Rate-limiting step in the actomyosin adenosinetriphosphatase cycle: studies with myosin subfragment 1 cross-linked to actin

Although there is agreement that actomyosin can hydrolyze ATP without dissociation of the actin from myosin, there is still controversy about the nature of the rate-limiting step in the ATPase cycle. Two models, which differ in their rate-limiting step, can account for the kinetic data. In the four-state model, which has four states containing bound ATP or ADP . Pi, the rate-limiting step is ATP hydrolysis (A . M . ATP in equilibrium A . M . ADP . Pi). In the six-state model, which we previously proposed, the rate-limiting step is a conformational change which occurs before Pi release but after ATP hydrolysis. A difference between these models is that only the four-state model predicts that almost no acto-subfragment 1 (S-1) . ADP . Pi complex will be formed when ATP is mixed with acto . S-1. In the present study, we determined the amount of acto . S-1 . ADP . Pi formed when ATP is mixed with S-1 cross-linked to actin [Mornet, D., Bertrand, R., Pantel, P., Audemard, E., & Kassab, R. (1981) Nature (London) 292, 301-306]. The amount of acto . S-1 . ADP . Pi was determined both from intrinsic fluorescence enhancement and from direct measurement of Pi. We found that at mu = 0.013 M, the fluorescence magnitude in the presence of ATP of the cross-linked actin . S-1 preparation was about 50% of the value obtained with S-1, while at mu = 0.053 M the fluorescence magnitude was about 70% of that obtained with S-1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Presence of a unit for actin-myosin interaction during the superprecipitation of actomyosin.

The interaction of actin with myosin was studied in the presence of ATP at low ionic strength by means of measurements of the actin-activated ATPase activity of myosin and superprecipitation of actomyosin. At high ATP concentrations the ATPase activities of myosin, heavy meromyosin (HMM) and myosin subfragment 1 (S-1) were activated by actin in the same extent. At low ATP concentrations the myosin ATPase activity was activated about 30-fold by actin, whereas those of HMM and S-1 were stimulated only several-fold. This high actin activation of myosin ATPase was coupled with the occurrence of superprecipitation. The activation of HMM or S-1 ATPase by actin shows a simple hyperbolic dependence on actin concentration, but the myosin ATPase was maximally activated by actin at a 2:1 molar ratio of actin to myosin, and a further increase in the actin concentration had no effect on the activation. These results suggest the presence of a unit for actin-myosin interaction, composed of two actin monomers and one myosin molecule in the filaments.     

Oxygen exchange kinetics of porcine cardiac acto-subfragment 1

Recent studies have shown that the KATPase of porcine cardiac S-1 is severalfold stronger than Kbinding. As with skeletal S-1, the four-state model can only explain this observation with the assumption that the release of the products of hydrolysis is rapid and not rate limiting. However, if the release of products is fast, the four-state model predicts that the extent of oxygen exchange with porcine cardiac S-1 should fall toward zero at high actin concentrations, as previously observed with skeletal acto-S-1. In the current work, we show that, in fact, the extent of oxygen exchange for porcine cardiac S-1 remains significant even at infinite actin concentration (i.e., with cross-linked actin-S-1) and that, therefore, the four-state model cannot adequately account for the oxygen exchange data and the ratio of Kbinding to KATPase simultaneously. As in the skeletal case, in order for the six-state model to account for these data, it is necessary to assume that Pi rotation in the acto-S-1.ADP.Pi state is rate limiting for oxygen exchange.     

Mechanism of actomyosin adenosine triphosphatase. Evidence that adenosine 5'-triphosphate hydrolysis can occur without dissociation of the actomyosin complex.

