Characterization of the motor and enzymatic properties of smooth muscle long S1 and short HMM: role of the two-headed structure on the activity and regulation of the myosin motor

Phosphorylation-induced structural changes in smooth muscle myosin regulatory light chain

We have performed complementary time-resolved fluorescence resonance energy transfer (TR-FRET) experiments and molecular dynamics (MD) simulations to elucidate structural changes in the phosphorylation domain (PD) of smooth muscle regulatory light chain (RLC) bound to myosin. PD is absent in crystal structures, leaving uncertainty about the mechanism of regulation. Donor-acceptor pairs of probes were attached to three site-directed di-Cys mutants of RLC, each having one Cys at position 129 in the C-terminal lobe and the other at position 2, 3, or 7 in the N-terminal PD. Labeled RLC was reconstituted onto myosin subfragment 1 (S1). TR-FRET resolved two simultaneously populated structural states of RLC, closed and open, in both unphosphorylated and phosphorylated biochemical states. All three FRET pairs show that phosphorylation shifts the equilibrium toward the open state, increasing its mol fraction by approximately 20%. MD simulations agree with experiments in remarkable detail, confirming the coexistence of two structural states, with phosphorylation shifting the system toward the more dynamic open structural state. This agreement between experiment and simulation validates the additional structural details provided by MD simulations: In the closed state, PD is bent onto the surface of the C-terminal lobe, stabilized by interdomain salt bridges. In the open state, PD is more helical and straight, resides farther from the C-terminal lobe, and is stabilized by an intradomain salt bridge. The result is a vivid atomic-resolution visualization of the first step in the molecular mechanism by which phosphorylation activates smooth muscle.

Smooth muscle myosin phosphorylated at single head shows sustained mechanical activity

Smooth muscle contraction is regulated by the phosphorylation of myosin. It is well known that tonic smooth muscles can maintain force with low energy consumption (latch state); however, the molecular mechanism underlying this phenomenon is unresolved. Here we show that single-head phosphorylated smooth myosin (SHPMII) exhibits fast ( approximately 24 s(-1)) and slow prolonged ( approximately 1 s(-1)) actin interactions, whereas double-head phosphorylated myosin (DHPMII) predominantly exhibits the fast ( approximately 29 s(-1)) interaction, suggesting that the phosphorylated head of SHPMII is mechanically as active as that of DHPMII. Both the fast and the slow actin interactions of SHPMII support the positive net mechanical displacement of actin. The actin translocating velocity of SHPMII was much slower than that of DHPMII, which is consistent with the slow actin interaction of SHPMII. We propose that the "latch state" can be explained by the motor characteristics of SHPMII that is present during the sustained phase of contraction.

Molecular dynamics simulations reveal a disorder-to-order transition on phosphorylation of smooth muscle myosin

We have performed molecular dynamics simulations of the phosphorylated (at S-19) and the unphosphorylated 25-residue N-terminal phosphorylation domain of the regulatory light chain (RLC) of smooth muscle myosin to provide insight into the structural basis of regulation. This domain does not appear in any crystal structure, so these simulations were combined with site-directed spin labeling to define its structure and dynamics. Simulations were carried out in explicit water at 310 K, starting with an ideal alpha-helix. In the absence of phosphorylation, large portions of the domain (residues S-2 to K-11 and R-16 through Y-21) were metastable throughout the simulation, undergoing rapid transitions among alpha-helix, pi-helix, and turn, whereas residues K-12 to Q-15 remained highly disordered, displaying a turn motif from 1 to 22.5 ns and a random coil pattern from 22.5 to 50 ns. Phosphorylation increased alpha-helical order dramatically in residues K-11 to A-17 but caused relatively little change in the immediate vicinity of the phosphorylation site (S-19). Phosphorylation also increased the overall dynamic stability, as evidenced by smaller temporal fluctuations in the root mean-square deviation. These results on the isolated phosphorylation domain, predicting a disorder-to-order transition induced by phosphorylation, are remarkably consistent with published experimental data involving site-directed spin labeling of the intact RLC bound to the two-headed heavy meromyosin. The simulations provide new insight into structural details not revealed by experiment, allowing us to propose a refined model for the mechanism by which phosphorylation affects the N-terminal domain of the RLC of smooth muscle myosin.

The elongation and contraction of actin bundles are induced by double-headed myosins in a motor concentration-dependent manner

Many types of myosin have been found and characterized to date, and already nearly 20 classes have been identified. However, these myosin motors can be classified more simply into two groups according to their head-structure, i.e. double- or single-headed myosins. Why do some myosin motors possess a double-headed structure? One obvious possible reason would be that the two heads improve the motor's processivity and sliding performance. Previously, to investigate the possibility that the double-headed myosins simultaneously interact with parallel arrayed two actin filaments in the presence of Mg-ATP, we developed an in vitro assay system using actin bundles formed by inert polymers. Using that system, we show here that skeletal muscle heavy meromyosin (HMM), a double-headed myosin derivative, but not subfragment-1 (S-1), a single-headed one, was able to contract or elongate actin bundles in a concentration-dependent manner. Similar elongation or contraction of actin bundles can also be induced by other double-headed myosin species isolated in the native state from Dictyostelium, from green algae Chara or from chicken brain. The results of this study confirm that double-headed myosin motors can induce sliding movements among neighboring actin filaments. The double-headed structure of myosins may also be important for generating tension or elongation in actin bundles or gels, and for organizing polarity-sorted actin networks, not just for improving their motor processivity or activity.

