Three conformational states of scallop myosin S1

A monomeric myosin VI with a large working stroke

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.

Structure of a kinesin microtubule depolymerization machine

With their ability to depolymerize microtubules (MTs), KinI kinesins are the rogue members of the kinesin family. Here we present the 1.6 A crystal structure of a KinI motor core from Plasmodium falciparum, which is sufficient for depolymerization in vitro. Unlike all published kinesin structures to date, nucleotide is not present, and there are noticeable differences in loop regions L6 and L10 (the plus-end tip), L2 and L8 and in switch II (L11 and helix4); otherwise, the pKinI structure is very similar to previous kinesin structures. KinI-conserved amino acids were mutated to alanine, and studied for their effects on depolymerization and ATP hydrolysis. Notably, mutation of three residues in L2 appears to primarily affect depolymerization, rather than general MT binding or ATP hydrolysis. The results of this study confirm the suspected importance of loop 2 for KinI function, and provide evidence that KinI is specialized to hydrolyze ATP after initiating depolymerization.

Myosin flexibility: structural domains and collective vibrations

The movement of the myosin motor along an actin filament involves a directed conformational change within the cross-bridge formed between the protein and the filament. Despite the structural data that has been obtained on this system, little is known of the mechanics of this conformational change. We have used existing crystallographic structures of three conformations of the myosin head, containing the motor domain and the lever arm, for structural comparisons and mechanical studies with a coarse-grained elastic network model. The results enable us to define structurally conserved domains within the protein and to better understand myosin flexibility. Notably they point to the role of the light chains in rigidifying the lever arm and to changes in flexibility as a consequence of nucleotide binding.

Dynamic reorganization of the motor domain of myosin subfragment 1 in different nucleotide states

Atomic models of the myosin motor domain with different bound nucleotides have revealed the open and closed conformations of the switch 2 element [Geeves, M.A. & Holmes, K.C. (1999) Annu. Rev. Biochem.68, 687-728]. The two conformations are in dynamic equilibrium, which is controlled by the bound nucleotide. In the present work we attempted to characterize the flexibility of the motor domain in the open and closed conformations in rabbit skeletal myosin subfragment 1. Three residues (Ser181, Lys553 and Cys707) were labelled with fluorophores and the probes identified three fluorescence resonance energy transfer pairs. The effect of ADP, ADP.BeFx, ADP.AlF4- and ADP.Vi on the conformation of the motor domain was shown by applying temperature-dependent fluorescence resonance energy transfer methods. The 50 kDa lower domain was found to maintain substantial rigidity in both the open and closed conformations to provide the structural basis of the interaction of myosin with actin. The flexibility of the 50 kDa upper domain was high in the open conformation and further increased in the closed conformation. The converter region of subfragment 1 became more rigid during the open-to-closed transition, the conformational change of which can provide the mechanical basis of the energy transduction from the nucleotide-binding pocket to the light-chain-binding domain.

Utility of genetic screening in hypertrophic cardiomyopathy: prevalence and significance of novel and double (homozygous and heterozygous) beta-myosin mutations

Genetic screening of the beta-myosin heavy chain gene (MYH7) was evaluated in 100 consecutive unrelated patients with hypertrophic cardiomyopathy (HCM) and 200 normal unrelated subjects. Seventeen beta-myosin mutations were identified in 19 patients. Notably, 13, or 76%, were novel. Mutations were detected in both alleles in two patients: homozygous for Lys207Gln in one, and heterozygous for Pro211 Leu and Arg663His in another. No mutation was detected in the controls. MYH7-associated HCM was associated with more marked left atrial enlargement and syncope than non-MYH7-related HCM. Our findings indicate that: (1) screening methods should allow identification of novel mutations; and (2) more than one sarcomeric mutation may be present in a patient more commonly than is appreciated. Further studies are necessary to ascertain the clinical consequences of the novel and compound gene abnormalities, and to determine whether correlating functional domain to phenotype provides more useful information about the clinical significance of the molecular defects.

Engineering Dictyostelium discoideum myosin II for the introduction of site-specific fluorescence probes

Dictyostelium discoideum is a useful host for the production of constructs for the analysis of structure-function relationships of myosin. Here we describe the use of myosin II constructs containing a single tryptophan residue, at different locations, for probing events at the nucleotide binding site, the relay loop and the communication path between them. GFP fusions have also been expressed at the N- and C-termini of the myosin motor to provide sensitive probes of the actomyosin dissociation reaction in microscope-based kinetic assays. We report on the fluorescence anisotropy of these constructs in the context of their use as resonance energy transfer probes.

A comparative study of motor-protein motions by using a simple elastic-network model

In this work, we report on a study of the structure-function relationships for three families of motor proteins, including kinesins, myosins, and F1-ATPases, by using a version of the simple elastic-network model of large-scale protein motions originally proposed by Tirion [Tirion, M. (1996) Phys. Rev. Lett. 77, 1905-1908]. We find a surprising dichotomy between kinesins and the other motor proteins (myosins and F1-ATPase). For the latter, there exist one or two dominant lowest-frequency modes (one for myosin, two for F1-ATPase) obtained from normal-mode analysis of the elastic-network model, which overlap remarkably well with the measured conformational changes derived from pairs of solved crystal structures in different states. Furthermore, we find that the computed global conformational changes induced by the measured deformation of the nucleotide-binding pocket also overlap well with the measured conformational changes, which is consistent with the "nucleotide-binding-induced power-stroke" scenario. In contrast, for kinesins, this simplicity breaks down. Multiple modes are needed to generate the measured conformational changes, and the computed displacements induced by deforming the nucleotide-binding pocket also overlap poorly with the measured conformational changes, and are insufficient to explain the large-scale motion of the relay helix and the linker region. This finding may suggest the presence of two different mechanisms for myosins and kinesins, despite their strong evolutionary ties and structural similarities.

Myosin cleft movement and its coupling to actomyosin dissociation

It has long been known that binding of actin and binding of nucleotides to myosin are antagonistic, an observation that led to the biochemical basis for the crossbridge cycle of muscle contraction. Thus ATP binding to actomyosin causes actin dissociation, whereas actin binding to the myosin accelerates ADP and phosphate release. Structural studies have indicated that communication between the actin- and nucleotide-binding sites involves the opening and closing of the cleft between the upper and lower 50K domains of the myosin head. Here we test the proposal that the cleft responds to actin and nucleotide binding in a reciprocal manner and show that cleft movement is coupled to actin binding and dissociation. We monitored cleft movement using pyrene excimer fluorescence from probes engineered across the cleft.

