Distinguishing inchworm and hand-over-hand processive kinesin movement by neck rotation measurements

Kinesin motors as molecular machines

Molecular motor proteins, fueled by energy from ATP hydrolysis, move along actin filaments or microtubules, performing work in the cell. The kinesin microtubule motors transport vesicles or organelles, assemble bipolar spindles or depolymerize microtubules, functioning in basic cellular processes. The mechanism by which motor proteins convert energy from ATP hydrolysis into work is likely to differ in basic ways from man-made machines. Several mechanical elements of the kinesin motors have now been tentatively identified, permitting researchers to begin to decipher the mechanism of motor function. The force-producing conformational changes of the motor and the means by which they are amplified are probably different for the plus- and minus-end kinesin motors.

Myosin V motor proteins: marching stepwise towards a mechanism

Mammalian myosin V motors transport cargo processively along actin filaments. Recent biophysical and structural studies have led to a detailed understanding of the mechanism of myosin V, making it perhaps the best understood cytoskeletal motor. In addition to describing the mechanism, this review will illustrate how "dynamic" single molecule measurements can synergize with "static" protein structural studies to produce amazingly clear information on the workings of a nanometer-scale machine.

Kinesin walks hand-over-hand

Kinesin is a processive motor that takes 8.3-nm center-of-mass steps along microtubules for each adenosine triphosphate hydrolyzed. Whether kinesin moves by a "hand-over-hand" or an "inchworm" model has been controversial. We have labeled a single head of the kinesin dimer with a Cy3 fluorophore and localized the position of the dye to within 2 nm before and after a step. We observed that single kinesin heads take steps of 17.3 +/- 3.3 nm. A kinetic analysis of the dwell times between steps shows that the 17-nm steps alternate with 0-nm steps. These results strongly support a hand-over-hand mechanism, and not an inchworm mechanism. In addition, our results suggest that kinesin is bound by both heads to the microtubule while it waits for adenosine triphosphate in between steps.

Kinesin moves by an asymmetric hand-over-hand mechanism

Kinesin is a double-headed motor protein that moves along microtubules in 8-nanometer steps. Two broad classes of model have been invoked to explain kinesin movement: hand-over-hand and inchworm. In hand-over-hand models, the heads exchange leading and trailing roles with every step, whereas no such exchange is postulated for inchworm models, where one head always leads. By measuring the stepwise motion of individual enzymes, we find that some kinesin molecules exhibit a marked alternation in the dwell times between sequential steps, causing these motors to "limp" along the microtubule. Limping implies that kinesin molecules strictly alternate between two different conformations as they step, indicative of an asymmetric, hand-over-hand mechanism.

Alternate fast and slow stepping of a heterodimeric kinesin molecule

A conventional kinesin molecule travels continuously along a microtubule in discrete 8-nm steps. This processive movement is generally explained by models in which the two identical heads of a kinesin move in a 'hand-over-hand' manner. Here, we show that a single heterodimeric kinesin molecule (in which one of the two heads is mutated in a nucleotide-binding site) exhibits fast and slow (with the dwell time at least 10 times longer than that of the fast step) 8-nm steps alternately, presumably corresponding to the displacement by the wild-type and mutant heads, respectively. Our results provide the first direct evidence for models in which the roles of the two heads alternate every 8-nm step.

Processivity of the single-headed kinesin KIF1A through biased binding to tubulin

Conventional isoforms of the motor protein kinesin behave functionally not as 'single molecules' but as 'two molecules' paired. This dimeric structure poses a barrier to solving its mechanism. To overcome this problem, we used an unconventional kinesin KIF1A (refs 5, 6) as a model molecule. KIF1A moves processively as an independent monomer, and can also work synergistically as a functional dimer. Here we show, by measuring its movement with an optical trapping system, that a single ATP hydrolysis triggers a single stepping movement of a single KIF1A monomer. The step size is distributed stochastically around multiples of 8 nm with a gaussian-like envelope and a standard deviation of 15 nm. On average, the step is directional to the microtubule's plus-end against a load force of up to 0.15 pN. As the source for this directional movement, we show that KIF1A moves to the microtubule's plus-end by approximately 3 nm on average on binding to the microtubule, presumably by preferential binding to tubulin on the plus-end side. We propose a simple physical formulation to explain the movement of KIF1A.

