Magnesium regulates ADP dissociation from myosin V

Switch-2 determines Mg2+ADP-release kinetics and fine-tunes the duty ratio of Dictyostelium class-1 myosins

Though myosins share a structurally conserved motor domain, single amino acid variations of active site elements, including the P-loop, switch-1 and switch-2, which act as nucleotide sensors, can substantially determine the kinetic signature of a myosin, i.e., to either perform fast movement or enable long-range transport and tension generation. Switch-2 essentially contributes to the ATP hydrolysis reaction and determines product release. With few exceptions, class-1 myosin harbor a tyrosine in the switch-2 consensus sequence DIYGFE, at a position where class-2 myosins and a selection of myosins from other classes have a substitution. Here, we addressed the role of the tyrosine in switch-2 of class-1 myosins as potential determinant of the duty ratio. We generated constitutively active motor domain constructs of two class-1 myosins from the social amoeba Dictyostelium discoideum, namely, Myo1E, a high duty ratio myosin and Myo1B, a low duty ratio myosin. In Myo1E we introduced mutation Y388F and in Myo1B mutation F387Y. The detailed functional characterization by steady-state and transient kinetic experiments, combined with in vitro motility and landing assays revealed an almost reciprocal relationship of a number of critical kinetic parameters and equilibrium constants between wild-type and mutants that dictate the lifetime of the strongly actin-attached states of myosin. The Y-to-F mutation increased the duty ratio of Moy1B by almost one order of magnitude, while the introduction of the phenylalanine in switch-2 of Myo1E transformed the myosin into a low duty ratio motor. These data together with structural considerations propose a role of switch-2 in fine-tuning ADP release through a mechanism, where the class-specific tyrosine together with surrounding residues contributes to the coordination of Mg2+ and ADP. Our results highlight the importance of conserved switch-2 residues in class-1 myosins for efficient chemo-mechanical coupling, revealing that switch-2 is important to adjust the duty ratio of the amoeboid class-1 myosins for performing movement, transport or gating functions.

Changes in the urinary proteome of rats after short-term intake of magnesium L-threonate(MgT)

Introduction

Magnesium (Mg) is an important mineral in living organisms. Magnesium has multiple functions in the human body, wherein it plays an important therapeutic and preventive role in a variety of diseases.

Methods

Urine samples of rats before and after gavage of magnesium L-threonate (MgT) were collected, and the urinary proteome was identified using the LC-MS/MS technique and analyzed using various databases.

Results and discussion

The results illustrated that the urinary proteome of rats was significantly altered after short-term intake of magnesium supplements and that the differential proteins and the biological functions were related to magnesium. This study innovatively establishes a method to study nutrients from the perspective of urine proteomics. This work demonstrates that the urinary proteome is capable of reflecting the effects of nutrient intake on the organism in a more systematic and comprehensive manner and has the potential to provide clues for clinical nutrition research and practice.

Conformational changes linked to ADP release from human cardiac myosin bound to actin-tropomyosin

Following binding to the thin filament, β-cardiac myosin couples ATP-hydrolysis to conformational rearrangements in the myosin motor that drive myofilament sliding and cardiac ventricular contraction. However, key features of the cardiac-specific actin-myosin interaction remain uncertain, including the structural effect of ADP release from myosin, which is rate-limiting during force generation. In fact, ADP release slows under experimental load or in the intact heart due to the afterload, thereby adjusting cardiac muscle power output to meet physiological demands. To further elucidate the structural basis of this fundamental process, we used a combination of cryo-EM reconstruction methodologies to determine structures of the human cardiac actin-myosin-tropomyosin filament complex at better than 3.4 Å-resolution in the presence and in the absence of Mg2+·ADP. Focused refinements of the myosin motor head and its essential light chains in these reconstructions reveal that small changes in the nucleotide-binding site are coupled to significant rigid body movements of the myosin converter domain and a 16-degree lever arm swing. Our structures provide a mechanistic framework to understand the effect of ADP binding and release on human cardiac β-myosin, and offer insights into the force-sensing mechanism displayed by the cardiac myosin motor.

Unraveling a Force-Generating Allosteric Pathway of Actomyosin Communication Associated with ADP and Pi Release

The actomyosin system generates mechanical work with the execution of the power stroke, an ATP-driven, two-step rotational swing of the myosin-neck that occurs post ATP hydrolysis during the transition from weakly to strongly actin-bound myosin states concomitant with P_i release and prior to ADP dissociation. The activating role of actin on product release and force generation is well documented; however, the communication paths associated with weak-to-strong transitions are poorly characterized. With the aid of mutant analyses based on kinetic investigations and simulations, we identified the W-helix as an important hub coupling the structural changes of switch elements during ATP hydrolysis to temporally controlled interactions with actin that are passed to the central transducer and converter. Disturbing the W-helix/transducer pathway increased actin-activated ATP turnover and reduced motor performance as a consequence of prolonged duration of the strongly actin-attached states. Actin-triggered P_i release was accelerated, while ADP release considerably decelerated, both limiting maximum ATPase, thus transforming myosin-2 into a high-duty-ratio motor. This kinetic signature of the mutant allowed us to define the fractional occupancies of intermediate states during the ATPase cycle providing evidence that myosin populates a cleft-closure state of strong actin interaction during the weak-to-strong transition with bound hydrolysis products before accomplishing the power stroke.

Structural and Computational Insights into a Blebbistatin-Bound Myosin•ADP Complex with Characteristics of an ADP-Release Conformation along the Two-Step Myosin Power Stoke

The motor protein myosin drives a wide range of cellular and muscular functions by generating directed movement and force, fueled through adenosine triphosphate (ATP) hydrolysis. Release of the hydrolysis product adenosine diphosphate (ADP) is a fundamental and regulatory process during force production. However, details about the molecular mechanism accompanying ADP release are scarce due to the lack of representative structures. Here we solved a novel blebbistatin-bound myosin conformation with critical structural elements in positions between the myosin pre-power stroke and rigor states. ADP in this structure is repositioned towards the surface by the phosphate-sensing P-loop, and stabilized in a partially unbound conformation via a salt-bridge between Arg131 and Glu187. A 5 Å rotation separates the mechanical converter in this conformation from the rigor position. The crystallized myosin structure thus resembles a conformation towards the end of the two-step power stroke, associated with ADP release. Computationally reconstructing ADP release from myosin by means of molecular dynamics simulations further supported the existence of an equivalent conformation along the power stroke that shows the same major characteristics in the myosin motor domain as the resolved blebbistatin-bound myosin-II·ADP crystal structure, and identified a communication hub centered on Arg232 that mediates chemomechanical energy transduction.

Molecular movie of nucleotide binding to a motor protein

BACKGROUND

The SecA DEAD (Asp-Glu-Ala-Asp) motor protein uses binding and hydrolysis of adenosine triphosphate (ATP) to push secretory proteins across the plasma membrane of bacteria. The reaction coordinate of nucleotide exchange is unclear at the atomic level of detail.

METHODS

We performed multiple atomistic computations of the DEAD motor domain of SecA with different occupancies of the nucleotide and magnesium ion sites, for a total of ~1.7 μs simulation time. To characterize dynamics at the active site we analyzed hydrogen-bond networks.