Rigor-force producing cross-bridges in skeletal muscle fibers activated by a substoichiometric amount of ATP

Isometric skinned muscle fibers were activated by the photogeneration of a substoichiometric amount of ATP and their cross-bridge configurations examined during the development of the rigor force by x-ray diffraction and electron microscopy. By the photogeneration of approximately 100 microM ATP, approximately 2/3 of the concentration of the myosin heads in a muscle fiber, muscle fibers originally in the rigor state showed a transient drop of the force and then produced a long-lasting rigor force (approximately 50% of the maximal active force), which gradually recovered to the original force level with a time constant of approximately 4 s. Associated with the photoactivation, muscle fibers revealed small but distinct changes in the equatorial x-ray diffraction that run ahead of the development of force. After reaching a plateau of force, long-lasting intensity changes in the x-ray diffraction pattern developed in parallel with the force decline. Two-dimensional x-ray diffraction patterns and electron micrographs of the sectioned muscle fibers taken during the period of 1-1.9 s after the photoactivation were basically similar to those from rigor preparations but also contained features characteristic of fully activated fibers. In photoactivated muscle fibers, some cross-bridges bound photogenerated ATP and underwent an ATP hydrolysis cycle whereas a significant population of the cross-bridges remained attached to the thin actin filaments with no available ATP to bind. Analysis of the results obtained indicates that, during the ATP hydrolysis reaction, the cross-bridges detached from actin filaments and reattached either to the same original actin monomers or to neighboring actin monomers. The latter cross-bridges contribute to produce the rigor force by interacting with the actin filaments, first producing the active force and then being locked in a noncycling state(s), transforming their configuration on the actin filaments to stably sustain the produced force as a passive rigor force.

Requirement of domain-domain interaction for conformational change and functional ATP hydrolysis in myosin

Coordination between the nucleotide-binding site and the converter domain of myosin is essential for its ATP-dependent motor activities. To unveil the communication pathway between these two sites, we investigated contact between side chains of Phe-482 in the relay helix and Gly-680 in the SH1-SH2 helix. F482A myosin, in which Phe-482 was changed to alanine with a smaller side chain, was not functional in vivo. In vitro, F482A myosin did not move actin filaments and the Mg2+-ATPase activity of F482A myosin was hardly activated by actin. Phosphate burst and tryptophan fluorescence analyses, as well as fluorescence resonance energy transfer measurements to estimate the movements of the lever arm domain, indicated that the transition from the open state to the closed state, which precedes ATP hydrolysis, is very slow. In contrast, F482A/G680F doubly mutated myosin was functional in vivo and in vitro. The fact that a larger side chain at the 680th position suppresses the defects of F482A myosin suggests that the defects are caused by insufficient contact between side chains of Ala-482 and Gly-680. Thus, the contact between these two side chains appears to play an important role in the coordinated conformational changes and subsequent ATP hydrolysis.

A mutant heterodimeric myosin with one inactive head generates maximal displacement

Each of the heads of the motor protein myosin II is capable of supporting motion. A previous report showed that double-headed myosin generates twice the displacement of single-headed myosin (Tyska, M.J., D.E. Dupuis, W.H. Guilford, J.B. Patlak, G.S. Waller, K.M. Trybus, D.M. Warshaw, and S. Lowey. 1999. Proc. Natl. Acad. Sci. USA. 96:4402-4407). To determine the role of the second head, we expressed a smooth muscle heterodimeric heavy meromyosin (HMM) with one wild-type head, and the other locked in a weak actin-binding state by introducing a point mutation in switch II (E470A). Homodimeric E470A HMM did not support in vitro motility, and only slowly hydrolyzed MgATP. Optical trap measurements revealed that the heterodimer generated unitary displacements of 10.4 nm, strikingly similar to wild-type HMM (10.2 nm) and approximately twice that of single-headed subfragment-1 (4.4 nm). These data show that a double-headed molecule can achieve a working stroke of approximately 10 nm with only one active head and an inactive weak-binding partner. We propose that the second head optimizes the orientation and/or stabilizes the structure of the motion-generating head, thereby resulting in maximum displacement.

The conformation of myosin head domains in rigor muscle determined by X-ray interference

In the absence of adenosine triphosphate, the head domains of myosin cross-bridges in muscle bind to actin filaments in a rigor conformation that is expected to mimic that following the working stroke during active contraction. We used x-ray interference between the two head arrays in opposite halves of each myosin filament to determine the rigor head conformation in single fibers from frog skeletal muscle. During isometric contraction (force T(0)), the interference effect splits the M3 x-ray reflection from the axial repeat of the heads into two peaks with relative intensity (higher angle/lower angle peak) 0.76. In demembranated fibers in rigor at low force (<0.05 T(0)), the relative intensity was 4.0, showing that the center of mass of the heads had moved 4.5 nm closer to the midpoint of the myosin filament. When rigor fibers were stretched, increasing the force to 0.55 T(0), the heads' center of mass moved back by 1.1-1.6 nm. These motions can be explained by tilting of the light chain domain of the head so that the mean angle between the Cys(707)-Lys(843) vector and the filament axis increases by approximately 36 degrees between isometric contraction and low-force rigor, and decreases by 7-10 degrees when the rigor fiber is stretched to 0.55 T(0).

Myosin head configuration in relaxed insect flight muscle: x-ray modeled resting cross-bridges in a pre-powerstroke state are poised for actin binding

Low-angle x-ray diffraction patterns from relaxed insect flight muscle recorded on the BioCAT beamline at the Argonne APS have been modeled to 6.5 nm resolution (R-factor 9.7%, 65 reflections) using the known myosin head atomic coordinates, a hinge between the motor (catalytic) domain and the light chain-binding (neck) region (lever arm), together with a simulated annealing procedure. The best head conformation angles around the hinge gave a head shape that was close to that typical of relaxed M*ADP*Pi heads, a head shape never before demonstrated in intact muscle. The best packing constrained the eight heads per crown within a compact crown shelf projecting at approximately 90 degrees to the filament axis. The two heads of each myosin molecule assume nonequivalent positions, one head projecting outward while the other curves round the thick filament surface to nose against the proximal neck of the projecting head of the neighboring molecule. The projecting heads immediately suggest a possible cross-bridge cycle. The relaxed projecting head, oriented almost as needed for actin attachment, will attach, then release Pi followed by ADP, as the lever arm with a purely axial change in tilt drives approximately 10 nm of actin filament sliding on the way to the nucleotide-free limit of its working stroke. The overall arrangement appears well designed to support precision cycling for the myogenic oscillatory mode of contraction with its enhanced stretch-activation response used in flight by insects equipped with asynchronous fibrillar flight muscles.

