Effect of phosphorylation on the binding of smooth muscle heavy meromyosin X ADP to actin

Local monomer levels and established filaments potentiate non-muscle myosin 2 assembly

The ability to dynamically assemble contractile networks is required throughout cell physiology, yet direct biophysical mechanisms regulating non-muscle myosin 2 filament assembly in living cells are lacking. Here, we use a suite of dynamic, quantitative imaging approaches to identify deterministic factors that drive myosin filament appearance and amplification. We find that actin dynamics regulate myosin assembly, but that the static actin architecture plays a less clear role. Instead, remodeling of actin networks modulates the local myosin monomer levels and facilitates assembly through myosin:myosin-driven interactions. Using optogenetically controlled myosin, we demonstrate that locally concentrating myosin is sufficient to both form filaments and jump-start filament amplification and partitioning. By counting myosin monomers within filaments, we demonstrate a myosin-facilitated assembly process that establishes filament stacks prior to partitioning into clusters that feed higher-order networks. Together, these findings establish the biophysical mechanisms regulating the assembly of non-muscle contractile structures that are ubiquitous throughout cell biology.

Local Monomer Levels and Established Filaments Potentiate Non-Muscle Myosin 2 Assembly

The ability to dynamically assemble contractile networks is required throughout cell physiology, yet the biophysical mechanisms regulating non-muscle myosin 2 filament assembly in living cells are lacking. Here we use a suite of dynamic, quantitative imaging approaches to identify deterministic factors that drive myosin filament appearance and amplification. We find that actin dynamics regulate myosin assembly, but that the actin architecture plays a minimal direct role. Instead, remodeling of actin networks modulates the local myosin monomer levels and facilitates assembly through myosin:myosin driven interactions. Using optogenetically controlled myosin, we demonstrate that locally concentrating myosin is sufficient to both form filaments and jump-start filament amplification and partitioning. By counting myosin monomers within filaments, we demonstrate a myosin-facilitated assembly process that establishes sub-resolution filament stacks prior to partitioning into clusters that feed higher-order networks. Together these findings establish the biophysical mechanisms regulating the assembly of non-muscle contractile structures that are ubiquitous throughout cell biology.

Kinetic characterization of nonmuscle myosin IIb at the single molecule level

Nonmuscle myosin IIB (NMIIB) is a cytoplasmic myosin, which plays an important role in cell motility by maintaining cortical tension. It forms bipolar thick filaments with ~14 myosin molecule dimers on each side of the bare zone. Our previous studies showed that the NMIIB is a moderately high duty ratio (~20-25%) motor. The ADP release step (~0.35 s(-1)) of NMIIB is only ~3 times faster than the rate-limiting phosphate release (0.13 ± 0.01 s(-1)). The aim of this study was to relate the known in vitro kinetic parameters to the results of single molecule experiments and to compare the kinetic and mechanical properties of single- and double-headed myosin fragments and nonmuscle IIB thick filaments. Examination of the kinetics of NMIIB interaction with actin at the single molecule level was accomplished using total internal reflection fluorescence (TIRF) with fluorescence imaging with 1-nm accuracy (FIONA) and dual-beam optical trapping. At a physiological ATP concentration (1 mm), the rate of detachment of the single-headed and double-headed molecules was similar (~0.4 s(-1)). Using optical tweezers we found that the power stroke sizes of single- and double-headed heavy meromyosin (HMM) were each ~6 nm. No signs of processive stepping at the single molecule level were observed in the case of NMIIB-HMM in optical tweezers or TIRF/in vitro motility experiments. In contrast, robust motility of individual fluorescently labeled thick filaments of full-length NMIIB was observed on actin filaments. Our results are in good agreement with the previous steady-state and transient kinetic studies and show that the individual nonprocessive nonmuscle myosin IIB molecules form a highly processive unit when polymerized into filaments.