RESULTS

ATP and ADP can bind spontaneously at the interface between the nucleotide binding domains, albeit at an intermediate binding site distinct from the native site. Binding of the nucleotide is facilitated by the presence of a magnesium ion close to the glutamic group of the conserved DEAD motif. In the absence of the magnesium ion, protein interactions of the ADP molecule are perturbed.

CONCLUSIONS

A protein hydrogen-bond network whose dynamics couples to the occupancy of the magnesium ion site helps guide the nucleotide along the nucleotide exchange path. In SecA, release of magnesium might be required to destabilize the ADP binding site prior to release of the nucleotide.

GENERAL SIGNIFICANCE

We identified dynamic hydrogen-bond networks that help control nucleotide exchange in SecA, and stabilize ADP at an intermediate site that could explain slow release. The reaction coordinate of the protein motor involves complex rearrangements of a hydrogen-bond network at the active site, with perturbation of the magnesium ion site likely occurring prior to the release of ADP.

Unconventional Myosins: How Regulation Meets Function

Unconventional myosins are multi-potent molecular motors that are assigned important roles in fundamental cellular processes. Depending on their mechano-enzymatic properties and structural features, myosins fulfil their roles by acting as cargo transporters along the actin cytoskeleton, molecular anchors or tension sensors. In order to perform such a wide range of roles and modes of action, myosins need to be under tight regulation in time and space. This is achieved at multiple levels through diverse regulatory mechanisms: the alternative splicing of various isoforms, the interaction with their binding partners, their phosphorylation, their applied load and the composition of their local environment, such as ions and lipids. This review summarizes our current knowledge of how unconventional myosins are regulated, how these regulatory mechanisms can adapt to the specific features of a myosin and how they can converge with each other in order to ensure the required tight control of their function.

Force Generation by Myosin Motors: A Structural Perspective

Generating force and movement is essential for the functions of cells and organisms. A variety of molecular motors that can move on tracks within cells have evolved to serve this role. How these motors interact with their tracks and how that, in turn, leads to the generation of force and movement is key to understanding the cellular roles that these motor-track systems serve. This review is focused on the best understood of these systems, which is the molecular motor myosin that moves on tracks of filamentous (F-) actin. The review highlights both the progress and the limits of our current understanding of how force generation can be controlled by F-actin-myosin interactions. What has emerged are insights they may serve as a framework for understanding the design principles of a number of types of molecular motors and their interactions with their tracks.

Dilated cardiomyopathy mutation in the converter domain of human cardiac myosin alters motor activity and response to omecamtiv mecarbil

We investigated a dilated cardiomyopathy (DCM) mutation (F764L) in human β-cardiac myosin by determining its motor properties in the presence and absence of the heart failure drug omecamtive mecarbil (OM). The mutation is located in the converter domain, a key region of communication between the catalytic motor and lever arm in myosins, and is nearby but not directly in the OM-binding site. We expressed and purified human β-cardiac myosin subfragment 1 (M2β-S1) containing the F764L mutation, and compared it to WT with in vitro motility as well as steady-state and transient kinetics measurements. In the absence of OM we demonstrate that the F764L mutation does not significantly change maximum actin-activated ATPase activity but slows actin sliding velocity (15%) and the actomyosin ADP release rate constant (25%). The transient kinetic analysis without OM demonstrates that F764L has a similar duty ratio as WT in unloaded conditions. OM is known to enhance force generation in cardiac muscle while it inhibits the myosin power stroke and enhances actin-attachment duration. We found that OM has a reduced impact on F764L ATPase and sliding velocity compared with WT. Specifically, the EC₅₀ for OM induced inhibition of in vitro motility was 3-fold weaker in F764L. Also, OM reduces maximum actin-activated ATPase 2-fold in F764L, compared with 4-fold with WT. Overall, our results suggest that F764L attenuates the impact of OM on actin-attachment duration and/or the power stroke. Our work highlights the importance of mutation-specific considerations when pursuing small molecule therapies for cardiomyopathies.

Interplay of actin, ADP and Mg2+ interactions with striated muscle myosin: Implications of their roles in ATPase

The effects of Mg²⁺ on the interaction between ADP, a product of the ATPase reaction, and striated muscle myosin-subfragment 1 (S1) were investigated with both functional and spectroscopic methods. Mg²⁺ inhibited striated muscle myosin ATPase in the presence of F-actin. Significant effects of Mg²⁺ were observed in both rate constants of NOE build-up and maximal intensities in WaterLOGSY NMR experiments as F-actin concentration increased. In the absence of F-actin, myosin S1 with Mg²⁺ bound to a fluorescent ADP analog about five-times tighter than without Mg²⁺. In the presence of F-actin, the affinity of myosin S1 toward the ADP analog significantly decreased both with and without Mg²⁺. The equilibrium titration of myosin-S1 into F-actin revealed that in the presence of ADP the apparent dissociation constant (K_d) without Mg²⁺ was more than five-fold smaller than with Mg²⁺. Further, we examined effects of F-actin, ADP and Mg²⁺ binding to myosin on the tertiary structure of myosin-S1 using near UV circular dichroism (CD) spectroscopy. Both in the presence and absence of ADP, there was a Mg²⁺-dependent difference in the near UV CD spectra of actomyosin. Our results show that Mg²⁺ affects myosin-ADP and actin-myosin interactions which may be reflected in myosin ATPase activity.

Nup159 Weakens Gle1 Binding to Dbp5 But Does Not Accelerate ADP Release

Dbp5, DDX19 in humans, is an essential DEAD-box protein involved in mRNA export, which has also been linked to other cellular processes, including rRNA export and translation. Dbp5 ATPase activity is regulated by several factors, including RNA, the nucleoporin proteins Nup159 and Gle1, and the endogenous small-molecule inositol hexakisphosphate (InsP₆). To better understand how these factors modulate Dbp5 activity and how this modulation relates to in vivo RNA metabolism, a detailed characterization of the Dbp5 mechanochemical cycle in the presence of those regulators individually or together is necessary. In this study, we test the hypothesis that Nup159 controls the ADP-bound state of Dbp5. In addition, the contributions of Mg²⁺ to the kinetics and thermodynamics of ADP binding to Dbp5 were assessed. Using a solution based in vitro approach, Mg²⁺ was found to slow ADP and ATP release from Dbp5 and increased the overall ADP and ATP affinities, as observed with other NTPases. Furthermore, Nup159 did not accelerate ADP release, while Gle1 actually slowed ADP release independent of Mg²⁺. These findings are not consistent with Nup159 acting as a nucleotide exchange factor to promote ADP release and Dbp5 ATPase cycling. Instead, in the presence of Nup159, the interaction between Gle1 and ADP-bound Dbp5 was found to be reduced by ~18-fold, suggesting that Nup159 alters the Dbp5-Gle1 interaction to aid Gle1 release from Dbp5.