Refined model of the 10S conformation of smooth muscle myosin by cryo-electron microscopy 3D image reconstruction

The actin-activated ATPase activity of smooth muscle myosin and heavy meromyosin (smHMM) is regulated by phosphorylation of the regulatory light chain (RLC). Complete regulation requires two intact myosin heads because single-headed myosin subfragments are always active. 2D crystalline arrays of the 10S form of intact myosin, which has a dephosphorylated RLC, were produced on a positively charged lipid monolayer and imaged in 3D at 2.0 nm resolution by cryo-electron microscopy of frozen, hydrated specimens. An atomic model of smooth muscle myosin was constructed from the X-ray structures of the smooth muscle myosin motor domain and essential light chain and a homology model of the RLC was produced based on the skeletal muscle S1 structure. The initial model of the 10S myosin, based on the previous reconstruction of smHMM, was subjected to real space refinement to obtain a quantitative fit to the density. The smHMM was likewise refined and both refined models reveal the same asymmetric interaction between the upper 50 kDa domain of the "blocked" head and parts of the catalytic, converter domains and the essential light chain of the "free" head observed previously. This observation suggests that this interaction is not simply due to crystallographic packing but is enforced by elements of the myosin heads. The 10S reconstruction shows additional alpha-helical coiled-coil not seen in the earlier smHMM reconstruction, but the location of one segment of S2 is the same in both.

VirB11 ATPases are dynamic hexameric assemblies: new insights into bacterial type IV secretion

The coupling of ATP binding/hydrolysis to macromolecular secretion systems is crucial to the pathogenicity of Gram-negative bacteria. We reported previously the structure of the ADP-bound form of the hexameric traffic VirB11 ATPase of the Helicobacter pylori type IV secretion system (named HP0525), and proposed that it functions as a gating molecule at the inner membrane, cycling through closed and open forms regulated by ATP binding/hydrolysis. Here, we combine crystal structures with analytical ultracentrifugation experiments to show that VirB11 ATPases indeed function as dynamic hexameric assemblies. In the absence of nucleotide, the N-terminal domains exhibit a collection of rigid-body conformations. Nucleotide binding 'locks' the hexamer into a symmetric and compact structure. We propose that VirB11s use the mechanical leverage generated by such nucleotide-dependent conformational changes to facilitate the export of substrates or the assembly of the type IV secretion apparatus. Biochemical characterization of mutant forms of HP0525 coupled with electron microscopy and in vivo assays support such hypothesis, and establish the relevance of VirB11s ATPases as drug targets against pathogenic bacteria.

An actin-dependent conformational change in myosin

Conformational changes within myosin lead to its movement relative to an actin filament. Several crystal structures exist for myosin bound to various nucleotides, but none with bound actin. Therefore, the effect of actin on the structure of myosin is poorly understood. Here we show that the swing of smooth muscle myosin lever arm requires both ADP and actin. This is the first direct observation that a conformation of myosin is dependent on actin. Conformational changes within myosin were monitored using fluorescence resonance energy transfer techniques. A cysteine-reactive probe is site-specifically labeled on a 'cysteine-light' myosin variant, in which the native reactive cysteines were removed and a cysteine engineered at a desired position. Using this construct, we show that the actin-dependent ADP swing causes an 18 A change in distance between a probe on the 25/50 kDa loop on the catalytic domain and a probe on the regulatory light chain, corresponding to a 23 degrees swing of the light-chain domain.

Different degrees of lever arm rotation control myosin step size

Myosins are actin-based motors that are generally believed to move by amplifying small structural changes in the core motor domain via a lever arm rotation of the light chain binding domain. However, the lack of a quantitative agreement between observed step sizes and the length of the proposed lever arms from different myosins challenges this view. We analyzed the step size of rat myosin 1d (Myo1d) and surprisingly found that this myosin takes unexpectedly large steps in comparison to other myosins. Engineering the length of the light chain binding domain of rat Myo1d resulted in a linear increase of step size in relation to the putative lever arm length, indicative of a lever arm rotation of the light chain binding domain. The extrapolated pivoting point resided in the same region of the rat Myo1d head domain as in conventional myosins. Therefore, rat Myo1d achieves its larger working stroke by a large calculated approximately 90 degrees rotation of the light chain binding domain. These results demonstrate that differences in myosin step sizes are not only controlled by lever arm length, but also by substantial differences in the degree of lever arm rotation.

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.

Myosin isoforms show unique conformations in the actin-bound state

Crystallographic data for several myosin isoforms have provided evidence for at least two conformations in the absence of actin: a prehydrolysis state that is similar to the original nucleotide-free chicken skeletal subfragment-1 (S1) structure, and a transition-state structure that favors hydrolysis. These weak-binding states differ in the extent of closure of the cleft that divides the actin-binding region of the myosin and the position of the light chain binding domain or lever arm that is believed to be associated with force generation. Previously, we provided insights into the interaction of smooth-muscle S1 with actin by computer-based fitting of crystal structures into three-dimensional reconstructions obtained by electron cryomicroscopy. Here, we analyze the conformations of actin-bound chicken skeletal muscle S1. We conclude that both myosin isoforms in the nucleotide-free, actin-bound state can achieve a more tightly closed cleft, a more downward position of the lever arm, and more stable surface loops than those seen in the available crystal structures, indicating the existence of unique actin-bound conformations.

Chemical decoupling of ATPase activation and force production from the contractile cycle in myosin by steric hindrance of lever-arm movement

The myosin motor protein generates force in muscle by hydrolyzing Adenosine 5'-triphosphate (ATP) while interacting transiently with actin. Structural evidence suggests the myosin globular head (subfragment 1 or S1) is articulated with semi-rigid catalytic and lever-arm domains joined by a flexible converter domain. According to the prevailing hypothesis for energy transduction, ATP binding and hydrolysis in the catalytic domain drives the relative movement of the lever arm. Actin binding and reversal of the lever-arm movement (power stroke) applies force to actin. These domains interface at the reactive lysine, Lys84, where trinitrophenylation (TNP-Lys84-S1) was observed in this work to block actin activation of myosin ATPase and in vitro sliding of actin over myosin. TNP-Lys84-S1's properties and interactions with actin were examined to determine how trinitrophenylation causes these effects. Weak and strong actin binding, the rate of mantADP release from actomyosin, and actomyosin dissociation by ATP were equivalent in TNP-Lys84-S1 and native S1. Molecular dynamics calculations indicate that lever-arm movement inhibition during ATP hydrolysis and the power stroke is caused by steric clashes between TNP and the converter or lever-arm domains. Together these findings suggest that TNP uncouples actin activation of myosin ATPase and the power stroke from other steps in the contraction cycle by inhibiting the converter and lever-arm domain movements.