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.

Human deafness mutation of myosin VI (C442Y) accelerates the ADP dissociation rate

The missense mutation of Cys(442) to Tyr of myosin VI causes progressive postlingual sensorineural deafness. Here we report the affects of the C442Y mutation on the kinetics of the actomyosin ATP hydrolysis mechanism and motor function of myosin VI. The largest changes in the kinetic mechanism of ATP hydrolysis produced by the C442Y mutation are about 10-fold increases in the rate of ADP dissociation from both myosin VI and actomyosin VI. The rates of ADP dissociation from acto-C442Y myosin VI-ADP and C442Y myosin VI-ADP are 20-40 times more rapid than the steady state rates and cannot be the rate-limiting steps of the hydrolysis mechanism in the presence or absence of actin. The 2-fold increase in the actin gliding velocity of C442Y compared with wild type (WT) may be explained at least in part by the more rapid rate of ADP dissociation. The C442Y myosin VI has a significant increase ( approximately 10-fold) in the steady state ATPase rate in the absence of actin relative to WT myosin VI. The steady state rate of actin-activated ATP hydrolysis is unchanged by the C442Y mutation at low (<10(-7) m) calcium but is calcium-sensitive with a 1.6-fold increase at high ( approximately 10(-4) m) calcium that does not occur with WT. The actin gliding velocity of the C442Y mutant decreases significantly at low surface density of myosin VI, suggesting that the mutation hampers the processive movement of myosin VI.

Cryo-atomic force microscopy of unphosphorylated and thiophosphorylated single smooth muscle myosin molecules

The purpose of this study was to determine whether steric blockage of one head by the second head of native two-headed myosin was responsible for the inactivity of nonphosphorylated two-headed myosin compared with the high activity of single-headed myosin, as suggested on the basis of electron microscopy of two-dimensional crystals of heavy meromyosin (Wendt, T., Taylor, D., Messier, T., Trybus, K. M., and Taylor, K. A. (1999) J. Cell Biol. 147, 1385-1390; and Wendt, T., Taylor, D., Trybus, K. M., and Taylor, K. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 4361-4366). Our earlier cryo-atomic force microscopy (cryo-AFM) (Zhang, Y., Shao, Z., Somlyo, A. P., and Somlyo, A. V. (1997) Biophys. J. 72, 1308-1318) indicates that thiophosphorylation of the regulatory light chain increases the separation of the two heads of a single myosin molecule, but the thermodynamic probability of steric hindrance by strong binding between the two heads was not determined. We now report this probability determined by cryo-AFM of single whole myosin molecules shown to have normal low ATPase activity (0.007 s-1). We found that the thermodynamic probability of the relative head positions of nonphosphorylated myosin was approximately equal between separated heads as compared with closely apposed heads (energy difference of 0.24 kT (where k is a Boltzman constant and T is the absolute temperature)), and thiophosphorylation increased the number of molecules having separated heads (energy advantage of -1.2 kT (where k is a Boltzman constant and I is the absolute temperature)). Our results do not support the suggestion that strong binding of one head to the other stabilizes the blocked conformation against thermal fluctuations resulting in steric blockage that can account for the low activity of nonphosphorylated two-headed myosin.

Two functional heads are required for full activation of smooth muscle myosin

The motor activity of smooth muscle myosin II is regulated by the regulatory light chain phosphorylation, but it is not understood how phosphorylation activates motor activity. To address this question, we produced asymmetric heavy meromyosin (HMM), which is composed of a wild-type (WT) heavy chain and a mutant heavy chain having no motor activity (i.e. S236T or G457A). The actin-activated ATPase activities (Vmax) of asymmetric HMMs were only 21.8 and 8.4% of the wild-type HMM for S236A/WT HMM and G456A/WT HMM, respectively. If the two heads of HMM are independent for their ATPase activities, asymmetric HMM should show 50% of the activity of wild-type HMM; however, the activity of asymmetric HMM was much lower than the expected value. The results suggest that the activity of the wild-type head is attenuated by the presence of inactive head. Consistently, the actin-gliding velocity of the asymmetric HMM (i.e. S236T/WT or G457A/WT) was less than one-fifth of the wild-type HMM. The present study supports an idea that the two heads of smooth muscle myosin II interact with each other and the presence of two active heads is required for full activation.

