of the earliest events in establishment of the cytokinetic contractile ring. In Drosophila S2 cells, we previously showed that myosin II is recruited to the furrow independently of F-actin, and that Rho1 and Rok are essential for this recruitment [1]. Rok phosphorylates several cellular proteins, including the myosin regulatory light chain (RLC). Methodology/Principal Findings Here we express phosphorylation state mimic constructs of the RLC in S2 cells to examine the role of RLC phosphorylation involving Rok in the localization of myosin. Phosphorylation of the RLC is required for myosin localization to the equatorial cortex during mitosis, and the essential role of Rok in this localization and for cytokinesis is to maintain phosphorylation of the RLC. The ability to regulate the RLC phosphorylation state spatio-temporally is not essential for the myosin localization. Furthermore, the essential role of Citron in cytokinesis is not phosphorylation of the RLC. Conclusions/Significance We conclude that the Rho1 pathway leading to myosin localization to the future cytokinetic furrow is relatively straightforward, where only Rok is needed, and it is only needed to maintain phosphorylation of the myosin RLC.
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97
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Bryant Z, Altman D, Spudich JA. The power stroke of myosin VI and the basis of reverse directionality. Proc Natl Acad Sci U S A 2006; 104:772-7. [PMID: 17182734 PMCID: PMC1713167 DOI: 10.1073/pnas.0610144104] [Citation(s) in RCA: 75] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Myosin VI supports movement toward the (-) end of actin filaments, despite sharing extensive sequence and structural homology with (+)-end-directed myosins. A class-specific stretch of amino acids inserted between the converter domain and the lever arm was proposed to provide the structural basis of directionality reversal. Indeed, the unique insert mediates a 120 degrees redirection of the lever arm in a crystal structure of the presumed poststroke conformation of myosin VI [Ménétrey J, Bahloul A, Wells AL, Yengo CM, Morris CA, Sweeney HL, Houdusse A (2005) Nature 435:779-785]. However, this redirection alone is insufficient to account for the large (-)-end-directed stroke of a monomeric myosin VI construct. The underlying motion of the myosin VI converter domain must therefore differ substantially from the power stroke of (+)-end-directed myosins. To experimentally map out the motion of the converter domain and lever arm, we have generated a series of truncated myosin VI constructs and characterized the size and direction of the power stroke for each construct using dual-labeled gliding filament assays and optical trapping. Motors truncated near the end of the converter domain generate (+)-end-directed motion, whereas longer constructs move toward the (-) end. Our results directly demonstrate that the unique insert is required for directionality reversal, ruling out a large class of models in which the converter domain moves toward the (-) end. We suggest that the lever arm rotates approximately 180 degrees between pre- and poststroke conformations.
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98
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Abstract
Mechanical tension controls the function of a wide variety of eukaryotic motor proteins. Single-molecule analyses have revealed how some of these proteins sense and respond to tension. The single motor studies on dynein by Reck-Peterson et al (2006) described in this issue pave the way to understand molecular mechanisms used by this unique machine.
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99
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Churchman LS, Flyvbjerg H, Spudich JA. A non-Gaussian distribution quantifies distances measured with fluorescence localization techniques. Biophys J 2005; 90:668-71. [PMID: 16258038 PMCID: PMC1367071 DOI: 10.1529/biophysj.105.065599] [Citation(s) in RCA: 84] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
When single-molecule fluorescence localization techniques are pushed to their lower limits in attempts to measure ever-shorter distances, measurement errors become important to understand. Here we describe the non-Gaussian distribution of measured distances that is the key to proper interpretation of distance measurements. We test it on single-molecule high-resolution colocalization data for a known distance, 10 nm, and find that it gives the correct result, whereas interpretation of the same data with a Gaussian distribution gives a result that is systematically too large.
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100
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Dean SO, Rogers SL, Stuurman N, Vale RD, Spudich JA. Distinct pathways control recruitment and maintenance of myosin II at the cleavage furrow during cytokinesis. Proc Natl Acad Sci U S A 2005; 102:13473-8. [PMID: 16174742 PMCID: PMC1200093 DOI: 10.1073/pnas.0506810102] [Citation(s) in RCA: 95] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
The correct localization of myosin II to the equatorial cortex is crucial for proper cell division. Here, we examine a collection of genes that cause defects in cytokinesis and reveal with live cell imaging two distinct phases of myosin II localization. Three genes in the rho1 signaling pathway, pebble (a Rho guanidine nucleotide exchange factor), rho1, and rho kinase, are required for the initial recruitment of myosin II to the equatorial cortex. This initial localization mechanism does not require F-actin or the two components of the centralspindlin complex, the mitotic kinesin pavarotti/MKLP1 and racGAP50c/CYK-4. However, F-actin, the centralspindlin complex, formin (diaphanous), and profilin (chickadee) are required to stably maintain myosin II at the furrow. In the absence of these latter genes, myosin II delocalizes from the equatorial cortex and undergoes highly dynamic appearances and disappearances around the entire cell cortex, sometimes associated with abnormal contractions or blebbing. Our findings support a model in which a rho kinase-dependent event, possibly myosin II regulatory light chain phosphorylation, is required for the initial recruitment to the furrow, whereas the assembly of parallel, unbranched actin filaments, generated by formin-mediated actin nucleation, is required for maintaining myosin II exclusively at the equatorial cortex.
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