Hypertrophic Cardiomyopathy Mutations of Troponin Reveal Details of Striated Muscle Regulation

Striated muscle contraction is inhibited by the actin associated proteins tropomyosin, troponin T, troponin I and troponin C. Binding of Ca²⁺ to troponin C relieves this inhibition by changing contacts among the regulatory components and ultimately repositioning tropomyosin on the actin filament creating a state that is permissive for contraction. Several lines of evidence suggest that there are three possible positions of tropomyosin on actin commonly called Blocked, Closed/Calcium and Open or Myosin states. These states are thought to correlate with different functional states of the contractile system: inactive-Ca²⁺-free, inactive-Ca²⁺-bound and active. The inactive-Ca²⁺-free state is highly occupied at low free Ca²⁺ levels. However, saturating Ca²⁺ produces a mixture of inactive and active states making study of the individual states difficult. Disease causing mutations of troponin, as well as phosphomimetic mutations change the stabilities of the states of the regulatory complex thus providing tools for studying individual states. Mutants of troponin are available to stabilize each of three structural states. Particular attention is given to the hypertrophic cardiomyopathy causing mutation, Δ14 of TnT, that is missing the last 14 C-terminal residues of cardiac troponin T. Removal of the basic residues in this region eliminates the inactive-Ca²⁺-free state. The major state occupied with Δ14 TnT at inactivating Ca²⁺ levels resembles the inactive-Ca²⁺-bound state in function and in displacement of TnI from actin-tropomyosin. Addition of Ca²⁺, with Δ14TnT, shifts the equilibrium between the inactive-Ca²⁺-bound and the active state to favor that latter state. These mutants suggest a unique role for the C-terminal region of Troponin T as a brake to limit Ca²⁺ activation.

Inhibition of contraction and myosin light chain phosphorylation in guinea-pig smooth muscle by p21-activated kinase 1

The p21-activated protein kinases (PAKs) have been implicated in cytoskeletal rearrangements and modulation of non-muscle contractility. Little, however, is known about the role of the PAK family members in smooth muscle contraction. Therefore, we investigated the effect of the predominant isoform in vascular smooth muscle cells, PAK1, on contraction and phosphorylation of the regulatory light chains of myosin (r-MLC) in Triton-skinned guinea-pig smooth muscle. We also investigated which of the three putative substrates at the contractile apparatus - MLCK, caldesmon or r-MLC - is phosphorylated by PAK1 in smooth muscle tissue. Incubation of Triton-skinned carotid artery and taenia coli from guinea-pig with an active mutant of PAK1 in relaxing solution for 30-60 min resulted in inhibition of submaximal force by about 50 %. The mechanism of inhibition of force was studied in the Triton-skinned taenia coli. In this preparation, inhibition of force was associated with a respective inhibition of r-MLC phosphorylation. In the presence of the myosin phosphatase inhibitor, microcystin-LR (10 microM), the rate of contraction and r-MLC phosphorylation elicited at pCa 6.79 were both decreased. Because under these conditions the rate of r-MLC phosphorylation is solely dependent on MLCK activity, this result suggests that the inhibitory effect of PAK1 on steady-state force and r-MLC phosphorylation is due to inhibition of MLCK. In line with this, we found that MLCK was significantly phosphorylated by PAK1 while there was very little 32P incorporation into caldesmon. PAK1 phosphorylated isolated r-MLC but not those in the skinned fibres or in purified smooth muscle myosin II. In conclusion, these results suggest that PAK1 attenuates contraction of skinned smooth muscle by phosphorylating and inhibiting MLCK.

Theoretical studies on competitive binding of caldesmon and myosin S1 to actin: prediction of apparent cooperativity in equilibrium and slow-down in kinetics of S1 binding by caldesmon

Several laboratories have reported cooperative binding of S1 to actin in the presence of caldesmon. This cooperative binding has been interpreted with a model similar to that proposed for the binding of S1 to regulated actins in which the binding affinity of S1 is controlled by the position of the tropomyosin filaments. In a recent paper [Sen, A., Chen, Y., Yan, B., and Chalovich, J. M. (2001) Biochemistry 40, 5757-64], we showed qualitatively that S1 binding resulted in rapid dissociation of caldesmon from actin or actin-tropomyosin. This suggests that the cooperativity observed in the case of caldesmon is not due to a conformational change in actin-caldesmon but to the displacement of caldesmon. We show in this paper that the pure competitive binding model, in which both S1 and caldesmon are competing for the same binding sites on actin, can simulate quantitatively the effect of caldesmon on both the equilibrium and the kinetics of S1 binding to actin. This model successfully predicts an apparent cooperativity for the binding of S1 to actin-caldesmon without the need to assume multiple actin-caldesmon structures and produces a decreased rate of S1 binding to actin in the presence of caldesmon. This suggests that the inhibitory action of caldesmon on the actin-activated ATPase activity of myosin in solution and on the generation of active force in a contracting muscle may be simply due to the blocking of myosin binding sites on actin by caldesmon.