Neck length and processivity of myosin V

Myosin V is an unconventional myosin that transports cargo such as vesicles, melanosomes, or mRNA on actin filaments. It is a two-headed myosin with an unusually long neck that has six IQ motifs complexed with calmodulin. In vitro studies have shown that myosin V moves processively on actin, taking multiple 36-nm steps that coincide with the helical repeat of actin. This allows the molecule to "walk" across the top of an actin filament, a feature necessary for moving large vesicles along an actin filament bound to the cytoskeleton. The extended neck length of the two heads is thought to be critical for taking 36-nm steps for processive movements. To test this hypothesis we have expressed myosin V heavy meromyosin-like fragments containing 6IQ motifs, as well as ones that shorten (2IQ, 4IQ) or lengthen (8IQ) the neck region or alter the spacing between 3rd and 4th IQ motifs. The step size was proportional to neck length for the 2IQ, 4IQ, 6IQ, and 8IQ molecules, but the molecule with the altered spacing took shorter than expected steps. Total internal reflection fluorescence microscopy was used to determine whether the heavy meromyosin IQ molecules were capable of processive movements on actin. At saturating ATP concentrations, all molecules except for the 2IQ mutant moved processively on actin. When the ATP concentration was lowered to 10 microm or less, the 2IQ mutant demonstrated some processive movements but with reduced run lengths compared with the other mutants. Its weak processivity was also confirmed by actin landing assays.

Determination of human myosin III as a motor protein having a protein kinase activity

The class III myosin is the most divergent member of the myosin superfamily, having a domain with homology to a protein kinase. However, the function of class III myosin at a molecular level is not known at all, and it has been questioned whether it is actually an actin-based motor molecule. Here, we showed that human myosin III has an ATPase activity that is significantly activated by actin (20-fold) with Kactin of 112 microm and Vmax of 0.34 s-1, indicating the mechanoenzymatic activity of myosin III. Furthermore, we found that human myosin III has actin translocating activity (0.11 +/- 0.05 microm/s) using an in vitro actin gliding assay, and it moves toward the plus end of actin filaments. Myosin III containing calmodulin as the light chain subunit showed a protein kinase activity and underwent autophosphorylation. The autophosphorylation was the intramolecular process, and the sites were at the C-terminal end of the motor domain. Autophosphorylation significantly activated the kinase activity, although it did not affect the ATPase activity. The present study is the first report that clearly demonstrates that the class III myosin is an actin-based motor protein having a protein kinase activity.

Structural model of the regulatory domain of smooth muscle heavy meromyosin

The goal of this study was to provide structural information about the regulatory domains of double-headed smooth muscle heavy meromyosin, including the N terminus of the regulatory light chain, in both the phosphorylated and unphosphorylated states. We extended our previous photo-cross-linking studies (Wu, X., Clack, B. A., Zhi, G., Stull, J. T., and Cremo, C. R. (1999) J. Biol. Chem. 274, 20328-20335) to determine regions of the regulatory light chain that are cross-linked by a cross-linker attached to Cys(108) on the partner regulatory light chain. For this purpose, we have synthesized two new biotinylated sulfhydryl reactive photo-cross-linking reagents, benzophenone, 4-(N-iodoacetamido)-4'-(N-biotinylamido) and benzophenone, 4-(N-maleimido)-4'-(N-biotinylamido). Cross-linked peptides were purified by avidin affinity chromatography and characterized by Edman sequencing and mass spectrometry. Labeled Cys(108) from one regulatory light chain cross-linked to (71)GMMSEAPGPIN(81), a loop in the N-terminal half of the regulatory light chain, and to (4)RAKAKTTKKRPQR(16), a region for which there is no atomic resolution data. Both cross-links were to the partner regulatory light chain and occurred in unphosphorylated but not phosphorylated heavy meromyosin. Using these data, data from our previous study, and atomic coordinates from various myosin isoforms, we have constructed a structural model of the regulatory domain in an unphosphorylated double-headed molecule that predicts the general location of the N terminus. The implications for the structural basis of the phosphorylation-mediated regulatory mechanism are discussed.

Myosin IXb is a single-headed minus-end-directed processive motor

Myosin is an actin-based molecular motor that constitutes a diverse superfamily. In contrast to conventional myosin, which binds to actin for only a short time during cross-bridge cycling, recent studies have demonstrated that class V myosin moves along actin filaments for a long distance without dissociating. This would make it suitable for supporting cargo movement in cells. Because myosin V has a two-headed structure with an expanded neck domain, it has been postulated to 'walk' along the 36-nm helical repeat of the actin filament, with one head attached to the actin and leading the other head to the neighbouring helical pitch. Here, we report that myosin IXb, a single-headed myosin, moves processively on actin filaments. Furthermore, we found that myosin IXb is a minus-end-directed motor. In addition to class VI myosin, this is the first myosin superfamily member identified that moves in the reverse direction. The processive movement of the single-headed myosin IXb cannot be explained by a 'hand-over-hand' mechanism. This suggests that an alternative mechanism must be operating for the processive movement of single-headed myosin IXb.