Interconversion of ATP binding and conformational free energies by tryptophanyl-tRNA synthetase: structures of ATP bound to open and closed, pre-transition-state conformations

Binding ATP to tryptophanyl-tRNA synthetase (TrpRS) in a catalytically competent configuration for amino acid activation destabilizes the enzyme structure prior to forming the transition state. This conclusion follows from monitoring the titration of TrpRS with ATP by small angle solution X-ray scattering, enzyme activity, and crystal structures. ATP induces a significantly smaller radius of gyration at pH=7 with a transition midpoint at approximately 8mM. A non-reciprocal dependence of Trp and ATP dissociation constants on concentrations of the second substrate show that Trp binding enhances affinity for ATP, while the affinity for Trp falls with the square of the [ATP] over the same concentration range ( approximately 5mM) that induces the more compact conformation. Two distinct TrpRS:ATP structures have been solved, a high-affinity complex grown with 1mM ATP and a low-affinity complex grown at 10mM ATP. The former is isomorphous with unliganded TrpRS and the Trp complex from monoclinic crystals. Reacting groups of the two individually-bound substrates are separated by 6.7A. Although it lacks tryptophan, the low-affinity complex has a closed conformation similar to that observed in the presence of both ATP and Trp analogs such as indolmycin, and resembles a complex previously postulated to form in the closely-related TyrRS upon induced-fit active-site assembly, just prior to catalysis. Titration of TrpRS with ATP therefore successively produces structurally distinct high- and low-affinity ATP-bound states. The higher quality X-ray data for the closed ATP complex (2.2A) provide new structural details likely related to catalysis, including an extension of the KMSKS loop that engages the second lysine and serine residues, K195 and S196, with the alpha and gamma-phosphates; interactions of the K111 side-chain with the gamma-phosphate; and a water molecule bridging the consensus sequence residue T15 to the beta-phosphate. Induced-fit therefore strengthens active-site interactions with ATP, substantially intensifying the interaction of the KMSKS loop with the leaving PP(i) group. Formation of this conformation in the absence of a Trp analog implies that ATP is a key allosteric effector for TrpRS. The paradoxical requirement for high [ATP] implies that Gibbs binding free energy is stored in an unfavorable protein conformation and can then be recovered for useful purposes, including catalysis in the case of TrpRS.

Modelling muscle motor conformations using low-angle X-ray diffraction

New results on myosin head organization using analysis of low-angle X-ray diffraction patterns from relaxed insect flight muscle (IFM) from a giant waterbug, building on previous studies of myosin filaments in bony fish skeletal muscle (BFM), show that the information content of such low-angle diffraction patterns is very high despite the 'crystallographically low' resolution limit (65 A) of the spacings of the Bragg diffraction peaks being used. This high information content and high structural sensitivity arises because: (i) the atomic structures of the domains of the myosin head are known from protein crystallography; and (ii) myosin head action appears to consist mainly of pivoting between domains which themselves stay rather constant in structure, thus (iii) the intensity distribution among diffraction peaks in even the low resolution diffraction pattern is highly determined by the high-resolution distribution of atomically modelled domain mass. A single model was selected among 5000+ computer-generated variations as giving the best fit for the 65 reflections recorded within the selected resolution limit of 65 A. Clear evidence for a change in shape of the insect flight muscle myosin motor between the resting (probably like the pre-powerstroke) state and the rigor state (considered to mimic the end-of-powerstroke conformation) has been obtained. This illustrates the power of the low-angle X-ray diffraction method. The implications of these new results about myosin motor action during muscle contraction are discussed.

The prepower stroke conformation of myosin V

We have used electron microscopy and single-particle image processing to study head conformation in myosin V molecules. We find that in the presence of ATP, many heads have a sharply angled conformation that is rare in its absence. The sharply angled conformation is similar to a myosin II atomic structure proposed to mimic the prepower stroke state. The leading head in molecules attached to actin by both heads has a similar conformation, but is also sharply angled in a second plane by tethering through the trail head. The lead head lever joins the motor domain approximately 5 nm axially from where it joins the trail motor. These positions locate the converter subdomain and show the lead motor is in the prepower stroke conformation. Tethering by the trail head places the lead head motor domain at the correct axial position along the actin for binding, but at the wrong orientation. Attachment is achieved either by bending the lead head lever throughout its length or at the pliant point. The microscopy shows that most of the walking stride is produced by changes in lever angle brought about by converter movement, but is augmented by distortion produced by thermal energy.

Mutations in human nonmuscle myosin IIA found in patients with May-Hegglin anomaly and Fechtner syndrome result in impaired enzymatic function

A family of autosomal-dominant diseases including May-Hegglin anomaly, Fechtner syndrome, Sebastian syndrome, Alport syndrome, and Epstein syndrome are commonly characterized by giant platelets and thrombocytopenia. In addition, there may be leukocyte inclusions, deafness, cataracts, and nephritis, depending on the syndrome. Mutations in the human nonmuscle myosin IIA heavy chain gene (MYH9) have been linked to these diseases. Two of the recently described mutations, N93K and R702C, are conserved in smooth and nonmuscle myosins from vertebrates and lie in the head domain of myosin. Interestingly, the two mutations lie within close proximity in the three-dimensional structure of myosin. These two mutations were engineered into a heavy meromyosin-like recombinant fragment of nonmuscle myosin IIA, which was expressed in baculovirus along with the appropriate light chains. The R702C mutant displays 25% of the maximal MgATPase activity of wild type heavy meromyosin and moves actin filaments at half the wild type rate. The effects of the N93K mutation are more dramatic. This heavy meromyosin has only 4% of the maximal MgATPase activity of wild type and does not translocate actin filaments in an in vitro motility assay. Biochemical characterization of the mutant is consistent with this mutant being unable to fully adopt the "on" conformation.

