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

ATP-dependent conformational dynamics in a photoactivated adenylate cyclase revealed by fluorescence spectroscopy and small-angle X-ray scattering

Structural insights into the photoactivated adenylate cyclases can be used to develop new ways of controlling cellular cyclic adenosine monophosphate (cAMP) levels for optogenetic and other applications. In this work, we use an integrative approach that combines biophysical and structural biology methods to provide insight on the interaction of adenosine triphosphate (ATP) with the dark-adapted state of the photoactivated adenylate cyclase from the cyanobacterium Oscillatoria acuminata (OaPAC). A moderate affinity of the nucleotide for the enzyme was calculated and the thermodynamic parameters of the interaction have been obtained. Stopped-flow fluorescence spectroscopy and small-angle solution scattering have revealed significant conformational changes in the enzyme, presumably in the adenylate cyclase (AC) domain during the allosteric mechanism of ATP binding to OaPAC with small and large-scale movements observed to the best of our knowledge for the first time in the enzyme in solution upon ATP binding. These results are in line with previously reported drastic conformational changes taking place in several class III AC domains upon nucleotide binding.

The biochemically defined super relaxed state of myosin-A paradox

The biochemical SRX (super-relaxed) state of myosin has been defined as a low ATPase activity state. This state can conserve energy when the myosin is not recruited for muscle contraction. The SRX state has been correlated with a structurally defined ordered (versus disordered) state of muscle thick filaments. The two states may be linked via a common interacting head motif (IHM) where the two heads of heavy meromyosin (HMM), or myosin, fold back onto each other and form additional contacts with S2 and the thick filament. Experimental observations of the SRX, IHM, and the ordered form of thick filaments, however, do not always agree, and result in a series of unresolved paradoxes. To address these paradoxes, we have reexamined the biochemical measurements of the SRX state for porcine cardiac HMM. In our hands, the commonly employed mantATP displacement assay was unable to quantify the population of the SRX state with all data fitting very well by a single exponential. We further show that mavacamten inhibits the basal ATPases of both porcine ventricle HMM and S1 (Ki, 0.32 and 1.76 μM respectively) while dATP activates HMM cooperatively without any evidence of an SRX state. A combination of our experimental observations and theories suggests that the displacement of mantATP in purified proteins is not a reliable assay to quantify the SRX population. This means that while the structurally defined IHM and ordered thick filaments clearly exist, great care must be employed when using the mantATP displacement assay.

Impact of lead tolerant plant growth promoting rhizobacteria on growth, physiology, antioxidant activities, yield and lead content in sunflower in lead contaminated soil

Present study was conducted to evaluate the effect of lead tolerant plant growth promoting rhizobacteria (LTPGPR) on growth, physiology, yield, antioxidant activities and lead uptake in sunflower in soil contaminated with lead under pot conditions. Three pre-characterized LTPGP strains (S2 (Pseudomonas gessardii strain BLP141), S5 (Pseudomonas fluorescens A506) and S10 (Pseudomonas fluorescens strain LMG 2189)) were used to inoculate sunflower growing in soil contaminated with different levels (300, 600 and 900 mg kg^-1) of lead by using lead nitrate salt as source of lead. Treatments were arranged according to completely randomized design with factorial arrangements. At harvesting, data regarding growth attributes (root shoot length, root shoot fresh and dry weights), yield per plant, physiological attributes (Chlorophyll 'a', 'b' and carotenoids content), antioxidant activities (Ascorbate peroxidase, catalase, superoxide dismutase and glutathione reductase), proline and malanodialdehyde content, and lead content in root, shoot and achenes of sunflower were recorded. Data were analysed by standard statistical procedures. Results showed that lead contamination reduced the plants growth, physiology and yield at all levels of lead stress. But application of LTPGPR in soil contaminated with lead improved plant growth, physiology, yield, and antioxidant activities, proline, and reduced the malanodialdehyde content (that is reduced by the application of different strains in lead contamination) of sunflower as compared to plants grown in soil without inoculation. Inoculation also promoted the uptake of lead in root, shoots and reduced the uptake of lead in achenes of plants as compared to plants in lead contamination without inoculation.