We have investigated the steps in the actomyosin ATPase cycle that determine the maximum ATPase rate (Vmax) and the binding between myosin subfragment one (S-1) and actin which occurs when the ATPase activity is close to Vmax. We find that the forward rate constant of the initial ATP hydrolysis (initial Pi burst) is about 5 times faster than the maximum turnover rate of the actin S-1 ATPase. Thus, another step in the cycle must be considerably slower than the forward rate of the initial Pi burst. If this slower step occurs only when S-1 is complexed with actin, as originally predicted by the Lymn-Taylor model, the ATPase activity and the fraction of S-1 bound to actin in the steady state should increase almost in parallel as the actin concentration is increased. As measured by turbidity determined in the stopped-flow apparatus, the fraction of S-1 bound to actin, like the ATPase activity, shows a hyperbolic dependence on actin concentration, approaching 100% asymptotically. However, the actin concentration required so that 50% of the S-1 is bound to actin is about 4 times greater than the actin concentration required for half-maximal ATPase activity. Thus, as previously found at 0 degrees C, at 15 degrees C much of the S-1 is dissociated from actin when the ATPase is close to Vmax, showing that a slow first-order transition which follows the initial Pi burst (the transition from the refractory to the nonrefractory state) must be the slowest step in the ATPase cycle. Stopped-flow studies also reveal that the steady-state turbidity level is reached almost instantaneously after the S-1, actin, and ATP are mixed, regardless of the order of mixing. Thus, the binding between S-1 and actin which is observed in the steady state is due to a rapid equilibrium between S-1--ATP and acto--S-1--ATP which is shifted toward acto-S-1--ATP at high actin concentration. Furthermore, both S-1--ATP and S-1--ADP.Pi (the state occurring immediately after the initial Pi burst) appear to have the same binding constant to actin. Thus, at high actin concentration both S-1--ATP and S-1--ADP.Pi are in rapid equilibrium with their respective actin complexes. Although at very high actin concentration almost complete binding of S-1--ATP and S-1--ADP.Pi to actin occurs, there is no inhibition of the ATPase activity at high actin concentration. This strongly suggests that both the initial Pi burst and the slow rate-limiting transition which follows (the transition from the refractory to the nonrefractory state) occur at about the same rates whether the S-1 is bound to or dissociated from actin. We, therefore, conclude that S-1 does not have to dissociate from actin each time an ATP molecule is hydrolyzed.     

Measurement of the reversibility of ATP binding to myosin in calcium-activated skinned fibers from rabbit skeletal muscle. Oxygen exchange between water and ATP released to the solution

We have measured the rate constant for ATP release from myosin heads of Ca2+-activated, demembranated muscle fibers using the technique of phosphate-water oxygen exchange. Single rabbit psoas fibers were held in an activating solution in [18O]water ([MgATP] = 8 mM, ionic strength = 0.2 M, pH = 7.0, 24 degrees C). After about 20% hydrolysis of ATP, product Pi and remaining ATP were isolated, and the distribution of 18O in both molecules was analyzed using a mass spectrometer. The exchange in Pi was similar to that previously reported (Hibberd, M. G., Webb, M. R., Goldman, Y. E., and Trentham, D. R. (1985) J. Biol. Chem. 260, 3496-3501). The amount of 18O in ATP gave a rate constant of about 4 s-1 for ATP release, if it is assumed that each rate constant in the pathway of ATP hydrolysis has the same value for all myosin ATPase sites. However, the distribution of 18O in both released Pi and ATP is not well explained by a single pathway for ATP hydrolysis. We present a model that indicates how such distributions could arise from a range of values for the rate constants for Pi and ATP release from actomyosin, and this range is determined by differences in the amounts of strain in attached crossbridges. The kinetic information obtained from these isotope exchange experiments is compared to show that they give a compatible set of rate constants for actomyosin in fibers.     

Oxygen exchange between phosphate and water accompanies calcium-regulated ATPase activity of skinned fibers from rabbit skeletal muscle

The extent of oxygen exchange between phosphate and water has been measured for the calcium-regulated magnesium-dependent ATPase activity of chemically skinned fibers from rabbit skeletal muscle. The oxygen exchange was determined for isometrically held fibers by measuring with a mass spectrometer the distribution of 18O atoms in the product inorganic phosphate when ATP hydrolysis was carried out in H2(18)O. The extent of exchange was much greater in relaxed muscle (free Ca2+ less than 10(-8) M) than in calcium-activated muscle (free Ca2+ approximately equal to 3 X 10(-5) M). Activated fibers had an ATPase activity at least 30-fold greater than the relaxed fibers. These results correlate well with the extents of oxygen exchange accompanying magnesium-dependent myosin and unregulated actomyosin ATPase activities, respectively. In relaxed fibers, comparison of the amount of exchange with the ATPase activity suggests that the rate constant for the reformation of myosin-bound ATP from the myosin products complex is about 10 s-1 at 20 degrees C and pH 7.1. In each experiment the distribution of 18O in the Pi formed was incompatible with a single pathway for ATP hydrolysis. In the case of the calcium-activated fibers, the multiple pathways for ATP hydrolysis appeared to be an intrinsic property of the actomyosin ATPase in the fiber. These results indicate that in muscle fibers, as in isolated actomyosin, cleavage of protein-bound ATP is readily reversible and that association of the myosin products complex with actin promotes Pi release.     