Factors contributing to troponin exchange in myofibrils and in solution

The troponin complex in a muscle fiber can be replaced with exogenous troponin by using a gentle exchange procedure in which the actin-tropomyosin complex is never devoid of a full complement of troponin (Brenner et al. (1999) Biophys J 77: 2677-2691). The mechanism of this exchange process and the factors that influence this exchange are poorly understood. In this study, the exchange process has now been examined in myofibrils and in solution. In myofibrils under rigor conditions, troponin exchange occurred preferentially in the region of overlap between actin and myosin when the free Ca2+ concentration was low. At higher concentrations of Ca2+, the exchange occurred uniformly along the actin. Ca2+ also accelerated troponin exchange in solution but the effect of S1 could not be confirmed in solution experiments. The rate of exchange in solution was insensitive to moderate changes in pH or ionic strength. Increasing the temperature resulted in a two-fold increase in rate with each 10 degrees C increase in temperature. A sequential two step model of troponin binding to actin-tropomyosin could simulate the observed association and dissociation transients. In the absence of Ca2+ or rigor S1, the following rate constants could describe the binding process: k1 = 7.12 microM(-1) s(-1), k(-1) = 0.65 s(-1), k2 = 0.07 s(-1), k(-2) = 0.0014 s(-1). The slow rate of detachment of troponin from actin (k(-2)) limits the rate of exchange in solution and most likely contributes to the slow rate of exchange in fibers.

Caldesmon reduces the apparent rate of binding of myosin S1 to actin-tropomyosin

Equilibrium measurements of the rate of binding of caldesmon and myosin S1 to actin-tropomyosin from different laboratories have yielded different results and have led to different models of caldesmon function. An alternate approach to answering these questions is to study the kinetics of binding of both caldesmon and S1 to actin. We observed that caldesmon decreased the rate of binding of S1 to actin in a concentration-dependent manner. The inhibition of the rate of S1 binding was enhanced by tropomyosin, but the effect of tropomyosin on the binding was small. Premixing actin with S1 reduced the amplitude (extent) of caldesmon binding in proportion to the fraction of actin that contained bound S1, but the rate of binding of caldesmon to free sites was not greatly altered. No evidence for a stable caldesmon-actin-tropomyosin-S1 complex was observed, although S1 did apparently bind to gaps between caldesmon molecules. These results indicate that experiments involving caldesmon, actin, tropomyosin, and myosin are inherently complex. When the concentration of either S1 or caldesmon is varied, the amount of the other component bound to actin-tropomyosin cannot be assumed to remain fixed. The results are not readily explained by a mechanism in which caldesmon acts only by stabilizing an inactive state of actin-tropomyosin. The results support regulatory mechanisms that involve changes in the actin-S1 interaction.

Theoretical kinetic studies of models for binding myosin subfragment-1 to regulated actin: Hill model versus Geeves model

It was previously shown that a one-dimensional Ising model could successfully simulate the equilibrium binding of myosin S1 to regulated actin filaments (T. L. Hill, E. Eisenberg and L. Greene, Proc. Natl. Acad. Sci. U.S.A. 77:3186-3190, 1980). However, the time course of myosin S1 binding to regulated actin was thought to be incompatible with this model, and a three-state model was subsequently developed (D. F. McKillop and M. A. Geeves, Biophys. J. 65:693-701, 1993). A quantitative analysis of the predicted time course of myosin S1 binding to regulated actin, however, was never done for either model. Here we present the procedure for the theoretical evaluation of the time course of myosin S1 binding for both models and then show that 1) the Hill model can predict the "lag" in the binding of myosin S1 to regulated actin that is observed in the absence of Ca++ when S1 is in excess of actin, and 2) both models generate very similar families of binding curves when [S1]/[actin] is varied. This result shows that, just based on the equilibrium and pre-steady-state kinetic binding data alone, it is not possible to differentiate between the two models. Thus, the model of Hill et al. cannot be ruled out on the basis of existing pre-steady-state and equilibrium binding data. Physical mechanisms underlying the generation of the lag in the Hill model are discussed.