Kinetic adaptation of human Myo19 for active mitochondrial transport to growing filopodia tips

Myosins are actin-based molecular motors which are enzymatically adapted for their cellular functions such as transportation and membrane tethering. Human Myo19 affects mitochondrial motility, and promotes their localization to stress-induced filopodia. Therefore, studying Myo19 enzymology is essential to understand how this motor may facilitate mitochondrial motility. Towards this goal, we have purified Myo19 motor domain (Myo19-3IQ) from a human-cell expression system and utilized transient kinetics to study the Myo19-3IQ ATPase cycle. We found that Myo19-3IQ exhibits noticeable conformational changes (isomerization steps) preceding both ATP and ADP binding, which may contribute to nucleotide binding regulation. Notably, the ADP isomerization step and subsequent ADP release contribute significantly to the rate-limiting step of the Myo19-3IQ ATPase cycle. Both the slow ADP isomerization and ADP release prolong the time Myo19-3IQ spend in the strong actin binding state and hence contribute to its relatively high duty ratio. However, the predicted duty ratio is lower than required to support motility as a monomer. Therefore, it may be that several Myo19 motors are required to propel mitochondria movement on actin filaments efficiently. Finally, we provide a model explaining how Myo19 translocation may be regulated by the local ATP/ADP ratio, coupled to the mitochondria presence in the filopodia.

Broad disorder and the allosteric mechanism of myosin II regulation by phosphorylation

Double electron electron resonance EPR methods was used to measure the effects of the allosteric modulators, phosphorylation, and ATP, on the distances and distance distributions between the two regulatory light chain of myosin (RLC). Three different states of smooth muscle myosin (SMM) were studied: monomers, the short-tailed subfragment heavy meromyosin, and SMM filaments. We reconstituted myosin with nine single cysteine spin-labeled RLC. For all mutants we found a broad distribution of distances that could not be explained by spin-label rotamer diversity. For SMM and heavy meromyosin, several sites showed two heterogeneous populations in the unphosphorylated samples, whereas only one was observed after phosphorylation. The data were consistent with the presence of two coexisting heterogeneous populations of structures in the unphosphorylated samples. The two populations were attributed to an on and off state by comparing data from unphosphorylated and phosphorylated samples. Models of these two states were generated using a rigid body docking approach derived from EM [Wendt T, Taylor D, Trybus KM, Taylor K (2001) Proc Natl Acad Sci USA 98:4361-4366] (PNAS, 2001, 98:4361-4366), but our data revealed a new feature of the off-state, which is heterogeneity in the orientation of the two RLC. Our average off-state structure was very similar to the Wendt model reveal a new feature of the off state, which is heterogeneity in the orientations of the two RLC. As found previously in the EM study, our on-state structure was completely different from the off-state structure. The heads are splayed out and there is even more heterogeneity in the orientations of the two RLC.

Modeling smooth muscle myosin's two heads: long-lived enzymatic roles and phosphorylation-dependent equilibria

Smooth muscle myosin has two heads, each capable of interacting with actin to generate force and/or motion as it hydrolyzes ATP. These heads are inhibited when their associated regulatory light chain is unphosphorylated (0P), becoming active and hydrolyzing ATP maximally when phosphorylated (2P). Interestingly, with only one of the two regulatory light chains phosphorylated (1P), smooth muscle myosin is active but its ATPase rate is <2P. To explain published 1P single ATP turnover and steady-state ATPase activities, we propose a kinetic model in which 1P myosin exists in an equilibrium between being fully active (2P) and inhibited (0P). Based on the single ATP turnover data, we also propose that each 2P head adopts a hydrolytic role distinct from its partner at any point in time, i.e., one head strongly binds actin and hydrolyzes ATP at its actin-activated rate while the other weakly binds actin. Surprisingly, the heads switch roles slowly (<0.1 s(-1)), suggesting that their activities are not independent. The phosphorylation-dependent equilibrium between active and inhibited states and the hydrolytic role that each head adopts during its interaction with actin may have implications for understanding regulation and mechanical performance of other members of the myosin family of molecular motors.

Modification of loop 1 affects the nucleotide binding properties of Myo1c, the adaptation motor in the inner ear