Oxygen exchange between Pi in the medium and water during ATP hydrolysis mediated by skinned fibers from rabbit skeletal muscle. Evidence for Pi binding to a force-generating state

Oxygen exchange between (18O4)Pi in the medium and water accompanies ATP hydrolysis catalyzed by the calcium-regulated MgATPase of vertebrate skeletal muscle. Exchange was observed in chemically skinned fibers from rabbit psoas muscle held isometrically and activated by 30 microM free Ca2+. The rate of exchange was approximately proportional to Pi concentration (up to 10 mM) and was characterized by an apparent second order rate constant greater than or equal to 475 M-1 S-1 (pH 7.1, ionic strength 0.2 M, 22 degrees C). Much less exchange occurred in the absence of Ca2+ or when ATP was replaced by ADP. It has been inferred from mechanical experiments that Pi can bind to a force-generating ADP-bound state of actomyosin with resultant suppression of force (Hibberd, M. G., Dantzig, J. A., Trentham, D. R., and Goldman, Y. E. (1985) Science 228, 1317-1319). The oxygen exchange results support this inference by providing direct evidence that Pi in the medium binds at the ATPase catalytic site in activated isometric fibers. The inter-relationship of these two effects involving Pi on mechanochemical coupling in muscle is discussed.     

Structure and function of the two heads of the myosin molecule. IV. Physiological functions of various reaction intermediates in myosin adenosinetriphosphatase, studied by the interaction between actomyosin and 8-bromoadenosine triphosphate.

The kinetic properties of the hydrolyses of 8-Br ATP and 8-SCH3 ATP by myosin [EC 3.6.1.3] and actomyosin were compared with those of ATP, and the following results were obtained. The Ca-NTPase activities of myosin using these two ATP analogs as substrates were smaller than that of ATPase, and the NTPase activities toward these analogs were strongly suppressed by EDTA. The Mg-NTPase activities toward these analogs were higher in a medium of high ionic strength than in a medium of low ionic strength, in contrast to the activity of Mg-ATPase. These analogs did not produce any initial burst of Pi liberation, activation of myosin NTPase by F-actin, or superprecipitation of actomyosin. The interactions between 8-Br ATP and HMM, acto-HMM, actomyosin, and myofibrils were studied in detail in the presence of Mg2+ in medium of low ionic strength. The Michaelis constant, Km, and the maximum rate, Vm, of 8-Br ATPase of HMM were 27 muM and 21 min-1, respectively. The fluorescence change of HMM induced by 8-Br ATP also followed the Michaelis-Menten equation, and the Michaelis constant, Kf1, was as low as 4 muM. Acto-HMM and acto-S-1 were fully dissociated by the addition of 8-Br ATP. The relation between the extent of dissociation of acto-HMM and the concentration of 8-Br ATP followed the Michaelis-Menten equation, and the apparent dissociation constant, Kd, was 22 muM. This Kd value is almost equal to the Km value of 8-Br ATPase of HMM described above. Myofibrillar contraction was not supported by 8-Br ATP. It was concluded that in the myosin NTPase reaction with 8-Br ATP as a substrate, M2NTP but not MNDPP is formed in route (1), while MNTP is formed in route (2). It was also concluded that the key intermediate for the actomyosin NTPase reaction is MNDPP, and that dissociation of acto-HMM is induced by the formation of M2NTP and MNTP in routes (1) and (2), respectively.     