Calponin interaction with alpha-actinin-actin: evidence for a structural role for calponin

The purpose of this study was to address the paradox of calponin localization with alpha-actinin and filamin, two proteins with tandem calponin homology (CH) domains, by determining the effect of these proteins on the binding of calponin to actin. The results show that actin can accommodate near-saturating concentrations of either calponin and alpha-actinin or calponin and filamin with little change or no change in ligand affinity. Little direct interaction occurred between alpha-actinin and calponin in the absence of actin, so this effect is not likely to explain the co-distribution of these proteins. Calponin, like alpha-actinin, induced elastic gel formation when added to actin. When alpha-actinin was added to newly formed calponin/actin gels, no change was seen in the mechanical properties of the gel compared to calponin and actin alone. However, when calponin was added to newly formed alpha-actinin/actin gels, the resulting gel was much stronger than the gels formed by either ligand alone. Furthermore, gels formed by the addition of calponin to alpha-actinin/actin exhibited a phenomenon known as strain hardening, a characteristic of mechanically resilient gels. These results add weight to the concept that one of the functions of calponin is to stabilize the actin cytoskeleton.

Thin filament activation probed by fluorescence of N-((2-(iodoacetoxy)ethyl)-N-methyl)amino-7-nitrobenz-2-oxa-1,3-diazole-labeled troponin I incorporated into skinned fibers of rabbit psoas muscle

A method is described for the exchange of native troponin of single rabbit psoas muscle fibers for externally applied troponin complexes without detectable impairment of functional properties of the skinned fibers. This approach is used to exchange native troponin for rabbit skeletal troponin with a fluorescent label (N-((2-(iodoacetoxy)ethyl)-N-methyl)amino-7-nitrobenz-2-oxa-1, 3-diazole, IANBD) on Cys(133) of the troponin I subunit. IANBD-labeled troponin I has previously been used in solution studies as an indicator for the state of activation of reconstituted actin filaments (. Proc. Natl. Acad. Sci. USA. 77:7209-7213). In the skinned fibers, the fluorescence of this probe is unaffected when cross-bridges in their weak binding states attach to actin filaments but decreases either upon the addition of Ca(2+) or when cross-bridges in their strong binding states attach to actin. Maximum reduction is observed when Ca(2+) is raised to saturating concentrations. Additional attachment of cross-bridges in strong binding states gives no further reduction of fluorescence. Attachment of cross-bridges in strong binding states alone (low Ca(2+) concentration) gives only about half of the maximum reduction seen with the addition of calcium. This illustrates that fluorescence of IANBD-labeled troponin I can be used to evaluate thin filament activation, as previously introduced for solution studies. In addition, at nonsaturating Ca(2+) concentrations IANBD fluorescence can be used for straightforward classification of states of the myosin head as weak binding (nonactivating) and strong binding (activating), irrespective of ionic strength or other experimental conditions. Furthermore, the approach presented here not only can be used as a means of exchanging native skeletal troponin and its subunits for a variety of fluorescently labeled or mutant troponin subunits, but also allows the exchange of native skeletal troponin for cardiac troponin.

Kinetics of thin filament activation probed by fluorescence of N-((2-(iodoacetoxy)ethyl)-N-methyl)amino-7-nitrobenz-2-oxa-1,3-diazole-labeled troponin I incorporated into skinned fibers of rabbit psoas muscle: implications for regulation of muscle contraction

Making use of troponin with fluorescently labeled troponin I subunit (N-((2-(iodoacetoxy)ethyl)-N-methyl)amino-7-nitrobenz-2-oxa-1, 3-diazole-troponin I, IANBD-TnI) that had previously been described in solution studies as a probe for thin filament activation (. Proc. Natl. Acad. Sci. 77:7209-7213), we present a new approach that allows the kinetics of thin filament activation to be studied in skinned muscle fibers. After the exchange of native troponin for fluorescently labeled troponin, the fluorescence intensity is sensitive to both changes in calcium concentration and actin attachment of cross-bridges in their strong binding states (. Biophys. J. 77:000-000). Imposing rapid changes in the fraction of strongly attached cross-bridges, e.g., by switching from isometric contraction to high-speed shortening, causes changes in thin filament activation at fixed Ca(2+) concentrations that can be followed by recording fluorescence intensity. Upon changing to high-speed shortening we observed small (<20%) changes in fluorescence that became faster at higher Ca(2+) concentrations. At all Ca(2+) concentrations, these changes are more than 10-fold faster than force redevelopment subsequent to the period of unloaded shortening. We interpret this as an indication that equilibration among different states of the thin filament is rapid and becomes faster as Ca(2+) is raised. Fast equilibration suggests that the rate constant of force redevelopment is not limited by changes in the activation level of thin filaments induced by the isotonic contraction before force redevelopment. Instead, our modeling shows that, in agreement with our previous proposal for the regulation of muscle contraction, a rapid and Ca(2+)-dependent equilibration among different states of the thin filament can fully account for the Ca(2+) dependence of force redevelopment and the fluorescence changes described in this study.