Evidence for absence of latch-bridge formation in muscular saphenous arteries

Large-diameter elastic arteries can produce strong contractions indefinitely at a high-energy economy by the formation of latch bridges. Whether downstream blood vessels also use latch bridges remains unknown. The zero-pressure medial thickness and lumen diameter of rabbit saphenous artery (SA), a muscular branch of the elastic femoral artery (FA), were, respectively, approximately twofold and half-fold that of the FA. In isolated FA and SA rings, KCl rapidly (< 16 s) caused strong increases in isometric stress (1.2 x 10(5) N/m2) and intracellular Ca2+ concentration ([Ca2+]i; 250 nM). By 10 min, [Ca2+]i declined to approximately 175 nM in both tissues, but stress was sustained in FA (1.3 x 10(5) N/m2) and reduced by 40% in SA (0.8 x 10(5) N/m2). Reduced tonic stress correlated with reduced myosin light chain (MLC) phosphorylation in SA (28 vs. 42% in FA), and simulations with the use of the four-state kinetic latch-bridge model supported the hypothesis that latch-bridge formation in FA, but not SA, permitted maintenance of high stress values at steady state. SA expressed more MLC phosphatase than FA, and permeabilized SA relaxed more rapidly than FA, suggesting that MLC phosphatase activity was greater in SA than in FA. The ratio of fast-to-slow myosin isoforms was greater for SA than FA, and on quick release, SA redeveloped isometric force faster than FA. These data support the hypothesis that maintained isometric force was 40% less in SA than in FA because expressed motor proteins in SA do not support latch-bridge formation.

Substrate and product dependence of force and shortening in fast and slow smooth muscle

To explore the molecular mechanisms responsible for the variation in smooth muscle contractile kinetics, the influence of MgATP, MgADP, and inorganic phosphate (P(i)) on force and shortening velocity in thiophosphorylated "fast" (taenia coli: maximal shortening velocity Vmax = 0.11 ML/s) and "slow" (aorta: Vmax = 0.015 ML/s) smooth muscle from the guinea pig were compared. P(i) inhibited active force with minor effects on the V(max). In the taenia coli, 20 mM P(i) inhibited force by 25%. In the aorta, the effect was markedly less (< 10%), suggesting differences between fast and slow smooth muscles in the binding of P(i) or in the relative population of P(i) binding states during cycling. Lowering of MgATP reduced force and V(max). The aorta was less sensitive to reduction in MgATP (Km for Vmax: 80 microM) than the taenia coli (Km for Vmax: 350 microM). Thus, velocity is controlled by steps preceding the ATP binding and cross-bridge dissociation, and a weaker binding of ATP is not responsible for the lower V(max) in the slow muscle. MgADP inhibited force and V(max). Saturating concentrations of ADP did not completely inhibit maximal shortening velocity. The effect of ADP on Vmax was observed at lower concentrations in the aorta compared with the taenia coli, suggesting that the ADP binding to phosphorylated and cycling cross-bridges is stronger in slow compared with fast smooth muscle.

Phosphorylation events associated with cyclic nucleotide-dependent inhibition of smooth muscle contraction

Activation of cyclic nucleotide-dependent signaling pathways leads to relaxation of bovine carotid artery smooth muscle contractions and is associated with increased phosphorylation of the small heat shock-related protein (HSP20). Previous reports have shown that human umbilical artery smooth muscle is uniquely resistant to cyclic nucleotide-dependent relaxation, and HSP20 is not phosphorylated. In this investigation, we determined the phosphorylation events associated with cyclic nucleotide-dependent inhibition of smooth muscle contraction. In carotid artery, activation of cyclic nucleotide-dependent signaling pathways inhibited contractile responses to serotonin but did not inhibit myosin light chain phosphorylation or oxygen consumption. The inhibition of contraction was associated with increases in HSP20 phosphorylation. In umbilical artery, activation of cyclic nucleotide-dependent signaling pathways did not inhibit serotonin-induced contraction or myosin light chain phosphorylation. The lack of contractile inhibition in umbilical artery was not associated with significant increases in HSP20 phosphorylation. In conclusion, cyclic nucleotide-dependent contractile inhibition is independent of the inhibition of myosin light chain phosphorylation or oxygen consumption but does correlate with increased HSP20 phosphorylation.