Myo1c is one of eight members of the mammalian myosin I family of actin-associated molecular motors. In stereocilia of the hair cells in the inner ear, Myo1c presumably serves as the adaptation motor, which regulates the opening and closing of transduction channels. Although there is conservation of sequence and structure among all myosins in the N-terminal motor domain, which contains the nucleotide- and actin-binding sites, some differences include the length and composition of surface loops, including loop 1, which lies near the nucleotide-binding domain. To investigate the role of loop 1, we expressed in insect cells mutants of a truncated form of Myo1c, Myo1c(1IQ), as well as chimeras of Myo1c(1IQ) with the analogous loop from other myosins. We found that replacement of the charged residues in loop 1 with alanines or the whole loop with a series of alanines did not alter the ATPase activity, transient kinetics properties, or Ca(2+) sensitivity of Myo1c(1IQ). Substitution of loop 1 with that of the corresponding region from tonic smooth muscle myosin II (Myo1c(1IQ)-tonic) or replacement with a single glycine (Myo1c(1IQ)-G) accelerated the release of ADP from A.M 2-3-fold in Ca(2+), whereas substitution with loop 1 from phasic muscle myosin II (Myo1c(1IQ)-phasic) accelerated the release of ADP 35-fold. Motility assays with chimeras containing a single alpha-helix, or SAH, domain showed that Myo1c(SAH)-tonic translocated actin in vitro twice as fast as Myo1c(SAH)-WT and 3-fold faster than Myo1c(SAH)-G. The studies show that changes induced in Myo1c via modification of loop 1 showed no resemblance to the behavior of the loop donor myosins or to the changes previously observed with similar Myo1b chimeras.

The myosin C-loop is an allosteric actin contact sensor in actomyosin

Actin and myosin form the molecular motor in muscle. Myosin is the enzyme performing ATP hydrolysis under the allosteric control of actin such that actin binding initiates product release and force generation in the myosin power stroke. Non-equilibrium Monte Carlo molecular dynamics simulation of the power stroke suggested that a structured surface loop on myosin, the C-loop, is the actin contact sensor initiating actin activation of the myosin ATPase. Previous experimental work demonstrated C-loop binds actin and established the forward and reverse allosteric link between the C-loop and the myosin active site. Here, smooth muscle heavy meromyosin C-loop chimeras were constructed with skeletal (sCl) and cardiac (cCl) myosin C-loops substituted for the native sequence. In both cases, actin-activated ATPase inhibition is indicated mainly by the lower V(max). In vitro motility was also inhibited in the chimeras. Motility data were collected as a function of myosin surface density, with unregulated actin, and with skeletal and cardiac isoforms of tropomyosin-bound actin for the wild type, cCl, and sCl. Slow and fast subpopulations of myosin velocities in the wild-type species were discovered and represent geometrically unfavorable and favorable actomyosin interactions, respectively. Unfavorable interactions are detected at all surface densities tested. Favorable interactions are more probable at higher myosin surface densities. Cardiac tropomyosin-bound actin promotes the favorable actomyosin interactions by lowering the inhibiting geometrical constraint barriers with a structural effect on actin. Neither higher surface density nor cardiac tropomyosin-bound actin can accelerate motility velocity in cCl or sCl, suggesting the element initiating maximal myosin activation by actin resides in the C-loop.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

The tail domain of myosin Va modulates actin binding to one head

Calcium activates full-length myosin Va steady-state enzymatic activity and favors the transition from a compact, folded "off" state to an extended "on" state. However, little is known of how a head-tail interaction alters the individual actin and nucleotide binding rate and equilibrium constants of the ATPase cycle. We measured the effect of calcium on nucleotide and actin filament binding to full-length myosin Va purified from chick brains. Both heads of nucleotide-free myosin Va bind actin strongly, independent of calcium. In the absence of calcium, bound ADP weakens the affinity of one head for actin filaments at equilibrium and upon initial encounter. The addition of calcium allows both heads of myosin Va.ADP to bind actin strongly. Calcium accelerates ADP binding to actomyosin independent of the tail but minimally affects ATP binding. Although 18O exchange and product release measurements favor a mechanism in which actin-activated Pi release from myosin Va is very rapid, independent of calcium and the tail domain, both heads do not bind actin strongly during steady-state cycling, as assayed by pyrene actin fluorescence. In the absence of calcium, inclusion of ADP favors formation of a long lived myosin Va.ADP state that releases ADP slowly, even after mixing with actin. Our results suggest that calcium activates myosin Va by allowing both heads to interact with actin and exchange bound nucleotide and indicate that regulation of actin binding by the tail is a nucleotide-dependent process favored by linked conformational changes of the motor domain.