Fesselin: a novel synaptopodin-like actin binding protein from muscle tissue

A novel actin binding protein has been isolated from chicken gizzard muscle. When isolated, a pair of proteins with apparent molecular weights of 79 kDa and 103 kDa are obtained; both proteins have a pI near 9.3. Peptide mapping indicates that these proteins are related. Antibodies against this protein cross-reacted with proteins from other smooth muscle containing tissues as well as skeletal and heart muscle. Traces of cross-reactive material were also detected in brain and kidney tissue. The affinity of this protein for actin is ca. 1x10(6) M(-1). Interestingly, this actin binding protein is a potent actin-bundling agent. A partial sequence analysis confirmed that there were no previously reported homologues in smooth muscle. However, considerable homology was found with the protein synaptopodin that is found in nervous tissue and kidney but is absent from muscle tissue. It is likely that the new protein is a member of the synaptopodin family. We call the smooth muscle actin binding protein fesselin.

Sarcomeric binding pattern of exogenously added intact caldesmon and its C-terminal 20-kDa fragment in skinned fibers of skeletal muscle

Intact caldesmon and particularly the actin-binding C-terminal fragment (20-kDa) of caldesmon have been shown in skeletal muscle fibers to selectively displace low affinity, weakly bound cross-bridges from actin without significantly altering the actin attachment of force producing, strong binding cross-bridges (Brenner et al., 1991; Kraft et al., 1995a). However, the sarcomeric distribution and the specific binding of externally added caldesmon to the myofilaments of skeletal muscle fibers was not known. It was e.g., unclear whether caldesmon binds along actin in a manner similar to tropomyosin or whether it also binds to myosin. In this study, we determined the binding pattern of exogenously added intact caldesmon and its C-terminal 20-kDa fragment, respectively, in MgATP-relaxed rabbit skeletal muscle fibers using electron (EM) and confocal fluorescence microscopy (CFM). EM showed that similar to what has been demonstrated earlier for smooth muscle thin filaments (Lehman et al., 1989), intact caldesmon binds periodically every 38 nm along the thin filaments. CFM revealed that rhodamine-labeled intact caldesmon and the 20-kDa caldesmon fragment bind along nearly the entire length of the thin filaments. A portion of the I-band near the Z-line appears unlabeled, both when equilibrated at normal and long sarcomere lengths. The width of the unlabeled region seems to depend on ionic strength. The 20-kDa C-terminal caldesmon fragment binds in essentially the same pattern as intact caldesmon. This indicates that the high fluorescence intensity in the overlap region seen with intact caldesmon does not depend on caldesmon binding to myosin. X-ray diffraction was used to monitor the effects of filament lattice. Intact caldesmon at > 0.3 mg/ml induced disorder in the myofilament lattice. No such disordering was observed, however, when fibers were equilibrated with up to 0.8 mg/ml of the 20-kDa caldesmon fragment.

X-ray diffraction studies of the cross-bridge intermediate states

Two dimensional x-ray diffraction was obtained from skinned rabbit psoas muscle fibers. The goal is to correlate structures of the cross-bridge population with various intermediate states in the cross-bridge cycle by using nucleotides and their analogs. It was found that in a relaxed muscle in ATP containing solutions, cross-bridges are distributed in three populations in equilibrium: those detached from actin and ordered on the myosin helix, those that are detached and disordered, and those weakly attached to actin in random orientations. The distribution among the three populations is highly dependent on temperature. Those that are detached and yet ordered in a helical structure surrounding the myosin backbone are very likely in the M.ADP.Pi state, supporting an earlier suggestion by Wray (1987). It was also found that the attached cross-bridges with bound MgADP are structurally distinct from those without nucleotide, in agreement with one of our earlier findings by osmotic compression (Xu et al., 1993). Another finding of interest is that the analog AMP-PNP was found to be an ATP analog, rather than an ADP analog as it has been reported previously by many research groups.

Fluorescence of NBD-labelled troponin-I as a probe for the kinetics of thin filament activation in skeletal muscle fibers

Using fluorescence of NBD-labelled troponin I incorporated into skinned fibers of the rabbit psoas muscle by chasing native troponin by troponin with the NBD-labelled TnI subunit we attempted to study kinetics of thin filament activation at different Ca(++)-concentrations. Since fluorescence of NBD-labelled TnI is sensitive to both, changes in Ca++, and strong cross-bridge attachment, we were able to induce changes thin filament activation by rapidly dropping the fraction of strongly attached cross-bridges to very low levels, e.g. by switching from isometric to isotonic contraction conditions. At any [Ca++], the time course of the resulting changes in fluorescence of NBD-labelled TnI was found at least an order of magnitude faster than the time course of force redevelopment subsequent to the period of isotonic contraction. Modelling shows that with the kinetics of thin filament activation derived from these studies, common kinetic schemes of the actomyosin ATPase predict regulation to act via changes in cross-bridge cycling kinetics, as we had previously proposed.