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

Relaxation of both smooth and skeletal muscles appears to be caused primarily by inhibition of the step associated with Pi release in the actomyosin ATPase cycle, rather than by a block in the binding of the myosin X ATP and myosin X ADP X Pi complexes to actin. In skeletal muscle, troponin-tropomyosin not only causes marked inhibition of Pi release, but it also markedly inhibits the binding of myosin subfragment-1 X ADP to actin, raising the possibility that the two phenomena are coupled in some way. In the present study we determined whether phosphorylation of smooth muscle heavy meromyosin (HMM) also affects both the binding of HMM X ADP to actin and the Pi release step. This was done by having phosphorylated and unphosphorylated HMM X ADP compete for sites on F-actin. At mu = 30 mM, phosphorylation increased the affinity of the HMM molecule for actin about 12-fold and at mu = 170 mM, there was less than a 3-fold increase in the affinity of HMM. If phosphorylation affects the binding of each head of HMM to the same extent, then phosphorylation caused about a 4- and 2-fold increase in the affinity of each head of HMM for actin at mu = 30 and 170 mM, respectively. In contrast, at both ionic strengths, phosphorylation caused more than 100-fold actin activation of the ATPase activity of smooth muscle HMM. Therefore, the marked activation of Pi release in the acto X HMM ATPase cycle upon phosphorylation of HMM is not accompanied by a comparable increase in the affinity of HMM X ADP for actin. We have also found that phosphorylation increases by only 4-fold the rate of Pi release from HMM alone. These results suggest that in smooth muscle, phosphorylation accelerates the step associated with the release of Pi both in the forward and the reverse direction without correspondingly affecting the binding of myosin X ADP to actin.     

Regulation of the adenosinetriphosphatase activity of cross-linked actin-myosin subfragment 1 by troponin-tropomyosin

Chalovich and Eisenberg [Chalovich, J. M., & Eisenberg, E. (1982) J. Biol. Chem. 257, 2432-2437] have suggested that at low ionic strength, troponin-tropomyosin regulates the actomyosin ATPase activity by inhibiting a kinetic step in the actomyosin ATPase cycle rather than by blocking the binding of myosin subfragment 1 (S-1) to actin. This leads to the prediction that troponin-tropomyosin should inhibit the ATPase activity of the complex of actin and S-1 (acto . S-1) even when S-1 is cross-linked to actin. We now find that the ATPase activity of cross-linked actin . S-1 prepared under milder conditions than those used by Mornet et al. [Mornet, D., Bertrand, R., Pantel, P., Audemard, E., & Kassab, R. (1981) Nature (London) 292, 301-306] is inhibited 90% by troponin-tropomyosin in the absence of Ca2+. At mu = 18 mM, 25 degrees C, the ATPase activity of this cross-linked preparation is only about 2-fold greater than the maximal actin-activated ATPase activity of S-1 obtained with regulated actin in the absence of Ca2+. At physiological ionic strength, the ATPase activity of this cross-linked actin . S-1 preparation is inhibited about 95% by troponin-tropomyosin. Since cross-linked S-1 behaves kinetically like S-1 in the presence of infinite actin concentration, it is very unlikely that inhibition of the ATPase activity of cross-linked actin . S-1 is due to blocking of the binding of S-1 to actin. Therefore, these results are in agreement with the suggestion that troponin-tropomyosin regulates primarily by inhibiting a kinetic step in the ATPase cycle.     

Modeling of the actomyosin ATPase activity

In the rapid “quench” kientics of myosin, the “initial phosphate burst” is the excess inorganic phosphate that is produced during the early time-course of ATP hydrolysis by myosin subfragment-1 (S-1) or HMM. In general, the existence of a Pi burst implies a rapid (i.e., generally an order of magnitude faster than the steady-state hydrolysis rate) lysis of the phospho-anhydride bond within the ATP molecule, followed by one or more slower steps that are rate limiting for the process. Thus, the presence of a Pi burst can provide an important clue to the mechanism of the reaction. However, in the case of actomyosin, this clue as long been the subject of controversy and misunderstanding. To measure the (initial) Pi burst, myosin S-1 (or HMM) is rapidly mixed with ATP and then the mixture is acid quenched after a specific time period. The medium produced contains free Pi generated from hydrolysis of the ATP. The quantitative measure of the phosphate generated in this way has always been significantly greater than that expected by steady-state “release” of Pi alone, and it is that very difference between this measured Pi after the quench and that amount of Pi expected to be released by steady-state considerations in that same time period that has been referred to as the “initial Pi burst”. Recent investigations of the kinetics of Pi release have used an entirely new method that directly measures the release of Pi from the enzyme-product complex. These studies have made reference to the properties of the “initial Pi burst” in the presence of actin, as well as to a new kinetic entity: the “burst of Pi release”, and have been often vague concerning the true nature of the initial Pi burst, as well as the properties of Pi release as predicted by the current models of the actin activation of the myosin ATPase activity. The purpose of the current article is to correct this oversight, to discuss the “burst” in some detail, and to display the kinetics predicted by the current models for the actin activation of myosin. Furthermore, predictions for the kinetics of the new “burst of Pi release” are discussed in terms of its ability to discriminate between the two current competing models for actin activation of the myosin ATPase activity.     