Theoretical model for motility and processivity of two-headed molecular motors

The processive motion of two-headed molecular motors is studied theoretically by introducing a model that takes into account the coordinated motion of the constituent heads and the detachment process of heads from linear molecular tracks. The mean velocity, the mean run length, and the mean run time of the motor along the track are calculated numerically based on the Langevin equation. It turns out that the model, with appropriate choice of model parameters, can explain qualitatively the dependence of these quantities on the external load and adenosin triphosphate concentration observed experimentally for kinesin motors. Furthermore, we discuss how the motility and processivity of the motor are affected by various model parameters, which may be tested by experiments.

Myosin V walks hand-over-hand: single fluorophore imaging with 1.5-nm localization

Myosin V is a dimeric molecular motor that moves processively on actin, with the center of mass moving approximately 37 nanometers for each adenosine triphosphate hydrolyzed. We have labeled myosin V with a single fluorophore at different positions in the light-chain domain and measured the step size with a standard deviation of <1.5 nanometers, with 0.5-second temporal resolution, and observation times of minutes. The step size alternates between 37 + 2x nm and 37 - 2x, where x is the distance along the direction of motion between the dye and the midpoint between the two heads. These results strongly support a hand-over-hand model of motility, not an inchworm model.

Processive and nonprocessive models of kinesin movement

Conventional kinesin is the prototypic member of a family of diverse proteins that use the chemical energy of ATP hydrolysis to generate force and move along microtubules. These proteins, which are involved in a wide range of cellular functions, have been identified in protozoa, fungi, plants, and animals and possess a high degree of sequence conservation among species in their motor domains. The biochemical properties of kinesin and its homologues, in conjunction with the recently solved three-dimensional structures of several kinesin motors, have contributed to our understanding of the mechanism of kinesin movement along microtubules. We discuss several models for movement, including the hand-over-hand, inchworm, and biased diffusion models of processive movement, as well as models of nonprocessive movement.

Stepping and stretching. How kinesin uses internal strain to walk processively

The ability of kinesin to travel long distances on its microtubule track without dissociating has led to a variety of models to explain how this remarkable degree of processivity is maintained. All of these require that the two motor domains remain enzymatically "out of phase," a behavior that would ensure that, at any given time, one motor is strongly attached to the microtubule. The maintenance of this coordination over many mechanochemical cycles has never been explained, because key steps in the cycle could not be directly observed. We have addressed this issue by applying several novel spectroscopic approaches to monitor motor dissociation, phosphate release, and nucleotide binding during processive movement by a dimeric kinesin construct. Our data argue that the major effect of the internal strain generated when both motor domains of kinesin bind the microtubule is to block ATP from binding to the leading motor. This effect guarantees the two motor domains remain out of phase for many mechanochemical cycles and provides an efficient and adaptable mechanism for the maintenance of processive movement.

Molecular motors

Life implies movement. Most forms of movement in the living world are powered by tiny protein machines known as molecular motors. Among the best known are motors that use sophisticated intramolecular amplification mechanisms to take nanometre steps along protein tracks in the cytoplasm. These motors transport a wide variety of cargo, power cell locomotion, drive cell division and, when combined in large ensembles, allow organisms to move. Motor defects can lead to severe diseases or may even be lethal. Basic principles of motor design and mechanism have now been derived, and an understanding of their complex cellular roles is emerging.