Caldesmon: binding to actin and myosin and effects on elementary steps in the ATPase cycle

The actin binding protein caldesmon inhibits the actin-activation of myosin ATPase activity. The steps in the cycle of ATP hydrolysis that caldesmon could inhibit include: (1) the binding of myosin to actin, (2) the transition between any two actin-myosin states and (3) the distribution between inactive and active states of actin. The analysis of these possibilities is complicated because caldesmon binds to both myosin and actin and because each caldesmon molecule binds to several actin monomers. This paper reviews procedures for analysing these interactions and summarizes current information on the stability and dynamics of the interaction of caldesmon with actin and myosin. Possible effects of caldesmon on transitions within the ATPase cycle of actomyosin are also discussed.

Characterizations of cross-bridges in the presence of saturating concentrations of MgAMP-PNP in rabbit permeabilized psoas muscle

Several earlier studies have led to different conclusions about the complex of myosin with MgAMP-PNP. It has been suggested that subfragment 1 of myosin (S1)-MgAMP-PNP forms an S1-MgADP-like state, an intermediate between the myosin S1-MgATP and myosin S1-MgADP states or a mixture of cross-bridge states. We suggest that the different states observed result from the failure to saturate S1 with MgAMP-PNP. At saturating MgAMP-PNP, the interaction of myosin S1 with actin is very similar to that which occurs in the presence of MgATP. 1) At 1 degrees C and 170 mM ionic strength the equatorial x-ray diffraction intensity ratio I11/I10 decreased with an increasing MgAMP-PNP concentration and leveled off by approximately 20 mM MgAMP-PNP. The resulting ratio was the same for MgATP-relaxed fibers. 2) The two dimensional x-ray diffraction patterns from MgATP-relaxed and MgAMP-PNP-relaxed bundles are similar. 3) The affinity of S1-MgAMP-PNP for the actin-tropomyosin-troponin complex in solution in the absence of free calcium is comparable with that of S1-MgATP. 4) In the presence of calcium, I11/I10 decreased toward the relaxed value with increasing MgAMP-PNP, signifying that the affinity between cross-bridge and actin is weakened by MgAMP-PNP. 5) The degree to which the equatorial intensity ratio decreases as the ionic strength increases is similar in MgAMP-PNP and MgATP. Therefore, results from both fiber and solution studies suggest that MgAMP-PNP acts as a non hydrolyzable MgATP analogue for myosin.

Caldesmon-actin-tropomyosin contains two types of binding sites for myosin S1

Caldesmon inhibits the activation of myosin ATPase activity by actin-tropomyosin. Caldesmon also inhibits the binding of myosin to actin. There is disagreement as to the degree to which competitive displacement of myosin subfragment binding to actin is responsible for the inhibition of ATPase activity. We have examined the possibility that one or more molecules of S1 may bind to actin-tropomyosin-caldesmon without having the normal actin activation of ATPase activity. The effect of caldesmon on the binding and ATPase activity of S1 was measured at several initial levels of saturation of S1 to determine if a fraction of the bound S1 was resistant to displacement by caldesmon. In the case of both unmodified S1 and rhoPDM-modified S1, most, but not all, of the S1 was displaced by caldesmon. The results are consistent with a single molecule of S1 binding with low affinity for each seven actin monomers that are fully saturated with caldesmon and tropomyosin. This single S1 is not necessarily bound directly to actin but may be attached to the NH2-terminal region of caldesmon.

Modulation of cross-bridge affinity for MgGTP by Ca2+ in skinned fibers of rabbit psoas muscle

Previously we reported that saturation of cross-bridges with MgATP gamma S in skinned muscle fibers was calcium sensitive. In the present study we investigate whether this observation can be generalized to other nucleotides by studying saturation of cross-bridges with MgGTP. In solution, myosin-subfragment 1 (S1) in the presence of 10 mM MgGTP was found to bind to actin with low affinity, similar to that in the presence of MgATP and MgATP gamma S. In EGTA buffer, the equatorial x-ray diffraction intensity ratio I11/I10 recorded in single skinned fibers decreased upon increasing MgGTP concentration from 0 to 10 mM (1 degree C and 170 mM ionic strength). The I11/I10 ratio leveled off at 10 mM MgGTP, indicating full saturation of cross-bridges with the nucleotide. Under these conditions, the value of I11/I10 is indistinguishable from that obtained in the presence of saturating [MgATP]. In CaEGTA buffer, however, the decrease in I11/I10 occurs over a wider range of concentrations, and there is no indication of I11/I10 leveling off at 10 mM MgGTP, suggesting that full saturation is not reached. The Ca2+ dependence of GTP binding appears to be a direct consequence of the differences in the affinities of the strongly bound cross-bridges to actin versus weakly bound cross-bridges to actin. A biochemical scheme that could qualitatively explain the titration behavior of ATP gamma S and GTP is presented.

