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The power stroke of myosin VI and the basis of reverse directionality.

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.

SUBMITTER: Bryant Z

PROVIDER: S-EPMC1713167 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Engineered myosin VI motors reveal minimal structural determinants of directionality and processivity.

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Myosin VI undergoes a 180 degrees power stroke implying an uncoupling of the front lever arm.

Project description:We simultaneously measure both the step size, via FIONA, and the 3-D orientation, via DOPI, of the light-chain domain of individual dimeric myosin VIs. This allows for the correlation of the change in orientation of the light chain domain to the stepping of the motor. Three different pairs of positions were tested using a rigid bifunctional rhodamine on the calmodulin of the IQ domain. The data for all three labeling positions support the model that the light chain domain undergoes a significant rotation of approximately 180 degrees . Contrary to an earlier study [Sun, Y. et al. (2007) Mol Cell 28, 954-964], our data does not support a model of multiple angles of the lever arm of the lead head, nor "wiggly" walking on actin. Instead, we propose that for the two heads of myosin VI to coordinate their processive movement, the lever arm of the lead head must be uncoupled from the converter until the rear head detaches. More specifically, intramolecular strain causes the myosin VI lever arm of the lead head to uncouple from the motor domain, allowing the motor domain to go through its product-release (phosphate and ADP) steps at an unstrained rate. The lever arm of the lead head rebinds to the motor and attains a rigor conformation when the rear head detaches. By coupling the orientation and position information with previously described kinetics, this allows us to explain how myosin VI coordinates its heads processively while maintaining the ability to move under load with a (semi-) rigid lever arm.

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The structure of the myosin VI motor reveals the mechanism of directionality reversal.

Project description:Here we solve a 2.4-A structure of a truncated version of the reverse-direction myosin motor, myosin VI, that contains the motor domain and binding sites for two calmodulin molecules. The structure reveals only minor differences in the motor domain from that in plus-end directed myosins, with the exception of two unique inserts. The first is near the nucleotide-binding pocket and alters the rates of nucleotide association and dissociation. The second unique insert forms an integral part of the myosin VI converter domain along with a calmodulin bound to a novel target motif within the insert. This serves to redirect the effective 'lever arm' of myosin VI, which includes a second calmodulin bound to an 'IQ motif', towards the pointed (minus) end of the actin filament. This repositioning largely accounts for the reverse directionality of this class of myosin motors. We propose a model incorporating a kinesin-like uncoupling/docking mechanism to provide a full explanation of the movements of myosin VI.

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A monomeric myosin VI with a large working stroke.

Project description:Myosin VI is involved in a wide variety of intracellular processes such as endocytosis, secretion and cell migration. Unlike almost all other myosins so far studied, it moves towards the minus end of actin filaments and is therefore likely to have unique cellular properties. However, its mechanism of force production and movement is not understood. Under our experimental conditions, both expressed full-length and native myosin VI are monomeric. Electron microscopy using negative staining revealed that the addition of ATP induces a large conformational change in the neck/tail region of the expressed molecule. Using an optical tweezers-based force transducer we found that expressed myosin VI is nonprocessive and produces a large working stroke of 18 nm. Since the neck region of myosin VI is short (it contains only a single IQ motif), it is difficult to reconcile the 18 nm working stroke with the classical 'lever arm mechanism', unless other structures in the molecule contribute to the effective lever. A possible model to explain the large working stroke of myosin VI is presented.

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Nucleotide-dependent shape changes in the reverse direction motor, myosin VI.

Project description:We have studied the shape of myosin VI, the actin minus-end directed motor, by negative stain and metal shadow electron microscopy. Single particle processing was used to make two-dimensional averages of the stain images, which greatly increases the clarity and allows detailed comparisons with crystal structures. A total of 169,964 particle images were obtained from two different constructs in six different states (four nucleotide states and with and without Ca(2+)). The shape of truncated apo myosin VI was very similar to the apo crystal structure, with the lever arm bent strongly backward and around the motor domain. In the full-length molecule, the C-terminal part of the tail has an additional bend taking it back across the motor domain, which may reflect a regulated state. Addition of ATP, ADP, or ATP-γS resulted in a large change, straightening the molecule from the bent shape and swinging the lever by ∼140°. Although these nucleotides would not be expected to produce the pre-powerstroke state, myosin VI in their presence was most similar to the truncated crystal structure with bound ADP-VO(4), which is thought to show the pre-powerstroke shape. The nucleotide data were therefore substantially different from expectation based on crystal structures. The full-length molecule was almost completely monomeric; only ∼1% were dimers, joined through the ends of the tail. Addition of calcium ions appeared to result in release of the second calmodulin light chain. In negatively stained molecules there was little indication of extended α-helical structure in the tail, but molecules viewed by metal shadowing had a tail ∼3× longer, 29 vs. 9 nm, part of which is likely to be a single α-helix.