Nucleotide-induced conformations in the neck region of dimeric kinesin

The neck region of kinesin constitutes a key component in the enzyme's walking mechanism. Here we applied cryoelectron microscopy and image reconstruction to investigate the location of the kinesin neck in dimeric and monomeric constructs complexed to microtubules. To this end we enhanced the visibility of this region by engineering an SH3 domain into the transition between neck linker and neck coiled coil. The resulting chimeric kinesin constructs remained functional as verified by physiology assays. In the presence of AMP-PNP the SH3 domains allowed us to identify the position of the neck in a well defined conformation and revealed its high flexibility in the absence of nucleotide. We show here the double-headed binding of dimeric kinesin along the same protofilament, which is characterized by the opposite directionality of neck linkers. In this configuration the neck coiled coil appears fully zipped. The position of the neck region in dimeric constructs is not affected by the presence of the tubulin C-termini as confirmed by subtilisin treatment of microtubules prior to motor decoration.

Three-dimensional structural dynamics of myosin V by single-molecule fluorescence polarization

The structural change that generates force and motion in actomyosin motility has been proposed to be tilting of the myosin light chain domain, which serves as a lever arm. Several experimental approaches have provided support for the lever arm hypothesis; however, the extent and timing of tilting motions are not well defined in the motor protein complex of functioning actomyosin. Here we report three-dimensional measurements of the structural dynamics of the light chain domain of brain myosin V using a single-molecule fluorescence polarization technique that determines the orientation of individual protein domains with 20-40-ms time resolution. Single fluorescent calmodulin light chains tilted back and forth between two well-defined angles as the myosin molecule processively translocated along actin. The results provide evidence for lever arm rotation of the calmodulin-binding domain in myosin V, and support a 'hand-over-hand' mechanism for the translocation of double-headed myosin V molecules along actin filaments. The technique is applicable to the study of real-time structural changes in other biological systems.

Cooperative behavior of molecular motors

Both experimental evidence and theoretical models for collective effects in the working mechanism of molecular motors are reviewed at three different levels, namely: (i) interaction between the two heads of double-headed motors, particularly in processive motors like kinesin, myosin V and myosin VI, (ii) cooperative regulation of muscle thin filaments by accessory proteins and the Ca2+ level, and (iii) collective dynamic effects stemming from the mechanical coupling of molecular motors within macroscopic structures such as muscle thick filaments or axonemes. We aim to bridge the gap between structural information at the molecular level and physiological data with accompanying specific models on the one hand, and general stochastic physical models for the action of molecular motors on the other hand. An underlying assumption is that while, ultimately, the function of molecular motors will be explainable by a quantitative description of specific intramolecular dynamics and intermolecular interactions, for some coarse grained larger scale dynamic features it will be sufficient and illuminating to construct physical models that are simplified to the bare essentials.

Thermodynamic properties of the kinesin neck-region docking to the catalytic core

Kinesin motors move on microtubules by a mechanism that involves a large, ATP-triggered conformational change in which a mechanical element called the neck linker docks onto the catalytic core, making contacts with the core throughout its length. Here, we investigate the thermodynamic properties of this conformational change using electron paramagnetic resonance (EPR) spectroscopy. We placed spin probes at several locations on the human kinesin neck linker and recorded EPR spectra in the presence of microtubules and either 5'-adenylylimidodiphosphate (AMPPNP) or ADP at temperatures of 4-30 degrees C. The free-energy change (DeltaG) associated with AMPPNP-induced docking of the neck linker onto the catalytic core is favorable but small, about 3 kJ/mol. In contrast, the favorable enthalpy change (DeltaH) and unfavorable entropy change (TDeltaS) are quite large, about 50 kJ/mol. A mutation in the neck linker, V331A/N332A, results in an unfavorable DeltaG for AMPPNP-induced zipping of the neck linker onto the core and causes motility defects. These results suggest that the kinesin neck linker folds onto the core from a more unstructured state, thereby paying a large entropic cost and gaining a large amount of enthalpy.