Inhibition of cross-bridge binding to actin by caldesmon fragments in skinned skeletal muscle fibers

Several regions within the 35-kDa COOH-terminal portion of caldesmon have been implicated in the ability of caldesmon to inhibit actin-activated myosin ATPase activity. To further define the functional regions of caldesmon, we have studied the effects of three chymotryptic fragments, one fragment produced by CNBr digestion and two fragments produced by digestion with submaxillaris arginase C protease, on the relaxed stiffness and active force of rabbit psoas fibers. Each of the regions of caldesmon studied had either direct or indirect effects on single-fiber mechanics. The 35-kDa and 20-kDa fragments of caldesmon, like intact caldesmon, were effective inhibitors of fiber stiffness, a measure of cross-bridge attachment. The 7.3-kDa and 10-kDa fragments, which constitute the NH2 and COOH halves of the 20-kDa fragment, inhibited both relaxed fiber stiffness and active force production, but with a reduced efficacy compared to the 20-kDa fragment. These results suggest that several regions within the 35-kDa COOH-terminal region of caldesmon are required for optimum function of caldesmon and that function includes inhibition of weak cross-bridge attachment and force production.

Radial equilibrium lengths of actomyosin cross-bridges in muscle

Radial equilibrium lengths of the weakly attached, force-generating, and rigor cross-bridges are determined by recording their resistance to osmotic compression. Radial equilibrium length is the surface-to-surface distance between myosin and actin filaments at which attached cross-bridges are, on average, radially undistorted. We previously proposed that differences in the radial equilibrium length represent differences in the structure of the actomyosin cross-bridge. Until now the radial equilibrium length had only been determined for various strongly attached cross-bridge states and was found to be distinct for each state examined. In the present work, we demonstrate that weakly attached cross-bridges, in spite of their low affinity for actin, also exert elastic forces opposing osmotic compression, and they are characterized by a distinct radial equilibrium length (12.0 nm vs. 10.5 nm for force-generating and 13.0 nm for rigor cross-bridge). This suggests significant differences in the molecular structure of the attached cross-bridges under these conditions, e.g., differences in the shape of the myosin head or in the docking of the myosin to actin. Thus, the present finding supports our earlier conclusion that there is a structural change in the attached cross-bridge associated with the transition from a weakly bound configuration to the force-generating configuration. The implications for imposing spatial constraints on modeling actomyosin interaction in the filament lattice are discussed.

Adenosine 5'-(gamma-thiotriphosphate): an ATP analog that should be used with caution in muscle contraction studies

The slowly hydrolyzed ATP analog adenosine 5'-(gamma-thiotriphosphate) (ATP gamma S) has been used in many studies of the muscle motor protein myosin in order to form a stable "weak binding" state analogous to the actin-S1-ATP complex However, the results from studies using ATP gamma S do not always agree with the results of experiments using ATP. The binding of myosin subfragment-1-ATP gamma S to actin has now been studied in some detail to determine its relationship to the actin-S1-ATP state. The binding of myosin subfragment-1-ATP gamma S to actin-troponin-tropomyosin is similar in affinity to the binding of myosin subfragment-1-ATP. Like myosin subfragment-1-ATP, the binding is not Ca(2+)-dependent, and most importantly, myosin subfragment-1-ATP gamma S does not stabilize the active configuration of actin-troponin-tropomyosin. Thus, myosin subfragment-1-ATP gamma S is an analog of myosin subfragment-1-ATP but must be used with caution for two reasons: (1) The binding of ATP gamma S to regulated actomyosin subfragment-1 is Ca(2+)-sensitive, and errors can be made in the interpretation of results if proteins are not fully saturated with nucleotide and a mixture of weak and strong binding states is present. (2) At the high concentrations of myosin subfragment-1 used in some experiments, significant amounts of ADP may form.(ABSTRACT TRUNCATED AT 250 WORDS)

Flexation of caldesmon: effect of conformation on the properties of caldesmon

The contribution of the extended and bent forms of caldesmon to its function was investigated by examining chemically modified forms of this protein. The bent 'hairpin' form of caldesmon was enhanced between pH 6.0 and 8.0 and at low ionic strengths, as reported by an increase in excimer fluorescence of pyrene-labelled caldesmon under these conditions. The presence of nucleotides also produced significant conformational changes in caldesmon, as detected by fluorescence measurements and protease digestions. Titrations of pyrene caldesmon with actin, heavy meromyosin, and calmodulin resulted in a decrease in excimer fluorescence. The function of the bent form of caldesmon was investigated by using intramolecular 1-ethyl-3-(3-dimethylamino propyl) carbodiimide-crosslinked caldesmon. The inhibition of acto-S-1 ATPase activity by crosslinked caldesmon was less efficient compared with that by pyrene modified and control caldesmons. Caldesmon's ability to switch from an activator to an inhibitor of actin-activated ATPase of myosin was also affected by the folding. Cosedimentation experiments revealed normal binding of crosslinked caldesmon to smooth muscle myosin. These results indicate the importance of caldesmon's transition from extended to folded forms and suggest possible functional roles for these different forms of caldesmon.