Regulatory and catalytic domain dynamics of smooth muscle myosin filaments

Domain dynamics of the chicken gizzard smooth muscle myosin catalytic domain (heavy chain Cys-717) and regulatory domain (regulatory light chain Cys-108) were determined in the absence of nucleotides using saturation-transfer electron paramagnetic resonance. In unphosphorylated synthetic filaments, the effective rotational correlation times, tau(r), were 24 +/- 6 micros and 441 +/- 79 micros for the catalytic and regulatory domains, respectively. The corresponding amplitudes of motion were 42 +/- 4 degrees and 24 +/- 9 degrees as determined from steady-state phosphorescence anisotropy. These results suggest that the two domains have independent mobility due to a hinge between the two domains. Although a similar hinge was observed for skeletal myosin (Adhikari and Fajer (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 9643-9647. Brown et al. (2001) Biochemistry 40, 8283-8291), the latter displayed higher regulatory domain mobility, tau(r)= 40 +/- 3 micros, suggesting a smooth muscle specific mechanism of constraining regulatory domain dynamics. In the myosin monomers the correlation times for both domains were the same (approximately 4 micros) for both smooth and skeletal myosin, suggesting that the motional difference between the two isoforms in the filaments was not due to intrinsic variation of hinge stiffness. Heavy chain/regulatory light chain chimeras of smooth and skeletal myosin pinpointed the origin of the restriction to the heavy chain and established correlation between the regulatory domain dynamics with the ability of myosin to switch off but not to switch on the ATPase and the actin sliding velocity. Phosphorylation of smooth muscle myosin filaments caused a small increase in the amplitude of motion of the regulatory domain (from 24 +/- 4 degrees to 36 +/- 7 degrees ) but did not significantly affect the rotational correlation time of the regulatory domain (441 to 408 micros) or the catalytic domain (24 to 17 micros). These data are not consistent with a stable interaction between the two catalytic domains in unphosphorylated smooth muscle myosin filaments in the absence of nucleotides.

Calcium regulates scallop muscle by changing myosin flexibility

Muscle myosins are molecular motors that convert the chemical free energy available from ATP hydrolysis into mechanical displacement of actin filaments, bringing about muscle contraction. Myosin cross-bridges exert force on actin filaments during a cycle of attached and detached states that are coupled to each round of ATP hydrolysis. Contraction and ATPase activity of the striated adductor muscle of scallop is controlled by calcium ion binding to myosin. This mechanism of the so-called "thick filament regulation" is quite different to vertebrate striated muscle which is switched on and off via "thin filament regulation" whereby calcium ions bind to regulatory proteins associated with the actin filaments. We have used an optically based single molecule technique to measure the angular disposition adopted by the two myosin heads whilst bound to actin in the presence and absence of calcium ions. This has allowed us to directly observe the movement of individual myosin heads in aqueous solution at room temperature in real time. We address the issue of how scallop striated muscle myosin might be regulated by calcium and have interpreted our results in terms of the structures of smooth muscle myosin that also exhibit thick filament regulation.

Adenosine diphosphate and strain sensitivity in myosin motors

The release of adenosine diphosphate (ADP) from the actomyosin cross-bridge plays an important role in the adenosine-triphosphate-driven cross-bridge cycle. In fast contracting muscle fibres, the rate at which ADP is released from the cross-bridge correlates with the maximum shortening velocity of the muscle fibre, and in some models the rate of ADP release defines the maximum shortening velocity. In addition, it has long been thought that the rate of ADP release could be sensitive to the load on the cross-bridge and thereby provide a molecular explanation of the Fenn effect. However, direct evidence of a strain-sensitive ADP-release mechanism has been hard to come by for fast muscle myosins. The recently published evidence for a strain-sensing mechanism involving ADP release for slower muscle myosins, and in particular non-muscle myosins, is more compelling and can provide the mechanism of processivity for motors such as myosin V. It is therefore timely to examine the evidence for this strain-sensing mechanism. The evidence presented here will argue that a strain-sensitive mechanism of ADP release is universal for all myosins but the basic mechanism has evolved in different ways for different types of myosin. Furthermore, this strain-sensing mechanism provides a way of coordinating the action of multiple myosin motor domains in a single myosin molecule, or in complex assemblies of myosins over long distances without invoking a classic direct allosteric or cooperative communication between motors.