Control of cross-bridge cycling by myosin light chain phosphorylation in mammalian smooth muscle

This review focuses on experiments in which the single turnover of myosin-bound ADP is used to characterize the regulation of the cross-bridge cycle by myosin light chain phosphorylation in mammalian smooth muscle. Under isometric conditions, at rest, when the myosin light chain is not phosphorylated, myosin cycles very slowly (about 0.004 s-1), while phosphorylation of the light chain results in a 50-fold increase in cycling rate of 0.2 s-1. Experiments consistently show that some myosin does not increase its cycling rate although its light chain is phosphorylated. Studies at low levels of myosin light chain phosphorylation show that phosphorylation also induces an increase in the cycling rate of unphosphorylated myosin. The fast cycling phosphorylated myosin is the main determinant of suprabasal myosin ATPase activity, while the cycling rate of cooperatively activated unphosphorylated myosin is slow and appears to depend on the extent of phosphorylation of the entire thick filament. Single turnover experiments measuring the rate of phosphorylation and dephosphorylation of myosin light chain show that the turnover of light chain phosphate can be very rapid (0.3-0.4 s-1) at suprabasal calcium concentrations. The expected effect of such a rapid turnover of light chain phosphorylation on the turnover of myosin-bound ADP is not observed. The effects of low levels of myosin light chain phosphorylation on the single turnover of myosin suggest that the same small pool of myosin remains phosphorylated for relatively long periods of time rather than the entire pool of myosin spending a small fraction of its cycle time in the phosphorylated state.

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.

Calcium modulates conformational changes in F-actin induced by smooth muscle heavy meromyosin

The effect of Ca2+ on conformational changes in rhodamine-phalloidin-labeled F-actin induced by binding of smooth muscle heavy meromyosin (HMM) with either phosphorylated or dephosphorylated regulatory light chains (LC20) was studied by polarized fluorimetry. LC20 phosphorylation caused alterations in the F-actin structure typical of the force-producing (strong-binding) state, while dephosphorylation of the chains led to alterations typical of the formation of non-force-producing (weak-binding) state of the actomyosin complex. The presence of Ca2+ enhanced the effect of LC20 phosphorylation and weakened the effect of LC20 dephosphorylation. These data suggest that Ca2+ modulates actin-myosin interaction in smooth muscle by promoting formation of the strong-binding state.

Kinetic tuning of myosin via a flexible loop adjacent to the nucleotide binding pocket

A surface loop (25/50-kDa loop) near the nucleotide pocket of myosin has been proposed to be an important element in determining the rate of ADP release from myosin, and as a consequence, the rate of actin-myosin filament sliding (Spudich, J. A. (1991) Nature 372, 515-518). To test this hypothesis, loops derived from different myosin II isoforms that display a range of actin filament sliding velocities were inserted into a smooth muscle myosin backbone. Chimeric myosins were produced by baculovirus/Sf9 cell expression. Although the nature of this loop affected the rate of ADP release (up to 9-fold), in vitro motility (2.7-fold), and the Vmax of actin-activated ATPase activity (up to 2-fold), the properties of each chimera did not correlate with the relative speed of the myosin from which the loop was derived. Rather, the rate of ADP release was a function of loop size/flexibility with the larger loops giving faster rates of ADP release. The rate of actin filament translocation was altered by the rate of ADP release, but was not solely determined by it. Through a combination of solute quenching and transient fluorescence measurements, it is concluded that, as the loop gets smaller, access to the nucleotide pocket is more restricted, ATP binding becomes less favored, and ADP binding becomes more favored. In addition, the rate of ATP hydrolysis is slowed.

ADP release produces a rotation of the neck region of smooth myosin but not skeletal myosin

Current theories of muscle cross-bridge function suggest that force is generated by a change in the orientation of the myosin neck region. We attached a paramagnetic probe to a subunit in the neck region and measured the orientation of the probe using electron paramagnetic resonance spectroscopy. The angle of the probes on smooth myosin S1 were changed by 20 degrees +/- 4 degrees on addition of ADP (50% effect at 5 +/- 2 microM), but ADP produced little effect on skeletal S1. The orientation of smooth myosin, +ADP, resembled that of skeletal myosin, +/- ADP, suggesting that the release of ADP generates an extra rotation of the neck region in smooth muscle at the end of its power stroke.