Kinetic signatures of myosin-5B, the motor involved in microvillus inclusion disease

Myosin-5B is a ubiquitous molecular motor that transports cargo vesicles of the endomembrane system in intracellular recycling pathways. Myosin-5B malfunction causes the congenital enteropathy microvillus inclusion disease, underlining its importance in cellular homeostasis. Here we describe the interaction of myosin-5B with F-actin, nucleotides, and the pyrazolopyrimidine compound myoVin-1. We show that single-headed myosin-5B is an intermediate duty ratio motor with a kinetic ATPase cycle that is rate-limited by the release of phosphate. The presence of a second head generates strain and gating in the myosin-5B dimer that alters the kinetic signature by reducing the actin-activated ADP release rate to become rate-limiting. This kinetic transition into a high-duty ratio motor is a prerequisite for the proposed transport function of myosin-5B in cellular recycling pathways. Moreover, we show that the small molecule compound myoVin-1 inhibits the enzymatic and functional activity of myosin-5B in vitro Partial inhibition of the actin-activated steady-state ATPase activity and sliding velocity suggests that caution should be used when probing the effect of myoVin-1 on myosin-5-dependent transport processes in cells.

High-Dimensional Mutant and Modular Thermodynamic Cycles, Molecular Switching, and Free Energy Transduction

Understanding how distinct parts of proteins produce coordinated behavior has driven and continues to drive advances in protein science and enzymology. However, despite consensus about the conceptual basis for allostery, the idiosyncratic nature of allosteric mechanisms resists general approaches. Computational methods can identify conformational transition states from structural changes, revealing common switching mechanisms that impose multistate behavior. Thermodynamic cycles use factorial perturbations to measure coupling energies between side chains in molecular switches that mediate shear during domain motion. Such cycles have now been complemented by modular cycles that measure energetic coupling between separable domains. For one model system, energetic coupling between domains has been shown to be quantitatively equivalent to that between dynamic side chains. Linkages between domain motion, switching residues, and catalysis make nucleoside triphosphate hydrolysis conditional on domain movement, confirming an essential yet neglected aspect of free energy transduction and suggesting the potential generality of these studies.

Myosins: Domain Organisation, Motor Properties, Physiological Roles and Cellular Functions

Myosins are cytoskeletal motor proteins that use energy derived from ATP hydrolysis to generate force and movement along actin filaments. Humans express 38 myosin genes belonging to 12 classes that participate in a diverse range of crucial activities, including muscle contraction, intracellular trafficking, cell division, motility, actin cytoskeletal organisation and cell signalling. Myosin malfunction has been implicated a variety of disorders including deafness, hypertrophic cardiomyopathy, Usher syndrome, Griscelli syndrome and cancer. In this chapter, we will first discuss the key structural and kinetic features that are conserved across the myosin family. Thereafter, we summarise for each member in turn its unique functional and structural adaptations, cellular roles and associated pathologies. Finally, we address the broad therapeutic potential for pharmacological interventions that target myosin family members.

How Myosin Generates Force on Actin Filaments

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.

Kinetic Adaptations of Myosins for Their Diverse Cellular Functions

Members of the myosin superfamily are involved in all aspects of eukaryotic life. Their function ranges from the transport of organelles and cargos to the generation of membrane tension, and the contraction of muscle. The diversity of physiological functions is remarkable, given that all enzymatically active myosins follow a conserved mechanoenzymatic cycle in which the hydrolysis of ATP to ADP and inorganic phosphate is coupled to either actin-based transport or tethering of actin to defined cellular compartments. Kinetic capacities and limitations of a myosin are determined by the extent to which actin can accelerate the hydrolysis of ATP and the release of the hydrolysis products and are indispensably linked to its physiological tasks. This review focuses on kinetic competencies that - together with structural adaptations - result in myosins with unique mechanoenzymatic properties targeted to their diverse cellular functions.

Force-producing ADP state of myosin bound to actin

Molecular motors produce force when they interact with their cellular tracks. For myosin motors, the primary force-generating state has MgADP tightly bound, whereas myosin is strongly bound to actin. We have generated an 8-Å cryoEM reconstruction of this state for myosin V and used molecular dynamics flexed fitting for model building. We compare this state to the subsequent state on actin (Rigor). The ADP-bound structure reveals that the actin-binding cleft is closed, even though MgADP is tightly bound. This state is accomplished by a previously unseen conformation of the β-sheet underlying the nucleotide pocket. The transition from the force-generating ADP state to Rigor requires a 9.5° rotation of the myosin lever arm, coupled to a β-sheet rearrangement. Thus, the structure reveals the detailed rearrangements underlying myosin force generation as well as the basis of strain-dependent ADP release that is essential for processive myosins, such as myosin V.

Various Themes of Myosin Regulation

Members of the myosin superfamily are actin-based molecular motors that are indispensable for cellular homeostasis. The vast functional and structural diversity of myosins accounts for the variety and complexity of the underlying allosteric regulatory mechanisms that determine the activation or inhibition of myosin motor activity and enable precise timing and spatial aspects of myosin function at the cellular level. This review focuses on the molecular basis of posttranslational regulation of eukaryotic myosins from different classes across species by allosteric intrinsic and extrinsic effectors. First, we highlight the impact of heavy and light chain phosphorylation. Second, we outline intramolecular regulatory mechanisms such as autoinhibition and subsequent activation. Third, we discuss diverse extramolecular allosteric mechanisms ranging from actin-linked regulatory mechanisms to myosin:cargo interactions. At last, we briefly outline the allosteric regulation of myosins with synthetic compounds.

Biological Nanomotors with a Revolution, Linear, or Rotation Motion Mechanism

The ubiquitous biological nanomotors were classified into two categories in the past: linear and rotation motors. In 2013, a third type of biomotor, revolution without rotation (http://rnanano.osu.edu/movie.html), was discovered and found to be widespread among bacteria, eukaryotic viruses, and double-stranded DNA (dsDNA) bacteriophages. This review focuses on recent findings about various aspects of motors, including chirality, stoichiometry, channel size, entropy, conformational change, and energy usage rate, in a variety of well-studied motors, including FoF1 ATPase, helicases, viral dsDNA-packaging motors, bacterial chromosome translocases, myosin, kinesin, and dynein. In particular, dsDNA translocases are used to illustrate how these features relate to the motion mechanism and how nature elegantly evolved a revolution mechanism to avoid coiling and tangling during lengthy dsDNA genome transportation in cell division. Motor chirality and channel size are two factors that distinguish rotation motors from revolution motors. Rotation motors use right-handed channels to drive the right-handed dsDNA, similar to the way a nut drives the bolt with threads in same orientation; revolution motors use left-handed motor channels to revolve the right-handed dsDNA. Rotation motors use small channels (<2 nm in diameter) for the close contact of the channel wall with single-stranded DNA (ssDNA) or the 2-nm dsDNA bolt; revolution motors use larger channels (>3 nm) with room for the bolt to revolve. Binding and hydrolysis of ATP are linked to different conformational entropy changes in the motor that lead to altered affinity for the substrate and allow work to be done, for example, helicase unwinding of DNA or translocase directional movement of DNA.