Conformational dynamics of the SH1-SH2 helix in the transition states of myosin subfragment-1

The alpha-helix containing the thiols, SH1 (Cys-707) and SH2 (Cys-697), has been proposed to be one of the structural elements responsible for the transduction of conformational changes in the myosin head (subfragment-1 (S1)). Previous studies, using a method that isolated and measured the rate of the SH1-SH2 cross-linking step, showed that this helix undergoes ligand-induced conformational changes. However, because of long incubation times required for the formation of the transition state complexes (S1.ADP.BeF(x), S1.ADP.AlF(4)-, and S1.ADP.V(i)), this method could not be used to determine the cross-linking rate constants for such states. In this study, kinetic data from the SH1-SH2 cross-linking reaction were analyzed by computational methods to extract rate constants for the two-step mechanism. For S1.ADP.BeF(x), the results obtained were similar to those for S1.ATPgammaS. For reactions involving S1.ADP.AlF(4)- and S1.ADP.V(i), the first step (SH1 modification) is rate limiting; consequently, only lower limits could be established for the rate constants of the cross-linking step. Nevertheless, these results show that the cross-linking rate constants in the transition state complexes are increased at least 20-fold for all the reagents, including the shortest one, compared with nucleotide-free S1. Thus, the SH1-SH2 helix appears to be destabilized in the post-hydrolysis state.

Crystallographic findings on the internally uncoupled and near-rigor states of myosin: further insights into the mechanics of the motor

Here we report a 2.3-A crystal structure of scallop myosin S1 complexed with ADP.BeF(x), as well as three additional structures (at 2.8-3.8 A resolution) for this S1 complexed with ATP analogs, some of which are cross-linked by para-phenyl dimaleimide, a short intramolecular cross-linker. In all cases, the complexes are characterized by an unwound SH1 helix first seen in an unusual 2.5-A scallop myosin-MgADP structure and described as corresponding to a previously unrecognized actin-detached internally uncoupled state. The unwinding of the SH1 helix effectively uncouples the converter/lever arm module from the motor and allows cross-linking by para-phenyl dimaleimide, which has been shown to occur only in weak actin-binding states of the molecule. Mutations near the metastable SH1 helix that disable the motor can be accounted for by viewing this structural element as a clutch controlling the transmission of torque to the lever arm. We have also determined a 3.2-A nucleotide-free structure of scallop myosin S1, which suggests that in the near-rigor state there are two conformations in the switch I loop, depending on whether nucleotide is present. Analysis of the subdomain motions in the weak actin-binding states revealed by x-ray crystallography, together with recent electron microscopic results, clarify the mechanical roles of the parts of the motor in the course of the contractile cycle and suggest how strong binding to actin triggers both the power stroke and product release.

A kinetic model of the co-operative binding of calcium and ADP to scallop (Argopecten irradians) heavy meromyosin

Analysis of the kinetics of ATP and ADP binding to scallop (Argopecten irradians) heavy meromyosin (HMM) showed that the only calcium-dependent process is the rate of ADP release. At physiological ionic strength calcium accelerated ADP release about 20-fold. Notably in the absence of calcium only one ADP bound HMM, with an affinity of 0.5-1 microM. The second nucleotide site remained unoccupied at up to 50 microM ADP yet could bind ATP rapidly. The calcium dependence of ADP-release rates showed that calcium binds co-operatively to scallop HMM with an affinity of 0.78 microM and a Hill coefficient of 1.9. Detailed interpretation of the data suggests that HMM exists in equilibrium between the on and off states and that calcium and ADP modulate the equilibrium between the two states. The on state is favoured in the presence of calcium and in the absence of both calcium and nucleotide. The off state is favoured by ADP (or ADP * P(i)) in the absence of calcium. A detailed co-operative model of the interaction of ADP and calcium with HMM is presented.

Myosin II from rabbit skeletal muscle and Dictyostelium discoideum and its interaction with F-actin studied by (1)H NMR spectroscopy

Mg-F-actin occurs in two conformational states, I and M, where the N-terminal amino acids are either immobile or highly mobile. In the rigor or ADP complex of rabbit myosin S1 with Mg-F-actin the N-terminal acetyl group of actin stays in its highly mobile state. The same is true for the complexes with the myosin motor domain from Dictyostelium discoideum. This excludes a direct strong interaction of the N-terminal amino acids with myosin in the rigor state as suggested. An interaction of the N-terminus of F-actin with myosin is also not promoted by occupying its low-affinity binding site(s) with divalent ions. The N-terminal high-mobility region may be part of a structural system which has evolved for releasing inadequate stress applied to the actin filaments.

Human myosin XVBP is a transcribed pseudogene

A novel human myosin gene located at 17q25 was identified through evaluation of genomic DNA sequence and designated myosin XVBP since it resembled human myosin XVA. In humans, myosin XVBP along with an adjacent gene, Lethal Giant Larvae 2 (LLGL2) appears to have arisen from a genomic duplication of a chromosomal interval that included LLGL and an ancestral myosin XV. Inspection of human myosin XVBP predicted amino acid sequence from genomic DNA revealed that 36 of the 131 conserved amino acid residues of the motor domain are substituted or deleted, including sequence changes within the regions involved in the binding of ATP and actin. Twelve myosin XVBP overlapping cDNAs from kidney and stomach mRNA samples were cloned and sequenced. Analyses of these myosin XVBP cDNAs revealed numerous additional disablements including translational reading frame shifts resulting in stop codons. From these data we conclude that myosin XVBP is a transcribed, unprocessed pseudogene.

Orientation changes of the myosin light chain domain during filament sliding in active and rigor muscle

Structural changes in myosin power many types of cell motility including muscle contraction. Tilting of the myosin light chain domain (LCD) seems to be the final step in transducing the energy of ATP hydrolysis, amplifying small structural changes near the ATP binding site into nanometer-scale motions of the filaments. Here we used polarized fluorescence measurements from bifunctional rhodamine probes attached at known orientations in the LCD to describe the distribution of orientations of the LCD in active contraction and rigor. We applied rapid length steps to perturb the orientations of the population of myosin heads that are attached to actin, and thereby characterized the motions of these force-bearing myosin heads. During active contraction, this population is a small fraction of the total. When the filaments slide in the shortening direction in active contraction, the long axis of LCD tilts towards its nucleotide-free orientation with no significant twisting around this axis. In contrast, filament sliding in rigor produces coordinated tilting and twisting motions.