Characterization of calponin binding to actin

Calponin, a protein isolated from smooth muscle and nonmuscle cells, has previously been shown to inhibit the actin-activated ATPase activity of myosin. Reports of the stoichiometry of binding range from 1 calponin per actin to 1 calponin per 3 actin monomers. We now report a detailed study of the binding of [14C]iodoacetamide-labeled calponin to actin. The labeling procedure did not significantly alter the binding constant of calponin to actin. The stoichiometry of binding was variable and dependent on ionic strength. Below 110 mM ionic strength, the stoichiometry of binding was 1:1. As the ionic strength was increased above 110 mM ionic strength, the stoichiometry shifted from 1:1 to 1 calponin per 2 actin monomers. At physiological ionic strength, the binding exhibited a small degree of positive cooperativity and was adequately described by a single class of binding sites with an association constant of 6 x 10(6) M-1. The affinity decreased to 20% of this value in the presence of ATP. Irrespective of the ionic strength, actin formed bundles when saturation with calponin exceeded about 30%. Measurements of the rate of association were complicated by this bundling, but the upper limit for this reaction was placed at 10(6) M-1 s-1. The addition of calponin to actin-caldesmon complexes caused displacement of the caldesmon.

Parallel inhibition of active force and relaxed fiber stiffness by caldesmon fragments at physiological ionic strength and temperature conditions: additional evidence that weak cross-bridge binding to actin is an essential intermediate for force generation

Previously we showed that stiffness of relaxed fibers and active force generated in single skinned fibers of rabbit psoas muscle are inhibited in parallel by actin-binding fragments of caldesmon, an actin-associated protein of smooth muscle, under conditions in which a large fraction of cross-bridges is weakly attached to actin (ionic strength of 50 mM and temperature of 5 degrees C). These results suggested that weak cross-bridge attachment to actin is essential for force generation. The present study provides evidence that this is also true for physiological ionic strength (170 mM) at temperatures up to 30 degrees C, suggesting that weak cross-bridge binding to actin is generally required for force generation. In addition, we show that the inhibition of active force is not a result of changes in cross-bridge cycling kinetics but apparently results from selective inhibition of weak cross-bridge binding to actin. Together with our previous biochemical, mechanical, and structural studies, these findings support the proposal that weak cross-bridge attachment to actin is an essential intermediate on the path to force generation and are consistent with the concept that isometric force mainly results from an increase in strain of the attached cross-bridge as a result of a structural change associated with the transition from a weakly bound to a strongly bound actomyosin complex. This mechanism is different from the processes responsible for quick tension recovery that were proposed by Huxley and Simmons (Proposed mechanism of force generation in striated muscle. Nature. 233:533-538.) to represent the elementary mechanism of force generation.

Role of ATP in the binding of caldesmon to smooth muscle myosin

We have reported earlier that ATP causes both an increase in the affinity of caldesmon for smooth muscle myosin and a change in stoichiometry from 2 caldesmon molecules per myosin to 1:1 (Hemric & Chalovich, 1990). We now show that this ATP effect does not occur with skeletal muscle myosin, indicating that ATP has a specific effect on the structure of filamentous smooth muscle myosin. This ATP effect does not appear to be due to stabilization of a 10S type of filamentous smooth muscle myosin like that reported earlier (Ikebe & Hartshorne, 1984) since neither phosphorylation nor extensive modification of myosin with MalNEt (both which stabilize the 6S state of monomeric myosin) eliminates the effect of ATP. Caldesmon does bind more tightly to a form of smooth muscle myosin which is resistant to papain digestion. These results suggest that the ATP effect is due to stabilization of a local conformation of smooth muscle myosin which is independent of the larger 10S/6S conformational change (Suzuki et al., 1988). In the presence of ATP, the two heads of smooth muscle muscle myosin and the S-2 region form a single, higher affinity binding region for caldesmon.

Kinetics of binding of caldesmon to actin

The time course of interaction of caldesmon with actin may be monitored by fluorescence changes that occur upon the binding of 12-(N-methyl-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl))-labeled caldesmon to actin or to acrylodan actin. The concentration dependence of the observed rate of caldesmon-actin binding was analyzed to a first approximation as a single-step reaction using a Monte Carlo simulation. The derived association and dissociation rates were 10(7) M-1 s-1 and 18.2 s-1, respectively. Smooth muscle tropomyosin enhances the binding of caldesmon to actin, and this was found to be due to a reduction in the rate of dissociation to 6.3 s-1. There is no evidence from this study for a different mechanism of binding in the presence of tropomyosin. The fluorescence changes that occurred with the binding of 12-(N-methyl-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl))-labeled caldesmon to actin or actin-tropomyosin were reversed by the addition of myosin subfragment 1 as predicted by a competitive binding mechanism.