Myosin phosphorylation and the control of myometrial contraction/relaxation

In summary, phosphorylation of the regulatory light chain of myosin by Ca2+/CaM-dependent MLCK plays an important role in smooth muscle contraction. Although there have been major advances in our understanding of the regulation and physiological functions of contractile proteins in smooth muscle in recent years, very little information exists on the functional status of these proteins in human myometrium during pregnancy. The simple view that contractile force in smooth muscle is proportionate to cytoplasmic Ca2+ concentrations (Ca2+i) and myosin light chain phosphorylation is now more complex as more experiments provide insights into mechanisms of regulation of the contractile elements. MLCK can be phosphorylated, which desensitizes its activation by Ca2+/CaM, and protein phosphatase activity toward myosin may also be regulated. Examples in smooth muscle tissue are sparse, and the different mechanisms by which these processes may be adapted in uterine smooth muscle during pregnancy are not well-defined. Much research is needed to define further the cellular, biochemical, and molecular basis for these physiological processes involved in the regulation of uterine smooth muscle contraction and relaxation.

Smooth muscle myosin subfragment-1 is a kinetic analogue for heavy meromyosin in the extended conformation

The 10S-->6S (Flexed-->Extended) transition in smooth muscle myosin is related to increased ATPase activity, but there is controversy over whether the analogous 9S-->7S transition in HMM is also associated with ATPase activity. We therefore studied the association of ionic strength, phosphorylation, and ATPase activity for HMM as compared to S1 which has no apparent flexed conformation. In addition, we performed both steady state and single turnover analyses, to control for artifacts due to multiple subfragment populations that might skew steady state results. At low ionic strength where myosin and HMM are in the flexed conformation, HMM had a near zero ATPase activity while S-1 had a high ATPase rate (0.07 s-1). At 400 mM ionic strength, where both myosin and HMM are in the extended conformation, S1 and HMM had the same ATPase rate (0.04 s-1). Phosphorylation did not affect S1 significantly, but shifted the HMM curve to higher rates at lower ionic strengths. Both steady state and single turnover experiments gave the same results, indicating that steady state results were not skewed by multiple subfragment populations. These data indicate that HMM has a conformation-ATPase relation similar to that observed with myosin. Furthermore, these findings suggest that the S1 ATPase rate corresponds to that of HMM in the extended conformation.

In vitro studies of determinants of smooth muscle mechanics

Smooth muscle contraction is dependent upon phosphorylation of the 20,000 Da light chain subunits of myosin. Whereas the kinetics of the hydrolysis of MgATP by smooth muscle myosin suggest a simple phosphorylation-dependent "on-off" mechanism, the contractile response in smooth muscle tissue is complex. Experiments to unravel this complexity have been performed in vitro using a combination of motility assays and kinetic techniques. Some insight into this complexity is obtained, but the mechanism and the regulation of smooth muscle contraction is still not completely known.

Cooperative activation of myosin by light chain phosphorylation in permeabilized smooth muscle

The purpose of this study was to determine the quantitative relationship between the number of myosin molecules that increase their ATPase activity and the degree of myosin light chain phosphorylation in smooth muscle. Single turnover experiments on the nucleotide bound to myosin were performed in the permeabilized rabbit portal vein. In the resting muscle, the rate of exchange of bound nucleoside diphosphate was biphasic and complete in approximately 30 min. When approximately 80% of the myosin light chain was thiophosphorylated, the nucleoside diphosphate exchange occurred at a much faster rate and was almost complete in 2 min. Thiophosphorylation of 10% of the myosin light chains caused an increase in the rate of ADP exchange from much more than 10% of the myosin subfragment-1. Less than 20% thiophosphorylation of the total myosin light chains resulted in the maximum increase in ADP exchanged in 2 min. It appears that a small degree of myosin light chain phosphorylation cooperatively turns on the maximum number of myosin molecules. Interestingly, even though less than 20% thiophosphorylation of the myosin light chain caused the maximum exchange of ADP within 2 min, higher degrees of thiophosphorylation were associated with further increases in the ATPase rates. We conclude that a small degree of myosin light chain thiophosphorylation cooperatively activates the maximum number of myosin molecules, and a higher degree of thiophosphorylation makes the myosin cycle faster. A kinetic model is proposed in which the rate constant for attachment of unphosphorylated cross bridges varies as a function of myosin light chain phosphorylation.

Regulation of smooth muscle myosin

It is well established that light chain phosphorylation is required before a smooth muscle can generate force. The apparent modulation of shortening velocity by phosphorylation during sustained contractions may be accounted for by a mechanical interaction between rapidly cycling phosphorylated crossbridges and slowly or non-cycling dephosphorylated crossbridges. Latchbridges, force-producing dephosphorylated crossbridges, have been proposed to explain why force levels remain high at low levels of phosphorylation. The role of the thin-filament-associated proteins caldesmon and calponin in regulation remains enigmatic, but their inhibitory properties in solution would be consistent with a possible involvement in maintenance of a relaxed state.