Novel sensors of the regulatory switch on the regulatory light chain of smooth muscle Myosin

Smooth muscle myosin can be switched on by phosphorylation of Ser-19 of the regulatory light chain. Our previous photocross-linking results suggested that an element of the structural mechanism for the regulatory switch was a phosphorylation-induced motion of the regulatory light chain N terminus (Wahlstrom, J. L., Randall, M. A., Jr., Lawson, J. D., Lyons, D. E., Siems, W. F., Crouch, G. J., Barr, R., Facemyer, K. C., and Cremo, C. R. (2003) J. Biol. Chem. 278, 5123-5131). Here we used three different approaches to test this notion, which are reactivity of cysteine thiols, pyrene and acrylodan spectral analysis, and pyrene fluorescence quenching. All methods detected significant differences between the unphosphorylated and phosphorylated regulatory light chain N termini in heavy meromyosin, a double-headed subfragment with an intact regulatory switch. These differences were not observed for subfragment-1, a single-headed, unregulated subfragment. In the presence of either ATP or ADP, phosphorylation increased the solvent exposure and decreased the polarity of the environment about position 23 of the regulatory light chain of heavy meromyosin. These phosphorylation-induced structural changes were not as evident in the absence of nucleotides. Nucleotide binding to unphosphorylated heavy meromyosin caused a decrease in exposure and an increase in polarity of the N terminus, whereas the effects of nucleotide on phosphorylated heavy meromyosin were the opposite. We showed a direct correlation between the kinetics of nucleotide binding/turnover and the conformational change reported by acrylodan at position 23 of the regulatory light chain. Acrylodan-A23C also reports the heads up (extended) to flexed (folded) transition in unphosphorylated heavy meromyosin. This is the first demonstration of direct coupling of nucleotide binding to conformational changes in the N terminus of the regulatory light chain.

Evaluation of the symmetric model for myosin-linked regulation: effect of site-directed mutations in the regulatory light chain on scallop myosin

Regulatory myosins are controlled through mechanisms intrinsic to their structures and can alternate between activated and inhibited states. However, the structural difference between these two states is unclear. Scallop (Pecten maximus) striated adductor myosin is activated directly by calcium. It has been proposed that the two heads of scallop myosin are symmetrically arranged and interact through their regulatory light chains [Offer and Knight (1996) J. Mol. Biol. 256, 407-416], the interface being strengthened in the inhibited state. By contrast, vertebrate smooth-muscle myosin is activated by phosphorylation. Its structure in the inhibited state has been determined from two-dimensional crystalline arrays [Wendt, Taylor, Trybus and Taylor (2001) Proc. Natl. Acad. Sci. U.S.A. 98, 4361-4366] and is asymmetric, requiring no interaction between regulatory light chains. Using site-directed mutagenesis of the scallop regulatory light chain, we have tested the symmetric model for scallop adductor muscle myosin. Specifically, we have made myosin hybrid molecules from scallop (P. maximus) myosin, in which the normal regulatory light chains have been replaced by expressed light chains containing mutations in three residues proposed to participate in the interaction between regulatory light chains. The mutations were R126A (Arg126-->Ala), K130A and E131A; made singly, in pairs or all three together, these mutations were designed to eliminate hydrogen bonding or salt linkages between heads, which are key features of this model. Functional assays to address the competence of these hybrid myosins to bind calcium specifically, to exhibit a calcium-regulated myofibrillar Mg-ATPase and to display calcium-dependent actin sliding were performed. We conclude that the symmetrical model does not describe the inhibited state of scallop regulatory myosin and that an asymmetric structure is a plausible alternative.

Ionic interactions play a role in the regulatory mechanism of scallop heavy meromyosin

Heavy meromyosin from scallop (scHMM) striated muscle is regulated by calcium binding to the essential light chain. The regulation can be modeled with a calcium-dependent equilibrium between on and off scHMM conformations. The observed rate constant for mant-ADP dissociation from scHMM is calcium dependent, and we show here that it can be used to define the equilibrium constant (K(eq)) between on and off conformations. The data show that K(eq) is markedly ionic strength dependent, with high salt (>/=200 mM) abolishing the off state even in the absence of calcium and low salt (<50 mM) favoring the off state even in the presence of calcium. Debye-Huckel plots of the equilibrium constant (K(eq)) for the on and off forms gave parallel slopes (5.94 +/- 0.33 and 6.36 +/- 0.17 M(-0.5)) in the presence and absence of calcium. The presence of an equilibrium mixture of two conformations was confirmed by sedimentation data and the effects of ADP, calcium and ionic strength were in qualitative agreement. Thus scHMM exists in two conformations that can be distinguished in sedimentation profiles and by the rate of release of mant-ADP. Increasing salt concentrations biases the system toward the on state, suggesting a role for ionic interactions in stabilizing the off state.