Molecular modeling of averaged rigor crossbridges from tomograms of insect flight muscle

Electron tomography, correspondence analysis, molecular model building, and real-space refinement provide detailed 3-D structures for in situ myosin crossbridges in the nucleotide-free state (rigor), thought to represent the end of the power stroke. Unaveraged tomograms from a 25-nm longitudinal section of insect flight muscle preserved native structural variation. Recurring crossbridge motifs that repeat every 38.7 nm along the actin filament were extracted from the tomogram and classified by correspondence analysis into 25 class averages, which improved the signal to noise ratio. Models based on the atomic structures of actin and of myosin subfragment 1 were rebuilt to fit 11 class averages. A real-space refinement procedure was applied to quantitatively fit the reconstructions and to minimize steric clashes between domains introduced during the fitting. These combined procedures show that no single myosin head structure can fit all the in situ crossbridges. The validity of the approach is supported by agreement of these atomic models with fluorescent probe data from vertebrate muscle as well as with data from regulatory light chain crosslinking between heads of smooth muscle heavy meromyosin when bound to actin.

DOC-2/DAB2 is the binding partner of myosin VI

Myosin VI is a molecular motor that moves processively along actin filaments and is believed to play a role in cargo movement in cells. Here we found that DOC-2/DAB2, a signaling molecule inhibiting the Ras cascade, binds to myosin VI at the globular tail domain. DOC-2/DAB2 binds stoichiometrically to myosin VI with one molecule per one myosin VI heavy chain. The C-terminal 122 amino acid residues of DOC-2/DAB2, containing the Grb2 binding site, is identified to be critical for the binding to myosin VI. Actin gliding assay revealed that the binding of DOC-2/DAB2 to myosin VI can support the actin filament gliding by myosin VI, suggesting that it can function as a myosin VI anchoring molecule. The C-terminal domain but not the N-terminal domain of DOC-2/DAB2 functions as a myosin VI anchoring site. The present findings suggest that myosin VI plays a role in transporting DOC-2/DAB2, a Ras cascade signaling molecule, thus involved in Ras signaling pathways.

Phosphorylation-dependent regulation is absent in a nonmuscle heavy meromyosin construct with one complete head and one head lacking the motor domain

To understand the domain requirements of phosphorylation-dependent regulation, we prepared three recombinant constructs of nonmuscle heavy meromyosin IIB containing 1) two complete heads, 2) one complete head and one head lacking the motor domain, and 3) one complete head and one head lacking both motor and regulatory domains. Steady-state ATPase measurements showed that phosphorylation did not alter the affinity for actin by more than a factor of 2 for any construct. Phosphorylation increased V(max) by a factor of 10 for construct 1 and 1.5-3 for construct 2 but had no effect for construct 3. Single turnover measurements, a better measure of slow rates inherent to unphosphorylated regulated myosins, showed that the single-headed construct 2, like construct 3 retains less than 1% of the regulatory properties of the double-headed construct 1 (300-fold activation). Therefore, a complete head cannot be down-regulated by a regulatory domain (without the motor domain) on the partner head. Two motor domains are required for regulation. This result is predicted by a structural model (Wendt, T., Taylor, D., Messier, T., Trybus, K. M., and Taylor, K. A. (1999) J. Cell Biol. 147, 1385-1390) showing interaction between the motor domains for unphosphorylated smooth muscle myosin, if motor-motor interaction is the basis for down-regulation.

Dual regulation of mammalian myosin VI motor function

Myosin VI is expressed in a variety of cell types and is thought to play a role in membrane trafficking and endocytosis, yet its motor function and regulation are not understood. The present study clarified mammalian myosin VI motor function and regulation at a molecular level. Myosin VI ATPase activity was highly activated by actin with K(actin) of 9 microm. A predominant amount of myosin VI bound to actin in the presence of ATP unlike conventional myosins. K(ATP) was much higher than those of other known myosins, suggesting that myosin VI has a weak affinity or slow binding for ATP. On the other hand, ADP markedly inhibited the actin-activated ATPase activity, suggesting a high affinity for ADP. These results suggested that myosin VI is predominantly in a strong actin binding state during the ATPase cycle. p21-activated kinase 3 phosphorylated myosin VI, and the site was identified as Thr(406). The phosphorylation of myosin VI significantly facilitated the actin-translocating activity of myosin VI. On the other hand, Ca(2+) diminished the actin-translocating activity of myosin VI although the actin-activated ATPase activity was not affected by Ca(2+). Calmodulin was not dissociated from the heavy chain at high Ca(2+), suggesting that a conformational change of calmodulin upon Ca(2+) binding, but not its physical dissociation, determines the inhibition of the motility activity. The present results revealed the dual regulation of myosin VI by phosphorylation and Ca(2+) binding to calmodulin light chain.