Kinetic mechanism of Nicotiana tabacum myosin-11 defines a new type of a processive motor

The 175-kDa myosin-11 from Nicotiana tabacum (Nt(175kDa)myosin-11) is exceptional in its mechanical activity as it is the fastest known processive actin-based motor, moving 10 times faster than the structurally related class 5 myosins. Although this ability might be essential for long-range organelle transport within larger plant cells, the kinetic features underlying the fast processive movement of Nt(175kDa)myosin-11 still remain unexplored. To address this, we generated a single-headed motor domain construct and carried out a detailed kinetic analysis. The data demonstrate that Nt(175kDa)myosin-11 is a high duty ratio motor, which remains associated with actin most of its enzymatic cycle. However, different from other processive myosins that establish a high duty ratio on the basis of a rate-limiting ADP-release step, Nt(175kDa)myosin-11 achieves a high duty ratio by a prolonged duration of the ATP-induced isomerization of the actin-bound states and ADP release kinetics, both of which in terms of the corresponding time constants approach the total ATPase cycle time. Molecular modeling predicts that variations in the charge distribution of the actin binding interface might contribute to the thermodynamic fine-tuning of the kinetics of this myosin. Our study unravels a new type of a high duty ratio motor and provides important insights into the molecular mechanism of processive movement of higher plant myosins.

Magnesium modulates actin binding and ADP release in myosin motors

We examined the magnesium dependence of five class II myosins, including fast skeletal muscle myosin, smooth muscle myosin, β-cardiac myosin (CMIIB), Dictyostelium myosin II (DdMII), and nonmuscle myosin IIA, as well as myosin V. We found that the myosins examined are inhibited in a Mg(2+)-dependent manner (0.3-9.0 mm free Mg(2+)) in both ATPase and motility assays, under conditions in which the ionic strength was held constant. We found that the ADP release rate constant is reduced by Mg(2+) in myosin V, smooth muscle myosin, nonmuscle myosin IIA, CMIIB, and DdMII, although the ADP affinity is fairly insensitive to Mg(2+) in fast skeletal muscle myosin, CMIIB, and DdMII. Single tryptophan probes in the switch I (Trp-239) and switch II (Trp-501) region of DdMII demonstrate these conserved regions of the active site are sensitive to Mg(2+) coordination. Cardiac muscle fiber mechanic studies demonstrate cross-bridge attachment time is increased at higher Mg(2+) concentrations, demonstrating that the ADP release rate constant is slowed by Mg(2+) in the context of an activated muscle fiber. Direct measurements of phosphate release in myosin V demonstrate that Mg(2+) reduces actin affinity in the M·ADP·Pi state, although it does not change the rate of phosphate release. Therefore, the Mg(2+) inhibition of the actin-activated ATPase activity observed in class II myosins is likely the result of Mg(2+)-dependent alterations in actin binding. Overall, our results suggest that Mg(2+) reduces the ADP release rate constant and rate of attachment to actin in both high and low duty ratio myosins.

Molecular characterization and subcellular localization of Arabidopsis class VIII myosin, ATM1

Land plants possess myosin classes VIII and XI. Although some information is available on the molecular properties of class XI myosins, class VIII myosins are not characterized. Here, we report the first analysis of the enzymatic properties of class VIII myosin. The motor domain of Arabidopsis class VIII myosin, ATM1 (ATM1-MD), and the motor domain plus one IQ motif (ATM1-1IQ) were expressed in a baculovirus system and characterized. ATM1-MD and ATM1-1IQ had low actin-activated Mg(2+)-ATPase activity (Vmax = 4 s(-1)), although their affinities for actin were high (Kactin = 4 μM). The actin-sliding velocities of ATM1-MD and ATM1-1IQ were 0.02 and 0.089 μm/s, respectively, from which the value for full-length ATM1 is calculated to be ∼0.2 μm/s. The results of actin co-sedimentation assay showed that the duty ratio of ATM1 was ∼90%. ADP dissociation from the actin·ATM1 complex (acto-ATM1) was extremely slow, which accounts for the low actin-sliding velocity, low actin-activated ATPase activity, and high duty ratio. The rate of ADP dissociation from acto-ATM1 was markedly biphasic with fast and slow phase rates (5.1 and 0.41 s(-1), respectively). Physiological concentrations of free Mg(2+) modulated actin-sliding velocity and actin-activated ATPase activity by changing the rate of ADP dissociation from acto-ATM1. GFP-fused full-length ATM1 expressed in Arabidopsis was localized to plasmodesmata, plastids, newly formed cell walls, and actin filaments at the cell cortex. Our results suggest that ATM1 functions as a tension sensor/generator at the cell cortex and other structures in Arabidopsis.

Magnesium impacts myosin V motor activity by altering key conformational changes in the mechanochemical cycle

We investigated how magnesium (Mg) impacts key conformational changes during the ADP binding/release steps in myosin V and how these alterations impact the actomyosin mechanochemical cycle. The conformation of the nucleotide binding pocket was examined with our established FRET system in which myosin V labeled with FlAsH in the upper 50 kDa domain participates in energy transfer with mant labeled nucleotides. We examined the maximum actin-activated ATPase activity of MV FlAsH at a range of free Mg concentrations (0.1-9 mM) and found that the highest activity occurs at low Mg (0.1-0.3 mM), while there is a 50-60% reduction in activity at high Mg (3-9 mM). The motor activity examined with the in vitro motility assay followed a similar Mg-dependence, and the trend was similar with dimeric myosin V. Transient kinetic FRET studies of mantdADP binding/release from actomyosin V FlAsH demonstrate that the transition between the weak and strong actomyosin.ADP states is coupled to movement of the upper 50 kDa domain and is dependent on Mg with the strong state stabilized by Mg. We find that the kinetics of the upper 50 kDa conformational change monitored by FRET correlates well with the ATPase and motility results over a wide range of Mg concentrations. Our results suggest the conformation of the upper 50 kDa domain is highly dynamic in the Mg free actomyosin.ADP state, which is in agreement with ADP binding being entropy driven in the absence of Mg. Overall, our results demonstrate that Mg is a key factor in coupling the nucleotide- and actin-binding regions. In addition, Mg concentrations in the physiological range can alter the structural transition that limits ADP dissociation from actomyosin V, which explains the impact of Mg on actin-activated ATPase activity and in vitro motility.

Global fit analysis of myosin-5b motility reveals thermodynamics of Mg2+-sensitive acto-myosin-ADP states

Kinetic and thermodynamic studies of the mechanochemical cycle of myosin motors are essential for understanding the mechanism of energy conversion. Here, we report our investigation of temperature and free Mg²⁺-ion dependencies of sliding velocities of a high duty ratio class-5 myosin motor, myosin-5b from D. discoideum using in vitro motility assays. Previous studies have shown that the sliding velocity of class-5 myosins obeys modulation by free Mg²⁺-ions. Free Mg²⁺-ions affect ADP release kinetics and the dwell time of actin-attached states. The latter determines the maximal velocity of actin translocation in the sliding filament assay. We measured the temperature dependence of sliding velocity in the range from 5 to 55°C at two limiting free Mg²⁺-ion concentrations. Arrhenius plots demonstrated non-linear behavior. Based on this observation we propose a kinetic model, which explains both sensitivity towards free Mg²⁺-ions and non-linearity of the temperature dependence of sliding velocity. According to this model, velocity is represented as a simple analytical function of temperature and free Mg²⁺-ion concentrations. This function has been applied to global non-linear fit analysis of three data sets including temperature and magnesium (at 20°C) dependence of sliding velocity. As a result we obtain thermodynamic parameters (ΔH_Mg and ΔS_Mg) of a fast equilibrium between magnesium free (AM·D) and magnesium bound acto-myosin-ADP (AM· Mg²⁺D) states and the corresponding enthalpic barriers associated with ADP release (ΔH₁^‡ and ΔH₂^‡). The herein presented integrative approach of data analysis based on global fitting can be applied to the remaining steps of the acto-myosin ATPase cycle facilitating the determination of energetic parameters and thermodynamics of acto-myosin interactions.