Biochemical kinetics of skeletal actosubfragment-1 at high subfragment-1 concentrations.

The actomyosin ATPase activity of skeletal myosin subfragment-1 (S-1) is typically studied by keeping the S-1 concentration low and varying the actin concentration. General agreement exists over the kinetic data observed. Another way of studying the ATPase activity is to keep the actin concentration low and vary the S-1 concentration. The picture that has emerged is that the maximal ATPase rate (per micromolar actin), Vamax, is several fold greater than the Vsmax measured at fixed S-1. Likewise, the apparent activation constant Kam is several fold weaker than KATPase. In addition it is found that Kam, henceforth Kam(At), varies with the total actin concentration At, but controversy continues over the actin dependence of Vamax. Of particular interest is the fact that the Lymn-Taylor and refractory state models could not account for the data. Here we have repeated studies on the ATPase activity at fixed actin concentration in an attempt to determine if the current models for the actin activated myosin ATPase activity can account for both the constant actin and constant S-1 data simultaneously, or if these data imply that new kinetic models need be postulated. We conclude that the current kinetic models can account for the data.     

Structure and function of the two heads of the myosin molecule. III. Cooperativity of the two heads of the myosin molecule, shown by the effect of modification of head A with rho-chloromercuribenzoate on the interaction of head B with F-actin.

Subfragment-1 of HMM was prepared by tryptic [EC 3.4.21.4] digestion of HMM, which had been modified with 1 mole of CMB per mole of HMM at a specific SH group, SHr. S-1(T) obtained from CMB-HMM retained almost all the CMB, and the amount of bound CMB was about 0.8-0.9 mole per 2 moles of S-1(T). S-2 of CMB-HMM contained no bound CMB. The ATPase [EC 3.6.1.3] activity of HMM increased gradually with increase in the concentration of FA, and the acto-HMM ATPase was inhibited by excess substrate or removal of Ca2+ ions in the presence of RP. The ATPase activity of CMB-HMM increased to a maximum level on adding a small amount of FA, and the acto-CMB-HMM ATPase showed neither substrate inhibition nor Ca2+ sensitivity in the presence of RP. On the other hand, the dependence on the concentration of FA of the ATPase activity of acto-S-1(T) was unaffected by modification of S-1 with CMB. The Ca2+ sensitivity of the ATPase activity of acto-S-1(T) in the presence of RP was also unaffected by the modification. Acto-S-1(T) dissociated almost completely, while acto-CMB-S-1(T) was only 50% dissociated on adding ATP. More than 80% of the bound CMB was contained in S-1(T) undissociated from FA. Furthermore, superprecipitation of actomyosin induced by ATP was completely inhibited by adding about 2 moles of CMB-S-1(T) per mole of actin monomer. On the other hand, about 90% of the burst size of Pi liberation was retained in S-1(T) dissociated from FA. It was concluded that the two heads of the myosin molecule are different: one shows the initial burst of Pi liberation, and does not contain the SHr group which binds CMB (head B), and the other does not show the initial burst and contains the SHr group (head A). It was also concluded that modification of head A of HMM or myosin with CMB increases its binding strength to FA, and consequently the substrate inhibition and Ca2+ sensitivity of acto-HMM or actomyosin ATPase at head B are lost on modification of head A with CMB. CMB-S-1(CT) was prepared by chymotryptic [EC 3.4.21.1] digestion of CMB-myosin, and separated into two fractions by ultracentrifugation of acto-CMB-S-1(CT) in the presence of ATP. Three components of CMB-S-1(CT) with molecular weights of 9, 2.4, and 1.2 X 10(4) were separated by SDS-polyacrylamide gel electrophoresis. The ratios of the peak areas of the three components in electrophoretograms were the same in CMB-S-1(CT) and in the two fractions (1 : 0.18 : 0.09), indicating that heads A and B have the same subunit structure.     