Motor function and regulation of myosin X

Myosin X is a member of the diverse myosin superfamily that is ubiquitously expressed in various mammalian tissues. Although its association with actin in cells has been shown, little is known about its biochemical and mechanoenzymatic function at the molecular level. We expressed bovine myosin X containing the entire head, neck, and coiled-coil domain and purified bovine myosin X in Sf9 cells. The Mg(2+)-ATPase activity of myosin X was significantly activated by actin with low K(ATP). The actin-activated ATPase activity was reduced at Ca(2+) concentrations above pCa 5 in which 1 mol of calmodulin light chain dissociates from the heavy chain. Myosin X translocates F-actin filaments with the velocity of 0.3 microm/s with the direction toward the barbed end. The actin translocating activity was inhibited at concentrations of Ca(2+) at pCa 6 in which no calmodulin dissociation takes place, suggesting that the calmodulin dissociation is not required for the inhibition of the motility. Unlike class V myosin, which shows a high affinity for F-actin in the presence of ATP, the K(actin) of the myosin X ATPase was much higher than that of myosin V. Consistently nearly all actin dissociated from myosin X in the presence of ATP. ADP did not significantly inhibit the actin-activated ATPase activity of myosin X, suggesting that the ADP release step is not rate-limiting. These results suggest that myosin X is a nonprocessive motor. Consistently myosin X failed to support the actin translocation at low density in an in vitro motility assay where myosin V, a processive motor, supports the actin filament movement.

The core of the motor domain determines the direction of myosin movement

Myosins constitute a superfamily of at least 18 known classes of molecular motors that move along actin filaments. Myosins move towards the plus end of F-actin filaments; however, it was shown recently that a certain class of myosin, class VI myosin, moves towards the opposite end of F-actin, that is, in the minus direction. As there is a large, unique insertion in the myosin VI head domain between the motor domain and the light-chain-binding domain (the lever arm), it was thought that this insertion alters the angle of the lever-arm switch movement, thereby changing the direction of motility. Here we determine the direction of motility of chimaeric myosins that comprise the motor domain and the lever-arm domain (containing an insert) from myosins that have movement in the opposite direction. The results show that the motor core domain, but neither the large insert nor the converter domain, determines the direction of myosin motility.

Real space refinement of acto-myosin structures from sectioned muscle

We have adapted a real space refinement protocol originally developed for high-resolution crystallographic analysis for use in fitting atomic models of actin filaments and myosin subfragment 1 (S1) to 3-D images of thin-sectioned, plastic-embedded whole muscle. The rationale for this effort is to obtain a refinement protocol that will optimize the fit of the model to the density obtained by electron microscopy and correct for poor geometry introduced during the manual fitting of a high-resolution atomic model into a lower resolution 3-D image. The starting atomic model consisted of a rigor acto-S1 model obtained by X-ray crystallography and helical reconstruction of electron micrographs. This model was rebuilt to fit 3-D images of rigor insect flight muscle at a resolution of 7 nm obtained by electron tomography and image averaging. Our highly constrained real space refinement resulted in modest improvements in the agreement of model and reconstruction but reduced the number of conflicting atomic contacts by 70% without loss of fit to the 3-D density. The methodology seems to be well suited to the derivation of stereochemically reasonable atomic models that are consistent with experimentally determined 3-D reconstructions computed from electron micrographs.

Ca(2+)-dependent regulation of the motor activity of myosin V

Mouse myosin V constructs were produced that consisted of the myosin motor domain plus either one IQ motif (M5IQ1), two IQ motifs (M5IQ2), a complete set of six IQ motifs (SHM5), or the complete IQ motifs plus the coiled-coil domain (thus permitting formation of a double-headed structure, DHM5) and expressed in Sf9 cells. The actin-activated ATPase activity of all constructs except M5IQ1 was inhibited above pCa 5, but this inhibition was completely reversed by addition of exogenous calmodulin. At the same Ca(2+) concentration, 2 mol of calmodulin from SHM5 and DHM5 or 1 mol of calmodulin from M5IQ2 were dissociated, suggesting that the inhibition of the ATPase activity is due to dissociation of calmodulin from the heavy chain. However, the motility activity of DHM5 and M5IQ2 was completely inhibited at pCa 6, where no dissociation of calmodulin was detected. Inhibition of the motility activity was not reversed by the addition of exogenous calmodulin. These results indicate that inhibition of the motility is due to conformational changes of calmodulin upon the Ca(2+) binding to the high affinity site but is not due to dissociation of calmodulin from the heavy chain.

Kinetics of smooth muscle heavy meromyosin with one thiophosphorylated head

Actin-activated MgATPase of smooth muscle heavy meromyosin is activated by thiophosphorylation of two regulatory light chains, one on each head domain. To understand cooperativity between heads, we examined the kinetics of heavy meromyosin (HMM) with one thiophosphorylated head. Proteolytic gizzard heavy meromyosin regulatory light chains were partially exchanged with recombinant thiophosphorylated His-tagged light chains, and HMM with one thiophosphorylated head was isolated by nickel-affinity chromatography. In vitro motility was observed. By steady-state kinetic analysis, one-head thiophosphorylated heavy meromyosin had a similar K(m) value for actin but a V(max) value of approximately 50% of the fully thiophosphorylated molecule. However, single turnover analysis, which is not sensitive to small amounts of active heads, showed that one-head thiophosphorylated heavy meromyosin was 46-120 times more active than unphosphorylated HMM but only 7-19% as active as the fully thiophosphorylated molecule. Discrepancy between the single turnover and steady-state values could be explained by a small fraction of rigor heads. These rigor heads would have a large effect on the steady-state kinetics of one-head thiophosphorylated HMM. In summary, thiophosphorylation of one head leads to a molecule with unique intermediate kinetics suggesting that thiophosphorylation of one head cooperatively alters the kinetics of the partner head and vice versa.