Temperature dependent measurements reveal similarities between muscle and non-muscle myosin motility

We examined the temperature dependence of muscle and non-muscle myosin (heavy meromyosin, HMM) with in vitro motility and actin-activated ATPase assays. Our results indicate that myosin V (MV) has a temperature dependence that is similar in both ATPase and motility assays. We demonstrate that skeletal muscle myosin (SK), smooth muscle myosin (SM), and non-muscle myosin IIA (NM) have different temperature dependence in ATPase compared to in vitro motility assays. In the class II myosins we examined (SK, SM, and NM) the rate-limiting step in ATPase assays is thought to be attachment to actin or phosphate release, while for in vitro motility assays it is controversial. In MV the rate-limiting step for both in vitro motility and ATPase assays is known to be ADP release. Consequently, in MV the temperature dependence of the ADP release rate constant is similar to the temperature dependence of in vitro motility. Interestingly, the temperature dependence of the ADP release rate constant of SM and NM was shifted toward the in vitro motility temperature dependence. Our results suggest that the rate-limiting step in SK, SM, and NM may shift from attachment-limited in solution to detachment limited in the in vitro motility assay. Internal strain within the myosin molecule or by neighboring myosin motors may slow ADP release which becomes rate-limiting in the in vitro motility assay. Within this small subset of myosins examined, the in vitro sliding velocity correlates reasonably well with actin-activated ATPase activity, which was suggested by the original study by Barany (J Gen Physiol 50:197-218, 1967).

Switch II mutants reveal coupling between the nucleotide- and actin-binding regions in myosin V

Conserved active-site elements in myosins and other P-loop NTPases play critical roles in nucleotide binding and hydrolysis; however, the mechanisms of allosteric communication among these mechanoenzymes remain unresolved. In this work we introduced the E442A mutation, which abrogates a salt-bridge between switch I and switch II, and the G440A mutation, which abolishes a main-chain hydrogen bond associated with the interaction of switch II with the γ phosphate of ATP, into myosin V. We used fluorescence resonance energy transfer between mant-labeled nucleotides or IAEDANS-labeled actin and FlAsH-labeled myosin V to examine the conformation of the nucleotide- and actin-binding regions, respectively. We demonstrate that in the absence of actin, both the G440A and E442A mutants bind ATP with similar affinity and result in only minor alterations in the conformation of the nucleotide-binding pocket (NBP). In the presence of ADP and actin, both switch II mutants disrupt the formation of a closed NBP actomyosin.ADP state. The G440A mutant also prevents ATP-induced opening of the actin-binding cleft. Our results indicate that the switch II region is critical for stabilizing the closed NBP conformation in the presence of actin, and is essential for communication between the active site and actin-binding region.

A master switch couples Mg²⁺-assisted catalysis to domain motion in B. stearothermophilus tryptophanyl-tRNA Synthetase

We demonstrate how tryptophanyl-tRNA synthetase uses conformation-dependent Mg(2+) activation to couple catalysis of tryptophan activation to specific, functional domain movements. Rate acceleration by Mg(2+) requires ∼-6.0 kcal/mol in protein⋅Mg(2+) interaction energy, none of which arises from the active site. A highly cooperative interaction between Mg(2+) and four residues from a remote, conserved motif that mediates the shear of domain movement (1) destabilizes the pretransition state conformation, thereby (2) inducing the Mg(2+) to stabilize the transition state for k(cat) by ∼-5.0 kcal/mol. Cooperative, long-range conformational effects on the metal therefore convert an inactive Mg(2+) coordination into one that can stabilize the transition state if, and only if, domain motion occurs. Transient, conformation-dependent Mg(2+) activation, analogous to the escapement in mechanical clocks, explains vectorial coupling.

Functional characterization of the human myosin-7a motor domain

Myosin-7a participates in auditory and visual processes. Defects in MYO7A, the gene encoding the myosin-7a heavy chain, are causative for Usher syndrome 1B, the most frequent cause of deaf-blindness in humans. In the present study, we performed a detailed kinetic and functional characterization of the isolated human myosin-7a motor domain to elucidate the details of chemomechanical coupling and the regulation of motor function. A rate-limiting, slow ADP release step causes long lifetimes of strong actin-binding intermediates and results in a high duty ratio. Moreover, our results reveal a Mg²⁺-sensitive regulatory mechanism tuning the kinetic and mechanical properties of the myosin-7a motor domain. We obtained direct evidence that changes in the concentration of free Mg²⁺ ions affect the motor properties of human myosin-7a using an in vitro motility assay system. Our results suggest that in a cellular environment, compartment-specific fluctuations in free Mg²⁺ ions can mediate the conditional switching of myosin-7a between cargo moving and tension bearing modes.

EPR spectra and molecular dynamics agree that the nucleotide pocket of myosin V is closed and that it opens on binding actin

We have used EPR spectroscopy and computational modeling of nucleotide-analog spin probes to investigate conformational changes at the nucleotide site of myosin V. We find that, in the absence of actin, the mobility of a spin-labeled diphosphate analog [spin-labeled ADP (SLADP)] bound at the active site is strongly hindered, suggesting a closed nucleotide pocket. The mobility of the analog increases when the MV·SLADP complex (MV=myosin V) binds to actin, implying an opening of the active site in the A·MV·SLADP complex (A=actin). The probe mobilities are similar to those seen with myosin II, despite the fact that myosin V has dramatically altered kinetics. Molecular dynamics (MD) simulation was used to understand the EPR spectra in terms of the X-ray database. The X-ray structure of MV·ADP·BeFx shows a closed nucleotide site and has been proposed to be the detached state. The MV·ADP structure shows an open nucleotide site and has been proposed to be the A·MV·ADP state at the end of the working powerstroke. MD simulation of SLADP docked in the closed conformation gave a probe mobility comparable to that seen in the EPR spectrum of the MV·SLADP complex. The simulation of the open conformation gave a probe mobility that was 35-40° greater than that observed experimentally for the A·MV·SLADP state. Thus, EPR, X-ray diffraction, and computational analysis support the closed conformation as a myosin V state that is detached from actin. The MD results indicate that the MV·ADP crystal structure, which may correspond to the strained actin-bound post-powerstroke conformation resulting from head-head interaction in the dimeric processive motor, is superopened.