Catalysis of oxygen-18 exchange between inorganic phosphate and water by the gastric H,K-ATPase

The gastric H,K-ATPase is shown to catalyze 18O exchange between Pi and HOH. Mg2+ is the only ion required for the reaction. K+ increases the rate of isotope exchange, which is directly proportional to specific ATPase activity. Ouabain, which potently inhibits the Na,K-ATPase, has no effect on the exchange reaction. Conversely, omeprazole, which is specific for the H,K-ATPase, completely inhibits 18O exchange. Vanadate inhibition of exchange can be explained by competitive binding with Pi. The rate of 18O exchange is faster than the hydrolytic rate and about equal to the dephosphorylation rate. Thus, the ionic requirements for exchange, inhibition of exchange, and the rate of exchange are all compatible with catalysis occurring via the same phosphoenzyme intermediate formed during hydrolysis of ATP. The distribution of 18O-labeled Pi species formed with time indicates that Pi loss is only about twice as fast as covalent bond formation. This kinetic pattern is unaffected by K+, temperature, or the specific activity of the enzyme preparation. Invariance of the kinetic pattern could mean isotope exchange is always catalyzed by the same form of the enzyme, and K+ and higher temperature accelerate the reaction by increasing the relative amount of the active conformer. Independence of the kinetic pattern from specific activity implies that the catalytic mechanism of active enzyme molecules is unaffected by inactive proteins in gastric microsomal membranes.     

Comparison of the reaction of N-ethylmaleimide with myosin and heavy meromyosin subfragment 1

Myosin reacted at low ionic strength with NEM forms an actomyosin which is Ca⁺⁺ insensitive. With HMM S-1 the reaction with NEM causes a marked loss of the actin activated ATPase activity and the Ca⁺⁺ sensitivity is reduced but not eliminated. The presence of actin during the sulfhydryl reaction does not significantly alter this result. HMM S-1 prepared from myosin previously desensitized by NEM regains Ca⁺⁺ sensitivity. These results indicate that the conformations of myosin and HMM S-1 are different and could reflect a difference between insoluble (filamentous) myosin and myosin, or its fragments, in solution.     

Pre-steady-state kinetic evidence for a cyclic interaction of myosin subfragment one with actin during the hydrolysis of adenosine 5'-triphosphate.

A single cycle of adenosine 5'-triphosphate (ATP) hydrolysis by a complex of actin and myosin subfragment one (acto-S-1) was studied in a stopped-flow apparatus at low temperature and low ionic strength, using light scattering to monitor the interaction of S-1 with actin and fluorescence to detect the formation of fluorescent intermediates. Our results show that the addition of a stoichiometric concentration of ATP to the acto-S-1 causes a cycle consisting of first, a rapid dissociation of the S-1 from actin by ATP; second, a slower fluorescence change in the S-1 that may be related to the initial phosphate burst; and third, a much slower rate limiting recombination of the S-1 with actin. This latter step equals the acto-S-1 steady-state adenosine 5'-triphosphatase (ATPase) rate at both low and high actin concentrations, and like the steady-state ATPase levels off at a V max of 0.9s-1 at high actin concentration. Therefore, the release of adenosine 5'-diphosphate and inorganic phosphate is not the rate-limiting step in the acto-S-1 ATPase. Rather, a slow first-order step corresponding to the previously postulated transition from the refractory to the nonrefractory state precedes the rebinding of the S-1 to the actin during each cycle of ATP hydrolysis.     

Kinetics of ATP and inorganic phosphate release during hydrolysis of ATP by rabbit skeletal actomyosin subfragment 1. Oxygen exchange between water and ATP or phosphate

We have used the technique of phosphate: water oxygen exchange to measure the rate of ATP and Pi release and Pi binding to myosin subfragment 1 and actomyosin subfragment 1 from rabbit skeletal muscle. The oxygen exchange distributions for ATP and Pi release fit a simple kinetic model with a single set of rate constants for each step. For actomyosin subfragment 1 (20 degrees C, pH 7.0, I = 50 mM), the rate constant governing ATP release is approximately 8 s-1, Pi release is at approximately 60 s-1 and Pi rebinds to an ADP state at greater than 120 M-1 s-1. These rate constants are similar to those that may occur for undistorted cross-bridges within glycerinated rabbit psoas fibers (Bowater, R., Webb, M. R., and Ferenczi, M. A. (1989) J. Biol. Chem. 264, 7193-7201.     