Phosphorylation-dependent structural changes in the regulatory light chain domain of smooth muscle heavy meromyosin

Smooth muscle heavy meromyosin, a double-headed proteolytic fragment of myosin lacking the COOH-terminal two-thirds of the tail, has been shown previously to be regulated by phosphorylation. To examine phosphorylation-dependent structural changes near the head-tail junction, we prepared five well regulated heavy meromyosins containing single-cysteine mutants of the human smooth muscle regulatory light chain labeled with the photocross-linking reagent, benzophenone-iodoacetamide. For those mutants that generated cross-links, only one type of cross-linked species was observed, a regulatory light chain dimer. Irradiated mutants fell into two classes. First, for Q15C, A23C, and wild type (Cys-108), a regulatory light chain dimer was formed for dephosphorylated but not thiophosphorylated heavy meromyosin. These data provide direct chemical evidence that in the dephosphorylated state, Gln-15, Ala-23, and Cys-108 on one head are positioned near (within 8.9 A) the regulatory light chain of the partner head and that thiophosphorylation abolishes proximity. This behavior was also observed for the Q15C mutant on a truncated heavy meromyosin lacking both catalytic domains. For the actin-heavy meromyosin complex, cross-links were formed in both de- and thiophosphorylated states. S59C and T134C mutants were in a second mutant class, where regulatory light chain dimers were not detected in dephosphorylated or thiophosphorylated heavy meromyosin, suggesting positions outside the region of interaction of the regulatory light chains.

Functional significance of the conserved residues in the flexible hinge region of the myosin motor domain

Analysis of the three-dimensional crystal structure of the Dictyostelium myosin motor domain revealed that the myosin head is required to bend at residues Ile-455 and Gly-457 to produce the conformation changes observed in the ternary complexes that resemble the pre- and post-hydrolysis states (Fisher, A. J., Smith, C. A., Thoden, J. B., Smith, R., Sutoh, K., Holden, H. M., and Rayment, I. (1995) Biochemistry 34, 8960-8972). Asp-454, Ile-455, and Gly-457 of smooth muscle myosin were substituted by Ala, Met, and Ala, respectively, and the mechano-enzymatic activities were determined to study the role of these residues in myosin motor function. Whereas the basal steady-state Mg2+-ATPase activity of D454A was higher than that of the wild type, the rate of the hydrolytic step is reduced approximately 2,000-fold and becomes rate-limiting. M-ATP rather than M-ADP-P is the predominant steady-state intermediate, and the initial Pi burst and the ATP-induced enhancement of intrinsic tryptophan fluorescence are absent in D454A. D454A binds actin in the absence of ATP but is not dissociated from actin by ATP. Moreover, actin inhibits rather than activates the ATPase activity; consequently, D454A does not support actin translocating activity. I455M has normal actin-activated ATPase activity, Pi burst, and ATP-induced enhancement of intrinsic tryptophan fluorescence, suggesting that the enzymatic properties are normal. However, the actin translocating activity was completely inhibited. This suggests that the side chain at Ile-455 is critical for myosin motor activity but not for relatively normal enzymatic function, which indicates an apparent uncoupling between enzymatic activity and motile function. Although G457A has normal ATP-dependent actin dissociation, ATP hydrolytic step is reduced by approximately 10(5)-fold in the presence or absence of actin; consequently, G457A does not have actin translocating activity. These results indicate the importance of these conserved residues at the hinge region for normal myosin motor function.

Dimerization of the head-rod junction of scallop myosin

We have compared the dimerization properties and coiled-coil stability of various recombinant fragments of scallop myosin around the head-rod junction. The heavy-chain peptide of the regulatory domain and its various extensions toward the alpha-helical rod region were expressed in Escherichia coli, purified, and reconstituted with the light chains. Rod fragments of the same length but without the light-chain binding domain were also expressed. Electron micrographs show that the regulatory domain complex containing 340 residues of the rod forms dimers with two knobs (two regulatory domains) at one end attached to an approximately 50-nm coiled coil. These parallel dimers are in equilibrium with monomers (Kd = 10.6 microM). By contrast, complexes with shorter rod extensions remain predominantly monomeric. Dimers are present, accounting for ca. 5% of the molecules containing a rod fragment of 87 residues and ca. 30% of those with a 180-residue peptide. These dimers appear to be antiparallel coiled coils, as judged by their length and the knobs observed at the two ends. The rod fragments alone do not dimerize and form a coiled-coil structure unless covalently linked by disulfide bridges. Our results suggest that the N-terminal end of the coiled-coil rod is stabilized by interactions with the regulatory domain, most likely with residues of the regulatory light chain. This labile nature of the coiled coil at the head-rod junction might be a structural prerequisite for regulation of scallop myosin by Ca2+-ions.