Coarse-grained modeling of conformational transitions underlying the processive stepping of myosin V dimer along filamentous actin

To explore the structural basis of processive stepping of myosin V along filamentous actin, we have performed comprehensive modeling of its key conformational states and transitions with an unprecedented residue level of details. We have built structural models for a myosin V monomer complexed with filamentous actin at four biochemical states [adenosine diphosphate (ATP)-, adenosine diphosphate (ADP)-phosphate-, ADP-bound or nucleotide-free]. Then we have modeled a myosin V dimer (consisting of lead and rear head) at various two-head-bound states with nearly straight lever arms rotated by intramolecular strain. Next, we have performed transition pathway modeling to determine the most favorable sequence of transitions (namely, phosphate release at the lead head followed by ADP release at the rear head, while ADP release at the lead head is inhibited), which underlie the kinetic coordination between the two heads. Finally, we have used transition pathway modeling to reveal the order of structural changes during three key biochemical transitions (phosphate release at the lead head, ADP release and ATP binding at the rear head), which shed lights on the strain-dependence of the allosterically coupled motions at various stages of myosin V's work cycle. Our modeling results are in agreement with and offer structural insights to many results of kinetic, single-molecule and structural studies of myosin V.

Switch I closure simultaneously promotes strong binding to actin and ADP in smooth muscle myosin

The motor protein myosin uses energy derived from ATP hydrolysis to produce force and motion. Important conserved components (P-loop, switch I, and switch II) help propagate small conformational changes at the active site into large scale conformational changes in distal regions of the protein. Structural and biochemical studies have indicated that switch I may be directly responsible for the reciprocal opening and closing of the actin and nucleotide-binding pockets during the ATPase cycle, thereby aiding in the coordination of these important substrate-binding sites. Smooth muscle myosin has displayed the ability to simultaneously bind tightly to both actin and ADP, although it is unclear how both substrate-binding clefts could be closed if they are rigidly coupled to switch I. Here we use single tryptophan mutants of smooth muscle myosin to determine how conformational changes in switch I are correlated with structural changes in the nucleotide and actin-binding clefts in the presence of actin and ADP. Our results suggest that a closed switch I conformation in the strongly bound actomyosin-ADP complex is responsible for maintaining tight nucleotide binding despite an open nucleotide-binding pocket. This unique state is likely to be crucial for prolonged tension maintenance in smooth muscle.

Detailed tuning of structure and intramolecular communication are dispensable for processive motion of myosin VI

Dimeric myosin VI moves processively hand-over-hand along actin filaments. We have characterized the mechanism of this processive motion by measuring the impact of structural and chemical perturbations on single-molecule processivity. Processivity is maintained despite major alterations in lever arm structure, including replacement of light chain binding regions and elimination of the medial tail. We present kinetic models that can explain the ATP concentration-dependent processivities of myosin VI constructs containing either native or artificial lever arms. We conclude that detailed tuning of structure and intramolecular communication are dispensable for processive motion, and further show theoretically that one proposed type of nucleotide gating can be detrimental rather than beneficial for myosin processivity.

Comparative kinetic and functional characterization of the motor domains of human nonmuscle myosin-2C isoforms

Nonmuscle myosins are widely distributed and play important roles in the maintenance of cell morphology and cytokinesis. In this study, we compare the detailed kinetic and functional characterization of naturally occurring transcript variants of the motor domain of human nonmuscle myosin heavy chain (NMHC)-2C. NMHC-2C is alternatively spliced both in loop-1 and loop-2. Isoform 2C0 contains no inserts in either of the loops and represents the shortest isoform. An 8-amino acid extension in the loop-1 region is present in isoforms 2C1 and 2C1C2. Isoform 2C1C2 additionally displays a 33-amino acid extension in the loop-2 region. Transient kinetic experiments indicate increased rate constants for F-actin binding by isoform 2C1C2 in the absence and presence of nucleotide, which can be attributed to the loop-2 extension. ADP binding shows only minor differences for the three transcript variants. In contrast, larger differences are observed for the rates of ADP release both in the absence and presence of F-actin. The largest differences are observed between isoforms 2C0 and 2C1C2. In the absence and presence of F-actin, isoform 2C1C2 displays a 5–7-fold increase in ADP affinity. Moreover, our results indicate that the ADP release kinetics of all three isoforms are modulated by changes in the concentration of free Mg²⁺ ions. The greatest responsiveness of the NMHC-2C isoforms is observed in the physiological range from 0.2 to 1.5 mm free Mg²⁺ ions, affecting their duty ratio, velocity, and tension-bearing properties.

Kinetics and thermodynamics of the rate-limiting conformational change in the actomyosin V mechanochemical cycle

We used FRET to examine the kinetics and thermodynamics of structural changes associated with ADP release in myosin V, which is thought to be a strain-sensitive step in many muscle and non-muscle myosins. We also explored essential dynamics using FIRST/FRODA starting with three different myosin V X-ray crystal structures to examine intrinsic flexibility and correlated motions. Our steady-state and time-resolved FRET analysis demonstrates a temperature-dependent reversible conformational change in the nucleotide-binding pocket (NBP). Our kinetic results demonstrate that the NBP goes from a closed to an open conformation prior to the release of ADP, while the actin-binding cleft remains closed. Interestingly, we find that the temperature dependence of the maximum actin-activated myosin V ATPase rate is similar to the pocket opening step, demonstrating that this is the rate-limiting structural transition in the ATPase cycle. Thermodynamic analysis demonstrates that the transition from the open to closed NBP conformation is unfavorable because of a decrease in entropy. The intrinsic flexibility analysis is consistent with conformational entropy playing a role in this transition as the MV.ADP structure is highly flexible compared to the MV.APO structure. Our experimental and modeling studies support the conclusion of a novel post-power-stroke actomyosin.ADP state in which the NBP and actin-binding cleft are closed. The novel state may be important for strain sensitivity as the transition from the closed to open NBP conformation may be altered by lever arm position.

Role of insert-1 of myosin VI in modulating nucleotide affinity

Myosin VI is unique in its directionality among myosin superfamily members and also displays a slow and strain-dependent rate of ATP binding that allows for gating between its heads. In this study we demonstrate that leucine 310 is positioned by a class VI-specific insert, insert-1, so as to account for the selective hindrance of ATP versus ADP binding. Mutation of leucine 310 to glycine removes all influence of insert-1 on ATP binding. Furthermore, by analyzing myosin VI structures with either leucine 310 substituted to a glycine or complete removal of insert-1, we conclude that nucleotides may initially bind to myosin by their purine rings before docking their phosphate moieties. Otherwise, insert-1 could not exert a differential influence on ATP versus ADP binding.

Nucleotide pocket thermodynamics measured by EPR reveal how energy partitioning relates myosin speed to efficiency

We have used spin-labeled ADP to investigate the dynamics of the nucleotide-binding pocket in a series of myosins, which have a range of velocities. Electron paramagnetic resonance spectroscopy reveals that the pocket is in equilibrium between open and closed conformations. In the absence of actin, the closed conformation is favored. When myosin binds actin, the open conformation becomes more favored, facilitating nucleotide release. We found that faster myosins favor a more closed pocket in the actomyosin•ADP state, with smaller values of ΔH(0) and ΔS(0), even though these myosins release ADP at a faster rate. A model involving a partitioning of free energy between work-generating steps prior to rate-limiting ADP release explains both the unexpected correlation between velocity and opening of the pocket and the observation that fast myosins are less efficient than slow myosins.