The elementary force generation process probed by temperature and length perturbations in muscle fibres from the rabbit

Single chemically permeabilized fibres from rabbit psoas muscle were activated maximally at 5-6 degrees C and then exposed to a rapid temperature increase ('T-jump') up to 37 degrees C by passing a high-voltage pulse (40 kHz AC, 0.15 ms duration) through the fibre length. Fibre cooling after the T-jump was compensated by applying a warming (40 kHz AC, 200 ms) pulse. Tension and changes in sarcomere length induced by the T-jumps and by fast length step perturbations of the fibres were monitored. In some experiments sarcomere length feedback control was used. After T-jumps tension increased from approximately 55 kN m(-2) at 5-6 degrees C to approximately 270 kN m(-2) at 36-37 degrees C, while stiffness rose by approximately 15 %, suggesting that at a higher temperature the myosin head generates more force. The temperature-tension relation became less steep at temperatures above 25 degrees C, but was not saturated even at near-physiological temperature. Comparison of tension transients induced by the T-jump and length steps showed that they are different. The T-jump transients were several times slower than fast partial tension recovery following length steps at low and high temperature (phase 2). The kinetics of the tension rise after the T-jumps was independent of the preceding length changes. When the length steps were applied during the tension rise induced by the T-jump, the observed complex tension transient was simply the sum of two separate responses to the mechanical and temperature perturbations. This demonstrates the absence of interaction between these processes. The data suggest that tension transients induced by the T-jumps and length steps are caused by different processes in myosin cross-bridges.

Changes in myosin S1 orientation and force induced by a temperature increase

Force generation in myosin-based motile systems is thought to result from an angular displacement of the myosin subfragment 1 (S1) tail domain with respect to the actin filament axis. In muscle, raised temperature increases the force generated by S1, implying a greater change in tail domain angular displacement. We used time-resolved x-ray diffraction to investigate the structural corollary of this force increase by measuring M3 meridional reflection intensity during sinusoidal length oscillations. This technique allows definition of S1 orientation with respect to the myofilament axis. M3 intensity changes were approximately sinusoid at low temperatures but became increasingly distorted as temperature was elevated, with the formation of a double intensity peak at maximum shortening. This increased distortion could be accounted for by assuming a shift in orientation of the tail domain of actin-bound S1 toward the orientation at which M3 intensity is maximal, which is consistent with a tail domain rotation model of force generation in which the tail approaches a more perpendicular projection from the thin filament axis at higher temperatures. In power stroke simulations, the angle between S1 tail mean position during oscillations and the position at maximum intensity decreased by 4.7 degrees, corresponding to a mean tail displacement toward the perpendicular of 0.73 nm for a temperature-induced force increase of 0.28 P(0) from 4 to 22 degrees C. Our findings suggest that at least 62% of crossbridge compliance is associated with the tail domain.

The biochemical kinetics underlying actin movement generated by one and many skeletal muscle myosin molecules

To better understand how skeletal muscle myosin molecules move actin filaments, we determine the motion-generating biochemistry of a single myosin molecule and study how it scales with the motion-generating biochemistry of an ensemble of myosin molecules. First, by measuring the effects of various ligands (ATP, ADP, and P(i)) on event lifetimes, tau(on), in a laser trap, we determine the biochemical kinetics underlying the stepwise movement of an actin filament generated by a single myosin molecule. Next, by measuring the effects of these same ligands on actin velocities, V, in an in vitro motility assay, we determine the biochemistry underlying the continuous movement of an actin filament generated by an ensemble of myosin molecules. The observed effects of P(i) on single molecule mechanochemistry indicate that motion generation by a single myosin molecule is closely associated with actin-induced P(i) dissociation. We obtain additional evidence for this relationship by measuring changes in single molecule mechanochemistry caused by a smooth muscle HMM mutation that results in a reduced P(i)-release rate. In contrast, we observe that motion generation by an ensemble of myosin molecules is limited by ATP-induced actin dissociation (i.e., V varies as 1/tau(on)) at low [ATP], but deviates from this relationship at high [ATP]. The single-molecule data uniquely provide a direct measure of the fundamental mechanochemistry of the actomyosin ATPase reaction under a minimal load and serve as a clear basis for a model of ensemble motility in which actin-attached myosin molecules impose a load.

A model of cross-bridge attachment to actin in the A*M*ATP state based on x-ray diffraction from permeabilized rabbit psoas muscle

A model of cross-bridges binding to actin in the weak binding A*M*ATP state is presented. The modeling was based on the x-ray diffraction patterns from the relaxed skinned rabbit psoas muscle fibers where ATP hydrolysis was inhibited by N-phenylmaleimide treatment (S. Xu, J. Gu, G. Melvin, L. C. Yu. 2002. Biophys. J. 82:2111-2122). Calculations included both the myosin filaments and the actin filaments of the muscle cells, and the binding to actin was assumed to be single headed. To achieve a good fit, considerable flexibility in the orientation of the myosin head and the position of the S1-S2 junction is necessary, such that the myosin head can bind to a nearby actin whereas the tail end was kept in the proximity of the helical track of the myosin filament. Hence, the best-fit model shows that the head binds to actin in a wide range of orientations, and the tail end deviates substantially from its lattice position in the radial direction (approximately 60 A). Surprisingly, the best fit model reveals that the detached head, whose location thus far has remained undetected, seems to be located close to the surface of the myosin filament. Another significant requirement of the best-fit model is that the binding site on actin is near the N terminus of the actin subunit, a position distinct from the putative rigor-binding site. The results support the idea that the essential role played by the weak binding states M*ATP <--> A*M*ATP for force generation lies in its flexibility, because the probability of attachment is greatly increased, compared with the weak binding M*ADP*P(i) <--> A*M*ADP*P(i) states.

Structural characterization of weakly attached cross-bridges in the A*M*ATP state in permeabilized rabbit psoas muscle

It is well established that in a skeletal muscle under relaxing conditions, cross-bridges exist in a mixture of four weak binding states in equilibrium (A*M*ATP, A*M*ADP*P(i), M*ATP, and M*ADP*P(i)). It has been shown that these four weak binding states are in the pathway to force generation. In the past their structural, biochemical, and mechanical properties have been characterized as a group. However, it was shown that the myosin heads in the M*ATP state exhibited a disordered distribution along the thick filament, while in the M*ADP*P(i) state they were well ordered. It follows that the structures of the weakly attached states of A*M*ATP and A*M*ADP*P(i) could well be different. Individual structures of the two attached states could not be assigned because protocol for isolating the two states has not been available until recently. In the present study, muscle fibers are reacted with N-phenylmaleimide such that ATP hydrolysis is inhibited, i.e., the cross-bridge population under relaxing conditions is distributed only between the two states of M*ATP and A*M*ATP. Two-dimensional x-ray diffraction was applied to determine the structural characteristics of the attached A*M*ATP state. Because the detached state of M*ATP is disordered and does not contribute to layer line intensities, changes as a result of increasing attachment in the A*M*ATP state are attributable to that state alone. The equilibrium toward the attached state was achieved by lowering the ionic strength. The results show that upon attachment, both the myosin and the first actin associated layer lines increased intensities, while the sixth actin layer line was not significantly affected. However, the intensities remain weak despite substantial attachment. The results, together with modeling (see J. Gu, S. Xu and L. C. Yu, 2002, Biophys. J. 82:2123-2133), suggest that there is a wide range of orientation of the attached A*M*ATP cross-bridges while the myosin heads maintain some degree of helical distribution on the thick filament, suggesting a high degree of flexibility in the actomyosin complex. Furthermore, the lack of sensitivity of the sixth actin layer line suggests that the binding site on actin differs from the putative site for rigor binding. The significance of the flexibility in the A*M*ATP complex in the process of force generation is discussed.