Structural and kinetic studies of phosphorylation-dependent regulation in smooth muscle myosin

In this study, we have examined the mechanism of phosphorylation-dependent regulation in smooth muscle myosin through the use of structural and kinetic methodologies applied to several myosin fragments. Fluorescence anisotropy decay measurements demonstrate that regulatory light chain phosphorylation significantly reduces the rotational correlation time of regulatable myosin preparations, whereas minimally regulated ones show little effect in this assay. Sedimentation equilibrium studies show that the regulatory domain can dimerize with a dissociation constant that is unaffected by regulatory light chain phosphorylation. Finally, kinetic studies on the interactions of myosin-ADP constructs with actin are also consistent with a model in which interactions occur between the two heads, which are lost with regulatory light chain phosphorylation. We propose that in the absence of regulatory light chain phosphorylation, the two heads of myosin interact with each other, due to a weak intrinsic dimerization of the regulatory domains that is significantly stabilized by the proximal rod. Regulatory light chain phosphorylation abolishes the stabilizing effect of the proximal rod, leading to a loss of this interaction.

A long, weakly charged actin-binding loop is required for phosphorylation-dependent regulation of smooth muscle myosin

Chimeric substitution of the weak actin-binding loop (ABL) from chicken skeletal muscle myosin for that of gizzard smooth muscle heavy meromyosin (HMM) causes activation of the dephosphorylated mutant (SABL HMM; Rovner, A. S., Freyzon, Y., and Trybus, K. M. (1995) J. Biol. Chem. 270, 30260-30263). The present study determined whether this loss of regulation is due to the greater positive charge density (5 versus 3 clustered lysine residues) or lesser length (14 versus 26 residues) of the mutant ABL. Charge augmentation had little effect on regulation of expressed mutants, but elimination of the 12 N-terminal amino acids from the wild-type ABL significantly increased actin-activated ATPase activity of the dephosphorylated relative to the phosphorylated molecule while conferring the ability to move actin filaments in vitro on the former. Addition of the same 12 residues to the SABL mutant increased the ratio of phosphorylated to dephosphorylated ATPase activity while imparting wild type-like regulation to motility. However, full actin activation of dephosphorylated ATPase activity required both the shorter length and greater positive charge density found in the SABL loop. These results demonstrate that, compared with skeletal, both the greater length and lesser positive charge density of the smooth muscle myosin ABL are required for proper phosphorylation-mediated regulation of the molecule.

Effects of mutations in the gamma-phosphate binding site of myosin on its motor function

The role of the highly conserved residues in the gamma-phosphate binding site of myosin upon myosin motor function was studied. Each of five residues (Ser181, Lys185, Asn235, Ser236, and Arg238) in smooth muscle myosin was mutated. K185Q has neither a steady state ATPase nor an initial Pi burst. Although ATP and actin bind to K185Q, it is not dissociated from actin by ATP. These results indicate that the hydrolysis of bound ATP by K185Q is inhibited. S236T has nearly normal basal Mg2+-ATPase activity, initial Pi burst, ATP-induced enhancement of intrinsic tryptophan fluorescence, and ATP-induced dissociation from actin. However, the actin activation of the Mg2+-ATPase activity and actin translocation of S236T were blocked. In contrast S236A has nearly normal enzymatic properties and actin-translocating activity. These results indicate that 1) the hydroxyl group of Ser236 is not critical as an intermediary of proton transfer during the ATP hydrolysis step, and 2) the bulk of the extra methyl group of the threonine residue in S236T blocks the acceleration of product release from the active site by actin. Arg238, which interacts with Glu459 at the Switch II region, was mutated to Lys and Ile, respectively. R238K has essentially normal enzymatic activity and motility. In contrast, R238I does not hydrolyze ATP or support motility, although it still binds ATP. These results indicate that the charge interaction between Glu459 and Arg238 is critical for ATP hydrolysis by myosin. Other mutants, S181A, S181T, and N235I, showed nearly normal enzymatic and motile activity.

Thiophosphorylation of myosin light chain increases rigor stiffness of rabbit smooth muscle

1. The effect of thiophosphorylation of the regulatory myosin light chain (MLC20) on rigor stiffness was determined in permeabilized rabbit bladder smooth muscle. 2. Rigor stiffness of alpha-toxin-permeabilized smooth muscle was significantly increased by thiophosphorylation of MLC20. This increase may have been due to partial shortening (melting) in the proximal rod region and/or stiffening of the regulatory domain of the myosin head. 3. We suggest that phosphorylation of MLC20, by increasing the stiffness of the S1 lever arm and/or S2 hinge regions of the myosin molecule, favours separation of the two phosphorylated heads and consequent deinhibition of motor domain activity.