Robust mechanosensing and tension generation by myosin VI

Myosin VI is a molecular motor that is thought to function both as a transporter and as a cytoskeletal anchor in vivo. Here we use optical tweezers to examine force generation by single molecules of myosin VI under physiological nucleotide concentrations. We find that myosin VI is an efficient transporter at loads of up to ∼2 pN but acts as a cytoskeletal anchor at higher loads. Our data and the resulting model are consistent with an indirect coupling of global structural motions to nucleotide binding and release. The model provides a mechanism by which load may regulate the dual functions of myosin VI in vivo. Our results suggest that myosin VI kinetics are tuned such that the motor maintains a consistent level of mechanical tension within the cell, a property potentially shared by other mechanosensitive proteins.

Functional adaptation of the switch-2 nucleotide sensor enables rapid processive translocation by myosin-5

Active site loops that are conserved across superfamilies of myosins, kinesins, and G proteins play key roles in allosteric coupling of NTP hydrolysis to interaction with track filaments or effector proteins. In this study, we investigated how the class-specific natural variation in the switch-2 active site loop contributes to the motor function of the intracellular transporter myosin-5. We used single-molecule, rapid kinetic and spectroscopic experiments and semiempirical quantum chemical simulations to show that the class-specific switch-2 structure including a tyrosine (Y439) in myosin-5 enables rapid processive translocation along actin filaments by facilitating Mg(2+)-dependent ADP release. Using wild-type control and Y439 point mutant myosin-5 proteins, we demonstrate that the translocation speed precisely correlates with the kinetics of nucleotide exchange. Switch-2 variants can thus be used to fine-tune translocation speed while maintaining high processivity. The class-specific variation of switch-2 in various NTPase superfamilies indicates its general role in the kinetic tuning of Mg(2+)-dependent nucleotide exchange.

Structural and functional insights into the Myosin motor mechanism

The general structural features of the motor region of myosin superfamily members are now well established, as is a subset of the structural and kinetic transitions of the actin-myosin catalytic cycle. Not yet visualized are the structural rearrangements triggered by actin binding that are coupled to force generation and product release. In this review we describe the recent progress in understanding these missing components of the mechanism of chemomechanical transduction by myosin motors. These insights come from a combination of kinetic and single-molecule studies on multiple classes of myosins, with additional insights from contracting muscle fibers. These recent studies have explored the effects of intermediate and high loads on the kinetics of the actin-bound myosin state transitions. We also describe studies that delineate how some classes of myosin motors are adapted for processive movement on actin.

Magnesium coordination controls the molecular switch function of DNA mismatch repair protein MutS

The DNA mismatch repair protein MutS acts as a molecular switch. It toggles between ADP and ATP states and is regulated by mismatched DNA. This is analogous to G-protein switches and the regulation of their “on” and “off” states by guanine exchange factors. Although GDP release in monomeric GTPases is accelerated by guanine exchange factor-induced removal of magnesium from the catalytic site, we found that release of ADP from MutS is not influenced by the metal ion in this manner. Rather, ADP release is induced by the binding of mismatched DNA at the opposite end of the protein, a long-range allosteric response resembling the mechanism of activation of heterotrimeric GTPases. Magnesium influences switching in MutS by inducing faster and tighter ATP binding, allowing rapid downstream responses. MutS mutants with decreased affinity for the metal ion are impaired in fast switching and in vivo mismatch repair. Thus, the G-proteins and MutS conceptually employ the same efficient use of the high energy cofactor: slow hydrolysis in the absence of a signal and fast conversion to the active state when required.

The Kinesin-1 tail conformationally restricts the nucleotide pocket

We have used electron paramagnetic resonance and fluorescence spectroscopy to study the interaction between the kinesin-1 head and its regulatory tail domain. The interaction between the tails and the enzymatically active heads has been shown to inhibit intrinsic and microtubule-stimulated ADP release. Here, we demonstrate that the probe mobility of two different spin-labeled nucleotide analogs in the kinesin-1 nucleotide pocket is restricted upon binding of the tail domain to kinesin-1 heads. This conformational restriction is distinct from the microtubule-induced changes in the nucleotide pocket. Unlike myosin V, this tail-induced restriction occurs independent of nucleotide state. We find that the head-tail interaction that causes the restriction only weakly stabilizes Mg(2+) in the nucleotide pocket. The conformational restriction also occurs when a tail construct containing a K922A point mutation is used. This mutation eliminates the tail's ability to inhibit ADP release, indicating that the tail does not inhibit nucleotide ejection from the pocket by simple steric hindrance. Together, our data suggest that the observed head-tail interaction serves as a scaffold to position K922 to exert its inhibitory effect, possibly by interacting with the nucleotide alpha/beta-phosphates in a manner analogous to the arginine finger regulators of some G proteins.

Structural model for strain-dependent microtubule activation of Mg-ADP release from kinesin

Mg-ADP release is considered to be a crucial process for the regulation and motility of kinesin. To gain insight into the structural basis of this process, we solved the atomic structures of kinesin superfamily protein-1A (KIF1A) during and after Mg(2+) release. On the basis of new structural and mutagenesis data, we propose a model mechanism for microtubule activation of Mg-ADP release from KIF1A. In our model, a specific interaction between loop L7 of KIF1A and beta-tubulin reconfigures the KIF1A active site by shifting the relative positions of switches I and II. This leads to the sequential release of a group of water molecules that sits over the Mg(2+) in the active site, followed by Mg(2+) and finally the ADP. We further propose that this set of events is linked to a strain-dependent docking of the neck linker to the motor core, which produces a two-step power stroke.

Allosteric communication in myosin V: from small conformational changes to large directed movements

The rigor to post-rigor transition in myosin, a consequence of ATP binding, plays an essential role in the Lymn–Taylor functional cycle because it results in the dissociation of the actomyosin complex after the powerstroke. On the basis of the X-ray structures of myosin V, we have developed a new normal mode superposition model for the transition path between the two states. Rigid-body motions of the various subdomains and specific residues at the subdomain interfaces are key elements in the transition. The allosteric communication between the nucleotide binding site and the U50/L50 cleft is shown to result from local changes due to ATP binding, which induce large amplitude motions that are encoded in the structure of the protein. The triggering event is the change in the interaction of switch I and the P-loop, which is stabilized by ATP binding. The motion of switch I, which is a relatively rigid element of the U50 subdomain, leads directly to a partial opening of the U50/L50 cleft; the latter is expected to weaken the binding of myosin to actin. The calculated transition path demonstrates the nature of the subdomain coupling and offers an explanation for the mutual exclusion of ATP and actin binding. The mechanism of the uncoupling of the converter from the motor head, an essential part of the transition, is elucidated. The origin of the partial untwisting of the central β-sheet in the rigor to post-rigor transition is described.

Myosins are molecular motor proteins that interact with actin filaments to perform a wide range of cellular functions. They use the universal energy storage molecule adenosine triphosphate (ATP). The functional cycle involves myosin binding to actin, a “powerstroke” leading to directed movement, and myosin release in preparation for the next step. A fundamental question concerns the mechanism by which the local structural changes due to ATP binding, hydrolysis, and products release can generate the large myosin changes of conformation required for this cycle. Here, we focus on the rigor to post-rigor transition of myosin V, which results in the release of myosin from actin. Starting from the X-ray structures of the two states, we have used the optimal superposition of normal modes to determine the transition path. The path shows the allosteric mechanism by which ATP binding leads to the opening of the U50/L50 cleft, the essential step in the unbinding of myosin from actin. More generally, the new normal-mode superposition model can be useful for describing large-amplitude conformational transitions encoded in protein structures by evolution.