The “Steric blocking model”, the “six-state model”, and the ATPase activity of regulated actomyosin

There has been a great deal of interest in the regulation of muscle contraction. Prior biochemical studies have demonstrated that the binding of regulated actin to S-1-ATP is unchanged at low Ca²⁺, even though the ATPase activity of regulated actomyosin is inhibited under these conditions. Prior structural studies using X-ray diffraction techniques have suggested that the tropomyosin-troponin complex may move and inhibit the actomyosin interaction at low Ca²⁺ (i.e., steric blocking). In physiologic fiber experiments, “weak” binding crossbridges have been found to bind to the actin filament at low Ca²⁺, especially at low ionic strength, and other experiments have suggested that Pi release is not directly regulated by calcium. In biochemical studies in the absence of ATP, inhibition of the binding of strong binding states have been reported in both equilibrium and transient kinetic studies. The current work suggests that all of these observations can be explained in terms of a six-state model in which regulation affects one particular actomyo sin state that contains both strongly bound ADP and Pi. This further implies that regulation affects both a kinetic transition as well as a weak binding constant.     

Caldesmon inhibits the cooperative turning-on of the smooth muscle heavy meromyosin by tropomyosin-actin

The 38-kDa chymotryptic fragment of caldesmon, which possesses the actin/calmodulin binding domain, was purified and utilized to study the mechanism for the inhibition of acto-myosin ATPase by caldesmon. The intact caldesmon inhibited the acto-HMM ATPase although it caused an increase in the binding of HMM to actin, presumably due to the interaction between the S-2 region of HMM and the caldesmon located on the actin filament. The 38-kDa fragment, which lacks the S-2 binding domain, inhibited both the acto-HMM ATPase and the HMM binding to actin. The ATPase and the HMM binding to actin decreased in parallel on increasing the 38-kDa fragment bound to actin. In the presence of tropomyosin, the ATPase activity fell more rapidly than did the HMM binding to actin. Binding of intact caldesmon or 38-kDa fragment to actin inhibited the cooperative turning-on of tropomyosin-actin by NEM.S-1, which forms rigor complexes in the presence of ATP. The absence of cooperative turning-on of the acto-HMM ATPase by rigor complexes in the presence of 38-kDa fragment was associated with an inhibition of the binding of HMM to tropomyosin-actin. Addition of NEM.S-1 to tropomyosin-actin-caldesmon caused a gradual decrease in the caldesmon-induced binding of HMM to actin. The calmodulin restored the caldesmon-induced binding of HMM to tropomyosin-actin, but it had only a slight effect on the acto-HMM ATPase. These data suggest that the cooperative turning-on of the smooth muscle tropomyosin-actin by rigor bonds is modulated by the interaction of caldesmon, tropomyosin, and calmodulin on the thin filament.     

Actomyosin interactions in the presence of ATP and the N-terminal segment of actin.

The binding of myosin subfragment 1 (S-1) to actin in the presence of ATP and the acto-S-1 ATPase activities of acto-S-1 complexes were determined at 5 degrees C under conditions of partial saturation of actin, up to 90%, by antibodies against the first seven N-terminal residues on actin. The antibodies [Fab(1-7)] inhibited strongly the acto-S-1 ATPase and the binding of S-1 to actin in the presence of ATP at low concentrations of S-1, up to 25 microM. Further increases in S-1 concentration resulted in a partial and cooperative recovery of both the binding of S-1 to actin and the acto-S-1 ATPase while causing only limited displacement of Fab(1-7) from actin. The extent to which the binding and the ATPase activity were recovered depended on the saturation of actin by Fab(1-7). The combined amounts of S-1 and Fab binding to actin suggested that the activation of the myosin ATPase activity was due to actin free of Fab. Examination of the acto-S-1 ATPase activities as a function of S-1 bound to actin at different levels of actin saturation by Fab(1-7) revealed that the antibodies inhibited the activation of the bound myosin. Thus, the binding of antibodies to the N-terminal segment of actin can act to inhibit both the binding of S-1 to actin in the presence of ATP and a catalytic step in ATP hydrolysis by actomyosin. The implications of these results to the regulation of actomyosin interaction are discussed.