Visualization of an unstable coiled coil from the scallop myosin rod

Alpha-helical coiled coils in muscle exemplify simplicity and economy of protein design: small variations in sequence lead to remarkable diversity in cellular functions. Myosin II is the key protein in muscle contraction, and the molecule's two-chain alpha-helical coiled-coil rod region--towards the carboxy terminus of the heavy chain--has unusual structural and dynamic features. The amino-terminal subfragment-2 (S2) domains of the rods can swing out from the thick filament backbone at a hinge in the coiled coil, allowing the two myosin 'heads' and their motor domains to interact with actin and generate tension. Most of the S2 rod appears to be a flexible coiled coil, but studies suggest that the structure at the N-terminal region is unstable, and unwinding or bending of the alpha-helices near the head-rod junction seems necessary for many of myosin's functional properties. Here we show the physical basis of a particularly weak coiled-coil segment by determining the 2.5-A-resolution crystal structure of a leucine-zipper-stabilized fragment of the scallop striated-muscle myosin rod adjacent to the head-rod junction. The N-terminal 14 residues are poorly ordered; the rest of the S2 segment forms a flexible coiled coil with poorly packed core residues. The unusual absence of interhelical salt bridges here exposes apolar core atoms to solvent.

Interactions of the two heads of scallop (Argopecten irradians) heavy meromyosin with actin: influence of calcium and nucleotides

We recently proposed a co-operative model for the influence of calcium and ADP on scallop ( Argopecten irradians ) muscle heavy meromyosin (scHMM), in which scHMM exists in two conformations (designated 'off' and 'on'), and calcium and ADP are allosteric effectors of the equilibrium between the off and on conformations [Nyitrai, Szent-Gyorgyi and Geeves (2002) Biochem. J. 365, 19-30]. Here we examine the influence of actin on scHMM. In the absence of nucleotide, both heads of scHMM bind very tightly to actin, independent of the presence of calcium. In the absence of calcium, ADP dissociates scHMM from actin completely, and little evidence of ternary complex formation can be found (actin affinity >20 microM). The off state of scHMM therefore does not interact with actin. In the presence of calcium, ADP and actin lower each other's affinity for scHMM by 30-50-fold, although both heads remain strongly attached to actin (actin affinity 0.17 microM). Detailed analysis suggests that the second head contributes far more to the overall binding energy than is the case for mammalian skeletal muscle HMM. This is consistent with a different stereochemical relationship between the two heads in scallop and mammalian HMM molecules.

Ca2+ causes release of myosin heads from the thick filament surface on the milliseconds time scale

We have used electron microscopy to study the structural changes induced when myosin filaments are activated by Ca2+. Negative staining reveals that when Ca2+ binds to the heads of relaxed Ca2+ -regulated myosin filaments, the helically ordered myosin heads become disordered and project further from the filament surface. Cryo-electron microscopy of unstained, frozen-hydrated specimens supports this finding, and shows that disordering is reversible on removal of Ca2+. The structural change is thus a result of Ca2+ binding alone and not an artifact of staining. Comparison of the two techniques suggests that negative staining preserves the structure induced by Ca2+ -binding. We therefore used a time-resolved negative staining technique to determine the time scale of the structural change. Full disordering was observed within 30 ms of Ca2+ addition, and had started to occur within 10 ms, showing that the change occurs on the physiological time scale. Comparison with studies of single heavy meromyosin molecules suggests that an increased mobility of myosin heads induced by Ca2+ binding underlies the changes in filament structure that we observe. We conclude that the loosening of the array of myosin heads that occurs on activation is real and physiological; it may function to make activated myosin heads freer to contact actin filaments during muscle contraction.

Capturing time-resolved changes in molecular structure by negative staining

Imaging structural intermediates of biological processes is a key step in understanding biological function. Because intermediates are commonly short-lived, lasting only milliseconds, the main methods used to capture them have been conventional imaging of analog or inhibited states, having extended lifetimes, or rapid (millisecond timescale) freezing of intermediates with subsequent observation by cryo-EM. We have developed a simpler method that fixes structure on the millisecond timescale. The procedure consists of briefly (milliseconds) exposing the macromolecular structure of interest on an EM grid to conditions that initiate the structural change, then immediately fixing with uranyl acetate or tannic acid. Specimens are then observed by negative staining. The key finding that validates this approach is our demonstration that uranyl acetate, and in some cases tannic acid, fixes protein molecular structure on the millisecond timescale. This is demonstrated by our observation that exposure of actin and myosin filaments to these fixatives for as little as 10 ms is sufficient to fully preserve them against changes that normally induce rapid and major alteration in their molecular structure. Fixation appears to stabilize both ionic and hydrophobic bonds. This approach should be of general utility for studying transient molecular changes in many systems.