Mechanistic implications for Escherichia coli cofactor-dependent phosphoglycerate mutase based on the high-resolution crystal structure of a vanadate complex

The structure of Escherichia coli cofactor-dependent phosphoglycerate mutase (dPGM), complexed with the potent inhibitor vanadate, has been determined to a resolution of 1.30 A (R-factor 0.159; R-free 0.213). The inhibitor is present in the active site, principally as divanadate, but with evidence of additional vanadate moieties at either end, and representing a different binding mode to that observed in the structural homologue prostatic acid phosphatase. The analysis reveals the enzyme-ligand interactions involved in inhibition of the mutase activity by vanadate and identifies a water molecule, observed in the native E.coli dPGM structure which, once activated by vanadate, may dephosphorylate the active protein. Rather than reflecting the active conformation previously observed for E.coli dPGM, the inhibited protein's conformation resembles that of the inactive dephosphorylated Saccharomyces cerevisiae dPGM. The provision of a high-resolution structure of both active and inactive forms of dPGM from a single organism, in conjunction with computational modelling of substrate molecules in the active site provides insight into the binding of substrates and the specific interactions necessary for three different activities, mutase, synthase and phosphatase, within a single active site. The sequence similarity of E.coli and human dPGMs allows us to correlate structure with clinical pathology.

Changes at the interface of the N- and C-terminal parts of the heavy chain of myosin subfragment 1

It has been previously shown that in the M-MgADP-P(i) state, where the myosin head adopts a pre-power stroke conformation, treatment of trypsin-split subfragment 1 of skeletal muscle myosin with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) results in cross-linking of the C-terminal fragment of the heavy chain of S1 -- most probably its converter region -- to the N-terminal S1 heavy-chain fragment, generating a product of 44 kDa [Biochim. Biophys. Acta 1481 (2000) 55]. The results described here show that this product is neither generated in the absence of nucleotide nor in the presence of MgADP or MgPP(i). The 44 kDa cross-linking product can be formed when S1 treated with EDC is complexed with MgADP-AlF(4) or MgADP-V(i) (MgADP-P(i) analogs) and with MgADP-BeF(x), MgATP gamma S or MgAMPPNP (MgATP analogs). The results suggest structural differences between MgATP- or MgADP-P(i)-bound S1, and MgADP-bound or nucleotide-free S1, in spatially close regions of their N- and C-terminal heavy-chain fragments.

Directionality and processivity of molecular motors

Analysis of a mutant with altered directionality has led to new insights into motor directionality. The prediction from current models for processivity of a two-heads-bound state has been confirmed by electron microscopy for myosin V and by unbinding experiments for kinesin. Evidence is emerging that non-processive motors bind their filament with one head, hydrolyze ATP and then release, requiring binding by a second motor to complete a step.

The myosin power stroke

Optical trapping technology now allows investigators in the motility field to measure the forces generated by single motor molecules. A handful of research groups have exploited this approach to further develop our understanding of the actin-based motor, myosin, an ATPase that is capable of converting chemical energy into mechanical work during a cyclical interaction with filamentous actin. In this regard, myosin-II from muscle is the most well-characterized myosin superfamily member. By combining the data obtained from optical trap assays with that from ensemble biochemical and mechanical assays, this review discusses the fundamental properties of the myosin-II power stroke and, perhaps more significantly, how these properties are governed by this molecule's atomic structure and the biochemical transitions that define its catalytic cycle.

Kinesin: switch I & II and the motor mechanism

New crystal structures of the kinesin motors differ from previously described motor-ADP atomic models, showing striking changes both in the switch I region near the nucleotide-binding cleft and in the switch II or ‘relay’ helix at the filament-binding face of the motor. The switch I region, present as a short helix flanked by two loops in previous motor-ADP structures, rearranges into a pseudo-β-hairpin or is completely disordered with melted helices to either side of the disordered switch I loop. The relay helix undergoes a rotational movement coupled to a translation that differs from the piston-like movement of the relay helix observed in myosin. The changes observed in the crystal structures are interpreted to represent structural transitions that occur in the kinesin motors during the ATP hydrolysis cycle. The movements of switch I residues disrupt the water-mediated coordination of the bound Mg2+, which could result in loss of Mg2+ and ADP, raising the intriguing possibility that disruption of the switch I region leads to release of nucleotide by the kinesins. None of the new structures is a true motor-ATP state, however, probably because conformational changes at the active site of the kinesins require interactions with microtubules to stabilize the movements.

Effects of substituting uridine triphosphate for ATP on the crossbridge cycle of rabbit muscle

1. Substituting uridine triphosphate (UTP) for ATP as a substrate for rabbit skeletal myosin and actin at 4 degrees C slowed the dissociation of myosin-S1 from actin by threefold, and hydrolysis of the nucleotide by sevenfold, without a decrease in the rates of phosphate or uridine diphosphate dissociation from actomyosin. 2. The same substitution in skinned rabbit psoas fibres at 2-3 degrees C reduced the maximum shortening velocity by 56 % and increased the force asymptote of the force-velocity curve relative to force (alpha/P(o)) by 112 % without altering the velocity asymptote, beta. It also decreased isometric force by 35 % and isometric stiffness by 20 %, so that the stiffness/force ratio was increased by 23 %. 3. Tension transient experiments showed that the stiffness/force increase was associated with a 10 % reduction in the amplitude of the rapid, partial (phase 2) recovery relative to the isometric force, and the addition of two new components, one that recovered at a step-size-independent rate of 100 s(-1) and another that did not recover following the length change. 4. The increased alpha/P(o) with constant beta suggests an internal load, as expected of attached crossbridges detained in their movement. An increased stiffness/force ratio suggests a greater fraction of attached bridges in low-force states, as expected of bridges with unhydrolyzed UTP detained in low-force states. Decreased phase 2 recovery suggests the detention of high-force bridges, as expected of slowed actomyosin dissociation by nucleotide. 5. These results suggest that the separation of hydrolysed phosphates from nucleotides occurs early in the attached phase of the crossbridge cycle, near and possibly identical to a transition to a firmly attached, low-force state from an initial state where bridges with hydrolysed nucleotides are easily detached by shortening.