Direct observation of the mechanochemical coupling in myosin Va during processive movement

Myosin Va transports intracellular cargoes along actin filaments in cells. This processive, two-headed motor takes multiple 36-nm steps in which the two heads swing forward alternately towards the barbed end of actin driven by ATP hydrolysis. The ability of myosin Va to move processively is a function of its long lever arm, the high duty ratio of its kinetic cycle and the gating of the kinetics between the two heads such that ADP release from the lead head is greatly retarded. Mechanical studies at the multiple- and the single-molecule level suggest that there is tight coupling (that is, one ATP is hydrolysed per power stroke), but this has not been directly demonstrated. We therefore investigated the coordination between the ATPase mechanism of the two heads of myosin Va and directly visualized the binding and dissociation of single fluorescently labelled nucleotide molecules, while simultaneously observing the stepping motion of the fluorescently labelled myosin Va as it moved along an actin filament. Here we show that preferential ADP dissociation from the trail head of mouse myosin Va is followed by ATP binding and a synchronous 36-nm step. Even at low ATP concentrations, the myosin Va molecule retained at least one nucleotide (ADP in the lead head position) when moving. Thus, we directly demonstrate tight coupling between myosin Va movement and the binding and dissociation of nucleotide by simultaneously imaging with near nanometre precision.

Dictyostelium myosin-5b is a conditional processive motor

Dictyostelium myosin-5b is the gene product of myoJ and one of two closely related myosin-5 isoenzymes produced in Dictyostelium discoideum. Here we report a detailed investigation of the kinetic and functional properties of the protein. In standard assay buffer conditions, Dictyostelium myosin-5b displays high actin affinity in the presence of ADP, fast ATP hydrolysis, and a high steady-state ATPase activity in the presence of actin that is rate limited by ADP release. These properties are typical for a processive motor that can move over long distances along actin filaments without dissociating. Our results show that a physiological decrease in the concentration of free Mg(2+)-ions leads to an increased rate of ADP release and shortening of the fraction of time the motor spends in the strong actin binding states. Consistently, the ability of the motor to efficiently translocate actin filaments at very low surface densities decreases with decreasing concentrations of free Mg(2+)-ions. In addition, we provide evidence that the observed changes in Dd myosin-5b motor activity are of physiological relevance and propose a mechanism by which this molecular motor can switch between processive and non-processive movement.

Kinesin tail domains and Mg2+ directly inhibit release of ADP from head domains in the absence of microtubules

Kinesin-1 is a vesicle motor that can fold into a compact inhibited conformation that is produced by interaction of the heavy chain C-terminal tail region with the N-terminal motor domains (heads). Binding of the tail domains to the heads inhibits net microtubule-stimulated ATPase activity by blocking the ability of the heads to bind to microtubules with coupled acceleration of ADP release. We now show that folding of kinesin-1 also directly inhibits ADP release even in the absence of microtubules. With long heavy chain constructs such as DKH960 that exhibit a high degree of regulation by folding, the basal rate of ADP release is inhibited up to 30-fold compared to that of truncated DKH894 which has lost the inhibitory tail domains and does not fold. Inhibition of ADP release is also observed when separate head and tail domain constructs are mixed at low salt concentrations. This inhibition of ADP release by tail domains is formally analogous to the action of nucleotide dissociation inhibitors (NDI or GDI) for regulatory GTPases. In contrast to their inhibition of ADP release, tail domains accelerate the rate of ADP binding to nucleotide-free kinesin-1. Inhibition of release of ADP by tail domains is reversed by Unc-76 (FEZ1) which is a potential regulator of kinesin-1. Tail domains only weakly inhibit the initial slow release of Mg (2+) from the kinesin-MgADP complex but strongly inhibit the fast release of Mg (2+)-free ADP.

Engineering the Processive Run Length of Myosin V

The processive motor myosin V has a high affinity for actin in the weak binding states when compared with non-processive myosins. Here we test whether this feature is essential for myosin V to walk processively along an actin filament. The net charge of loop 2, a surface loop implicated in the initial weak binding between myosin and actin, was increased or decreased to correspondingly change the affinity of myosin V for actin in the weak binding state, without changing the velocity of movement. Processive run lengths of single molecules were determined by total internal reflection fluorescence microscopy. Reducing the net positive charge of loop 2 significantly decreased both the affinity of myosin V for actin and the processive run length. Conversely, the addition of positive charge to loop 2 increased actin affinity and processive run length. We hypothesize that a high affinity for actin allows the detached head of a stepping myosin V to find its next actin binding site more quickly, thus decreasing the probability of run termination.

Reversible movement of switch 1 loop of myosin determines actin interaction

The conserved switch 1 loop of P-loop NTPases is implicated as a central element that transmits information between the nucleotide-binding pocket and the binding site of the partner proteins. Recent structural studies have identified two states of switch 1 in G-proteins and myosin, but their role in the transduction mechanism has yet to be clarified. Single tryptophan residues were introduced into the switch 1 region of myosin II motor domain and studied by rapid reaction methods. We found that in the presence of MgADP, two states of switch 1 exist in dynamic equilibrium. Actin binding shifts the equilibrium towards one of the MgADP states, whereas ATP strongly favors the other. In the light of electron cryo-microscopic and X-ray crystallographic results, these findings lead to a specific structural model in which the equilibrium constant between the two states of switch 1 is coupled to the strength of the actin-myosin interaction. This has implications for the enzymatic mechanism of G-proteins and possibly P-loop NTPases in general.

The Slow Skeletal Muscle Isoform of Myosin Shows Kinetic Features Common to Smooth and Non-muscle Myosins

Fast and slow mammalian muscle myosins differ in the heavy chain sequences (MHC-2, MHC-1) and muscles expressing the two isoforms contract at markedly different velocities. One role of slow skeletal muscles is to maintain posture with low ATP turnover, and MHC-1 expressed in these muscles is identical to heavy chain of the beta-myosin of cardiac muscle. Few studies have addressed the biochemical kinetic properties of the slow MHC-1 isoform. We report here a detailed analysis of the MHC-1 isoform of the rabbit compared with MHC-2 and focus on the mechanism of ADP release. We show that MHC-1, like some non-muscle myosins, shows a biphasic dissociation of actin-myosin by ATP. Most of the actin-myosin dissociates at up to approximately 1000 s(-1), a very similar rate constant to MHC-2, but 10-15% of the complex must go through a slow isomerization (approximately 20 s(-1)) before ATP can dissociate it. Similar slow isomerizations were seen in the displacement of ADP from actin-myosin.ADP and provide evidence of three closely related actin-myosin.ADP complexes, a complex in rapid equilibrium with free ADP, a complex from which ADP is released at the rate required to define the maximum shortening velocity of slow muscle fibers (approximately 20 s(-1)), and a third complex that releases ADP too slowly (approximately 6 s(-1)) to be on the main ATPase pathway. The role of these actin-myosin.ADP complexes in the mechanochemistry of slow muscle contraction is discussed in relation to the load dependence of ADP release.