High affinity Ca2+ binding sites of calmodulin are critical for the regulation of myosin Ibeta motor function

We coexpressed myosin Ibeta heavy chain with three different calmodulin mutants in which the two Ca2+-binding sites of the two N-terminal domain (E12Q), C-terminal domain (E34Q), or all four sites (E1234Q) are mutated in order to define the importance of these Ca2+ binding sites to the regulation of myosin Ibeta. The calmodulin mutated at the two Ca2+ binding sites in N-terminal domain and C-terminal domain lost its lower affinity Ca2+ binding site and higher affinity Ca2+ binding site, respectively. We found that, based upon the change in the actin-activated ATPase activities and actin translocating activities, myosin Ibeta with E12Q calmodulin has the regulatory characteristics similar to myosin Ibeta containing wild-type calmodulin, while myosin Ibeta with E34Q or E1234Q calmodulin lose all Ca2+ regulation. While the increase in myosin Ibeta ATPase activity paralleled the dissociation of 1 mol of calmodulin from myosin Ibeta heavy chain for both wild type (above pCa 5) and E12Q calmodulin (above pCa 6), the Ca2+ level required for the inhibition of actin-translocating activity of myosin Ibeta was lower than that required for dissociation of calmodulin, suggesting that the conformational change induced by the binding of Ca2+ at the high affinity site but not the dissociation of calmodulin is critical for the inhibition of the motor activity. Our results suggest that the regulation of unconventional myosins by Ca2+ is directly mediated by the Ca2+ binding to calmodulin, and that the C-terminal pair of Ca2+-binding sites are critical for this regulation.

A hinge at the central helix of the regulatory light chain of myosin is critical for phosphorylation-dependent regulation of smooth muscle myosin motor activity

The motor function of smooth muscle myosin is activated by phosphorylation of the regulatory light chain (RLC) at Ser19. However, the molecular mechanism by which the phosphorylation activates the motor function is not yet understood. In the present study, we focused our attention on the role of the central helix of RLC for regulation. The flexible region at the middle of the central helix (Gly95-Pro98) was substituted or deleted to various extents, and the effects of the deletion or substitution on the regulation of the motor activity of myosin were examined. Deletion of Gly95-Asp97, Gly95-Thr96, or Thr96-Asp97 decreased the actin-translocating activity of myosin a little, but the phosphorylation-dependent regulation of the motor activity was not disrupted. In contrast, the deletion of Gly95-Pro98 of RLC completely abolished the actin translocating activity of phosphorylated myosin. However, the unregulated myosin long subfragment 1 containing this RLC mutant showed motor activity the same as that containing the wild type RLC. Since long subfragment 1 motor activity is unregulated by phosphorylation, i.e. constitutively active, these results suggest that the deletion of these residues at the central helix of RLC disrupts the phosphorylation-mediated activation mechanism but not the motor function of myosin itself. On the other hand, the elimination of Pro98 or substitution of Gly95-Pro98 by Ala resulted in the activation of actin translocating activity of dephosphorylated myosin, whereas it did not affect the motor activity of phosphorylated myosin. Together, these results clearly indicate the importance of the hinge at the central helix of RLC on the phosphorylation-mediated regulation of smooth muscle myosin.

The motor domain and the regulatory domain of myosin solely dictate enzymatic activity and phosphorylation-dependent regulation, respectively

While the structures of skeletal and smooth muscle myosins are homologous, they differ functionally from each other in several respects, i.e., motor activities and regulation. To investigate the molecular basis for these differences, we have produced a skeletal/smooth chimeric myosin molecule and analyzed the motor activities and regulation of this myosin. The produced chimeric myosin is composed of the globular motor domain of skeletal muscle myosin (Met1-Gly773) and the C-terminal long alpha-helix domain of myosin subfragment 1 as well as myosin subfragment 2 (Gly773-Ser1104) and light chains of smooth muscle myosin. Both the actin-activated ATPase activity and the actin-translocating activity of the chimeric myosin were completely regulated by light chain phosphorylation. On the other hand, the maximum actin-activated ATPase activity of the chimeric myosin was the same as skeletal myosin and thus much higher than smooth myosin. These results show that the C-terminal light chain-associated domain of myosin head solely confers regulation by light chain phosphorylation, whereas the motor domain determines the rate of ATP hydrolysis. This is the first report, to our knowledge, that directly determines the function of the two structurally separated domains in myosin head.

Functional analysis of the mutations in the human cardiac beta-myosin that are responsible for familial hypertrophic cardiomyopathy. Implication for the clinical outcome

More than 30 missense mutations in the beta-cardiac myosin heavy chain gene have been shown to be responsible for familial hypertrophic cardiomyopathy. To clarify the effects of these point mutations on myosin motor function, we expressed wild-type and mutant human beta-cardiac myosin heavy chains in insect cells with human cardiac light chains. The wild-type myosin was well purified with similar enzymatic and motor activities to those of the naturally isolated V3 cardiac myosin. Arg249-->Gln and Arg453-->Cys mutations resulted in decreased actin translocating activity (61 and 23% of the wild-type, respectively) with decreased intrinsic ATPase activity. Arg403-->Gln mutation greatly decreased actin translocating activity (27% of wild type) with a 3.3-fold increased dissociation constant for actin, while intrinsic ATPase activity was unchanged. Val606-->Met mutation only mildly affected the actin translocating activity as well as ATPase activity of myosin. The degree of deterioration by each mutation was closely correlated with the prognosis of the affected kindreds, indicating that myosin dysfunction caused by the point mutations is responsible for the pathogenesis of the disease. Structure/function relationship of myosin is discussed.