The interdomain motions in myosin subfragment 1

The interdomain motions in myosin subfragment 1 (S1) were studied by steady-state and time-resolved fluorescence of tryptophan residues and N-(iodoacetyl)-N'-(5-sulfo-1-naphtyl)ethylenediamine (AEDANS) attached to Cys178 of alkali light chain 1 (A1) exchanged into S1. The efficiency of fluorescence resonance energy transfer (FRET) from tryptophan residues of motor domain to AEDANS at A1 decreased dramatically after addition of ATP to S1A1-AEDANS. The efficiency of FRET calculated from the crystal structure of chicken S1 corresponded to the experimental one measured in the presence of ATP. The results showed that AEDANS at Cys178 of A1 became more mobile and distant from the motor domain of S1 upon ATP binding. These findings led to the suggestion that a release of the products of ATP hydrolysis and power stroke might be associated with movement of light chain-binding domain towards the N-terminal domain of S1.

Master equation approach to molecular motors

A master equation approach to molecular motors allows us to describe a mechanochemical cyclic system where chemical and translational degrees of freedom are treated on an equal footing. A generalized detailed balance condition in the out-of-equilibrium regime is shown to be compatible with the Fokker-Planck equation in the continuum limit. The Onsager reciprocity relations hold for stationary states close to equilibrium, provided the generalized detailed balance condition is satisfied. Semiphenomenological considerations in the case of motor proteins lead to a discrete kinetics model, for which interesting observable quantities may be directly calculated and compared with experimental data.

Structure of a fast kinesin: implications for ATPase mechanism and interactions with microtubules

We determined the crystal structure of the motor domain of the fast fungal kinesin from Neurospora crassa (NcKin). The structure has several unique features. (i) Loop 11 in the switch 2 region is ordered and enables one to describe the complete nucleotide-binding pocket, including three inter-switch salt bridges between switch 1 and 2. (ii) Loop 9 in the switch 1 region bends outwards, making the nucleotide-binding pocket very wide. The displacement in switch 1 resembles that of the G-protein ras complexed with its guanosine nucleotide exchange factor. (iii) Loop 5 in the entrance to the nucleotide-binding pocket is remarkably long and interacts with the ribose of ATP. (iv) The linker and neck region is not well defined, indicating that it is mobile. (v) Image reconstructions of ice-embedded microtubules decorated with NcKin show that it interacts with several tubulin subunits, including a central beta-tubulin monomer and the two flanking alpha-tubulin monomers within the microtubule protofilament. Comparison of NcKin with other kinesins, myosin and G-proteins suggests that the rate-limiting step of ADP release is accelerated in the fungal kinesin and accounts for the unusually high velocity and ATPase activity.

Nucleotide-dependent conformational changes in a protease-associated ATPase HsIU

BACKGROUND

The bacterial heat shock locus ATPase HslU is an AAA(+) protein that has structures known in many nucleotide-free and -bound states. Nucleotide is required for the formation of the biologically active HslU hexameric assembly. The hexameric HslU ATPase binds the dodecameric HslV peptidase and forms an ATP-dependent HslVU protease.

RESULTS

We have characterized four distinct HslU conformational states, going sequentially from open to closed: the empty, SO(4), ATP, and ADP states. The nucleotide binds at a cleft formed by an alpha/beta domain and an alpha-helical domain in HslU. The four HslU states differ by a rotation of the alpha-helical domain. This classification leads to a correction of nucleotide identity in one structure and reveals the ATP hydrolysis-dependent structural changes in the HslVU complex, including a ring rotation and a conformational change of the HslU C terminus. This leads to an amended protein unfolding-coupled translocation mechanism.

CONCLUSIONS

The observed nucleotide-dependent conformational changes in HslU and their governing principles provide a framework for the mechanistic understanding of other AAA(+) proteins.

A fluorescence temperature-jump study of conformational transitions in myosin subfragment 1

The tryptophan fluorescence of unmodified myosin subfragment 1 (S1) from rabbit and chicken skeletal muscle with various nucleotides and phosphate analogues bound was measured after rapid temperature jumps. The fluorescence decreased during the temperature rise. Under some conditions, this decrease was followed by an increase, reflecting structural transitions within the protein. With adenosine 5'-[beta,gamma-imido]triphosphate (p[NH]ppA) or with ADP and BeF(x) bound, this rise was very rapid (reciprocal time constant approx. 2000 s(-1)) and varied only slightly with starting temperature, suggesting that, with these ligands, two different protein conformations were present in rapid equilibrium over a large temperature range. In the presence of ATP, the transient included several relaxation processes. Overall, the results suggest that complexes of S1 with ATP or with a number of other ligands exist as a mixture of two forms in temperature-dependent equilibrium. The results throw light on the finding of different forms of S1 in recent crystallographic studies and indicate a surprising lack of strong coupling between myosin's structural state and the nature of the nucleotide bound.

The myosin swinging cross-bridge model

No biological system has been studied by more diverse approaches than the actin-based molecular motor myosin. Biophysics, biochemistry, physiology, classical genetics and molecular genetics have all made their contributions, and myosin is now becoming one of the best-understood enzymes in biology.

Resolution of distinct rotational substeps by submillisecond kinetic analysis of F1-ATPase

The enzyme F1-ATPase has been shown to be a rotary motor in which the central gamma-subunit rotates inside the cylinder made of alpha3beta3 subunits. At low ATP concentrations, the motor rotates in discrete 120 degrees steps, consistent with sequential ATP hydrolysis on the three beta-subunits. The mechanism of stepping is unknown. Here we show by high-speed imaging that the 120 degrees step consists of roughly 90 degrees and 30 degrees substeps, each taking only a fraction of a millisecond. ATP binding drives the 90 degrees substep, and the 30 degrees substep is probably driven by release of a hydrolysis product. The two substeps are separated by two reactions of about 1 ms, which together occupy most of the ATP hydrolysis cycle. This scheme probably applies to rotation at full speed ( approximately 130 revolutions per second at saturating ATP) down to occasional stepping at nanomolar ATP concentrations, and supports the binding-change model for ATP synthesis by reverse rotation of F1-ATPase.