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The unique insert at the end of the myosin VI motor is the sole determinant of directionality.

Project description:Myosin VI moves toward the pointed (minus) end of actin filaments, the reverse direction of other myosin classes. The myosin VI structure demonstrates that a unique insert at the end of the motor repositions its lever arm and is at least in part responsible for the reversal of directionality. However, it has been proposed that there must be additional modifications within the motor that contribute to its large step size and to the reversal of directionality. To ascertain the inherent directionality of the motor core, we attached the myosin V lever arm to myosin VI, with and without the unique insert. If the insert was maintained, the motor moved toward the minus end of actin filaments, but if removed, movement was redirected toward the plus end. Single-molecule studies revealed that further adaptations within the motor increase the magnitude and variability of the plus-end directed converter movements, and unexpectedly provide the source of the highly variable myosin VI step size. Thus, the unique insert is necessary and sufficient to reverse an inherently plus-end directed myosin.

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Electron microscopic recording of myosin head power stroke in hydrated myosin filaments.

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Direct observation of the myosin-Va power stroke and its reversal.

Project description:Complex forms of cellular motility, including cell division, organelle trafficking or signal amplification in the auditory system, require strong coordination of the myosin motors involved. The most basic mechanism of coordination is via direct mechanical interactions of individual motor heads leading to modification of their mechanochemical cycles. Here we used an optical trap-based assay to investigate the reversibility of the force-generating conformational change (power stroke) of single myosin-Va motor heads. By applying load to the head shortly after binding to actin, we found that, at a certain load, the power stroke could be reversed, and the head fluctuated between an actin-bound pre- and a post-power stroke conformation. This load-dependent mechanical instability might be critical to coordinate the heads of processive, dimeric myosin-Va. Nonlinear response to load leading to coordination or oscillations amongst motors might be relevant for many cellular functions.

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The post-rigor structure of myosin VI and implications for the recovery stroke.

Project description:Myosin VI has an unexpectedly large swing of its lever arm (powerstroke) that optimizes its unique reverse direction movement. The basis for this is an unprecedented rearrangement of the subdomain to which the lever arm is attached, referred to as the converter. It is unclear at what point(s) in the myosin VI ATPase cycle rearrangements in the converter occur, and how this would effect lever arm position. We solved the structure of myosin VI with an ATP analogue (ADP.BeF3) bound in its nucleotide-binding pocket. The structure reveals that no rearrangement in the converter occur upon ATP binding. Based on previously solved myosin structures, our structure suggests that no reversal of the powerstroke occurs during detachment of myosin VI from actin. The structure also reveals novel features of the myosin VI motor that may be important in maintaining the converter conformation during detachment from actin, and other features that may promote rapid rearrangements in the structure following actin detachment that enable hydrolysis of ATP.

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Irrelevance of the power stroke for the directionality, stopping force, and optimal efficiency of chemically driven molecular machines.

Project description:A simple model for a chemically driven molecular walker shows that the elastic energy stored by the molecule and released during the conformational change known as the power-stroke (i.e., the free-energy difference between the pre- and post-power-stroke states) is irrelevant for determining the directionality, stopping force, and efficiency of the motor. Further, the apportionment of the dependence on the externally applied force between the forward and reverse rate constants of the power-stroke (or indeed among all rate constants) is irrelevant for determining the directionality, stopping force, and efficiency of the motor. Arguments based on the principle of microscopic reversibility demonstrate that this result is general for all chemically driven molecular machines, and even more broadly that the relative energies of the states of the motor have no role in determining the directionality, stopping force, or optimal efficiency of the machine. Instead, the directionality, stopping force, and optimal efficiency are determined solely by the relative heights of the energy barriers between the states. Molecular recognition--the ability of a molecular machine to discriminate between substrate and product depending on the state of the machine--is far more important for determining the intrinsic directionality and thermodynamics of chemo-mechanical coupling than are the details of the internal mechanical conformational motions of the machine. In contrast to the conclusions for chemical driving, a power-stroke is very important for the directionality and efficiency of light-driven molecular machines and for molecular machines driven by external modulation of thermodynamic parameters.

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