Single fungal kinesin motor molecules move processively along microtubules

Conventional kinesins are two-headed molecular motors that move as single molecules micrometer-long distances on microtubules by using energy derived from ATP hydrolysis. The presence of two heads is a prerequisite for this processive motility, but other interacting domains, like the neck and K-loop, influence the processivity and are implicated in allowing some single-headed kinesins to move processively. Neurospora kinesin (NKin) is a phylogenetically distant, dimeric kinesin from Neurospora crassa with high gliding speed and an unusual neck domain. We quantified the processivity of NKin and compared it to human kinesin, HKin, using gliding and fluorescence-based processivity assays. Our data show that NKin is a processive motor. Single NKin molecules translocated microtubules in gliding assays on average 2.14 micro m (N = 46). When we tracked single, fluorescently labeled NKin motors, they moved on average 1.75 micro m (N = 182) before detaching from the microtubule, whereas HKin motors moved shorter distances (0.83 micro m, N = 229) under identical conditions. NKin is therefore at least twice as processive as HKin. These studies, together with biochemical work, provide a basis for experiments to dissect the molecular mechanisms of processive movement.

Coordination of kinesin's two heads studied with mutant heterodimers

A conventional kinesin molecule has two identical catalytic domains (heads) and is thought to use them alternately to move processively, with 8-nm steps. To clarify how each head contributes to the observed steps, we have constructed heterodimeric kinesins that consist of two distinct heads. The heterodimers in which one of the heads is mutated in a microtubule-binding loop moved processively, even when the parent mutant homodimers bound too weakly to retain microtubules in microtubule-gliding assays. The velocities of the heterodimers were only slightly higher than those of the mutant homodimers, although mixtures of these weak-binding mutant homodimers and the WT dimers moved microtubules at a velocity similar to the WT. Thus, the mutant head affects the motility of the WT head only when they are in the same molecule. The maximum force a single heterodimer produced in optical trapping nanometry was intermediate between the WT and mutant homodimers, indicating that both heads contribute to the maximum force at the same time. These results demonstrate close collaboration of kinesin's two heads in producing force and motility.

Microscopic evidence for a minus-end-directed power stroke in the kinesin motor ncd

We used cryo-electron microscopy and image reconstruction to investigate the structure and microtubule-binding configurations of dimeric non-claret disjunctional (ncd) motor domains under various nucleotide conditions, and applied molecular docking using ncd's dimeric X-ray structure to generate a mechanistic model for force transduction. To visualize the alpha-helical coiled-coil neck better, we engineered an SH3 domain to the N-terminal end of our ncd construct (296-700). Ncd exhibits strikingly different nucleotide-dependent three-dimensional conformations and microtubule-binding patterns from those of conventional kinesin. In the absence of nucleotide, the neck adapts a configuration close to that found in the X-ray structure with stable interactions between the neck and motor core domain. Minus-end-directed movement is based mainly on two key events: (i) the stable neck-core interactions in ncd generate a binding geometry between motor and microtubule which places the motor ahead of its cargo in the minus-end direction; and (ii) after the uptake of ATP, the two heads rearrange their position relative to each other in a way that promotes a swing of the neck in the minus-end direction.

Measuring kinesin's first step

A variety of models have recently emerged to explain how the molecular motor kinesin is able to maintain processive movement for over 100 steps. Although these models differ in significant features, they all predict that kinesin's catalytic domains intermittently separate from each other as the motor takes 8-nm steps along the microtubule. Furthermore, at some point in this process, one molecule of ATP is hydrolyzed per step. However, exactly when hydrolysis and product release occur in relation to this forward step have not been established. Furthermore, the rate at which this separation occurs as well as the speed of motor stepping onto and release from the microtubule have not been measured. In the absence of this information, it is difficult to critically evaluate competing models of kinesin function. We have addressed this issue by developing spectroscopic probes whose fluorescence is sensitive to motor-motor separation or microtubule binding. The kinetics of these fluorescence changes allow us to directly measure how fast kinesin steps onto and releases from the microtubule and provide insight into how processive movement is maintained by this motor.