Myosin-product complex in the resting state and during relaxation of smooth muscle

Previous findings suggested that in resting smooth muscle ADP is bound to myosin and that phosphorylation of the myosin, and its subsequent interaction with actin, increases the rate of ADP release. We have now extended these studies to include measurements of bound Pi as well as bound ADP in permeabilized rabbit portal vein. We report that in resting smooth muscle that has been exposed to [3H]ATP and [gamma-32P]ATP, followed by a chase in an unlabeled relaxing solution, the ratio of bound [3H]ADP to bound [32P]Pi is close to unity, and both are released at approximately the same rate. This suggests that myosin exists predominantly with both ADP and Pi bound under resting conditions and that the release of one is quickly followed by the release of the other. In contrast, there is a significant 30% excess of bound Pi over ADP in a muscle during relaxation from an isometric contraction. Under these conditions, while force output is slowly decreasing, both light chain phosphorylation and adenosinetriphosphatase (ATPase) activity have decreased to near-resting values. The time course of relaxation is similar to the time course of Pi release from both the resting and relaxing muscle. We propose that during relaxation the dephosphorylated cross bridges which are bearing force have Pi but not ADP bound and that detachment of the cross bridge (and thus force decay) is limited by Pi release from myosin which occurs at the same rate as in the resting muscle.

ADP release from myosin in permeabilized smooth muscle

The purpose of this study was to determine the nucleotide bound to myosin and its rate of release under relaxed and activated conditions in permeabilized rabbit portal veins. Incubation of the muscles in a relaxing solution containing [3H]-ATP resulted in the formation of 60-70 microM radiolabeled ADP in the muscle whether or not the myosin light chains had been thiophosphorylated. This value was similar to the estimate of the concentration of myosin subfragment 1. Upon transfer of the muscles to a chase solution containing no labeled ATP, there was a very slow loss of labeled ADP when the light chains were unphosphorylated, but a much faster release occurred when the light chains were thiophosphorylated. The results suggest that smooth muscle myosin exists primarily in a complex with ADP under both relaxed and phosphorylated conditions and that phosphorylation of all of the light chains results in a large increase in the rate of release of the products of ATP splitting from all of the myosin. Interestingly, the exponential release of ADP in relaxed muscle shows two components, one of which contains about two-thirds of the total ADP and is 5- to 10-fold faster than the other. If the difference in rates of ADP release observed in relaxed muscle persists when the myosin is phosphorylated, then it is possible that there is a 5- to 10-fold difference in rates of cycling for different phosphorylated cross bridges in smooth muscle.

Active site trapping of nucleotide by smooth and non-muscle myosins

The folded 10 S monomer conformation of smooth muscle myosin traps the hydrolysis products ADP and Pi in its active sites. To test the significance of this, we have searched for equivalent trapping in other conformational and assembly states of avian gizzard and brush border myosins, using formycin triphosphate (FTP) as an ATP analogue. When myosin monomers were in the straight-tail 6 S conformation, the hydrolysis products were released at about 0.03 s-1. Adoption of the folded 10 S monomer conformation reduced this rate by more than 100-fold, effectively trapping the products FDP and Pi in the active sites. This profound inhibition of product release occurred only on formation of the looped tail monomer conformation. In vitro-assembled myosin filaments released products at a comparable rate to free straight-tail 6 S monomers, and smooth muscle heavy meromyosin, which lacks the C-terminal two-thirds of the myosin tail, also did not trap the products in this way. Phosphorylation of the myosin regulatory light chain had no effect on the rate of product release from straight-tail 6 S myosin monomers or from myosin filaments. Rather, it allowed actin to accelerate product release. Phosphorylation acted also to destabilize the folded monomer conformation, causing the recruitment of molecules from the pool of folded monomers into the myosin filaments. The two processes of contraction and filament assembly are thus both controlled in vitro by light-chain phosphorylation. A similar linked control in vivo would allow the organization of myosin in the cell to adapt itself continuously to the pattern of contractile activity.