Distinct molecular processes associated with isometric force generation and rapid tension recovery after quick release

It was proposed by Huxley and Simmons (Nature 1971, 233:533-538) that force-generating cross-bridges are attached to actin in several stable positions. In this concept, isometric force is generated by the same mechanism as the quick tension recovery after an abrupt release of length; i.e., when crossbridges proceed from the first postulated stable position to the second and/or subsequent positions, resulting in straining of the elastic elements within the cross-bridges. Therefore, isometric force is generated by cross-bridges in the second or even subsequent stable positions. However, through mechanical measurements of skinned rabbit psoas muscle fibers, we found that during isometric contraction only the first stable state is significantly occupied; i.e., isometric force is generated by cross-bridges in the first of the stable states. Thus, isometric force and the quick tension recovery appear to result from two distinctly different molecular processes. We propose that isometric force results from a structural change in the actomyosin complex associated with the transition from a weakly bound configuration to a strongly bound configuration before the reaction steps in the Huxley-Simmons model, whereas a major component of quick tension recovery originates from transitions among the subsequent strongly bound states. Mechanical, biochemical, and structural evidence for the two distinct processes is summarized and reviewed.

Turkey gizzard caldesmon molecular weight and shape

The molecular weight of chicken gizzard muscle caldesmon has been measured previously by sedimentation equilibrium in the analytical ultracentrifuge and found to be 93 +/- 4 kDa [P. Graceffa, C.-L. A. Wang, and W.F. Stafford (1988) J. Biol. Chem. 263, 14196-14202]. The molecular weight of turkey gizzard caldesmon has been determined by another group to be 75 +/- 2 kDa by the same technique [D.A. Malecik, J. Ausio, C.E. Byles, B. Modrell, and S.R. Anderson (1989) Biochemistry 28, 8227-8233]. We have reevaluated the molecular weight of the turkey protein by sedimentation equilibrium analysis and found a value of 90 +/- 3 kDa, indicating that turkey gizzard caldesmon is a typical muscle caldesmon and does not belong to the class of smaller nonmuscle caldesmons. The two muscle caldesmons do not comigrate during polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, indicating that they have different amino acid sequences.

Caldesmon, N-terminal yeast actin mutants, and the regulation of actomyosin interactions

N-Terminal yeast actin mutants were used to assess the role of N-terminal acidic residues in the interactions of caldesmon with actin. The yeast actins differed only in their N-terminal charge: wild type, two negative charges; 4Ac, four negative charges; DNEQ, neutral charge; delta DSE, one positive charge. Caldesmon inhibition of actomyosin subfragment 1 ATPase was affected by alterations in the N-terminus of actin. This inhibition was similar for skeletal muscle alpha-actin and the yeast 4Ac and wild-type actins (80%), but much smaller for the neutral and deletion mutants (15%). However, cosedimentation experiments revealed similar binding of caldesmon to polymerized rabbit skeletal muscle alpha-actin and each yeast actin. This result shows that the N-terminal acidic residues of actin are not required for the binding of caldesmon to F-actin. Caldesmon-actin interactions were also examined by monitoring the polymerization of G-actin induced by caldesmon. Although the final extent of polymerization was similar for all actins tested, the rates of polymerization differed. Skeletal muscle and 4Ac actins had similar rates of polymerization, and the wild-type actin polymerized at a slower rate. The neutral and deletion mutants had even slower rates of polymerization by caldesmon. The slow polymerization of DNEQ G-actin was traced to a greatly reduced binding of caldesmon to this mutant G-actin when compared to wild-type and alpha-actin. MgCl2-induced actin polymerization proceeded at identical rates for all actins.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of actin stimulation of skeletal muscle (A1)S-1 ATPase activity by caldesmon

We have previously shown that caldesmon inhibits the actin-activated ATPase activity of myosin subfragments in parallel with inhibition of myosin subfragment.ATP binding to actin (M. E. Hemric, and J. M. Chalovich, 1988, J. Biol. Chem. 263, 1878-1885; L. Velaz, R. H. Ingraham, and J. M. Chalovich, 1990, J. Biol. Chem. 265, 2929-2934). From these data, we suggested that caldesmon is a competitive inhibitor of binding of myosin subfragment-1 to actin. To confirm this result, we now show the effect of caldesmon on the steady-state parameters of ATP hydrolysis by (A1)S-1 at increasing actin concentrations. Low ionic strength conditions were used to maximize the interaction between (A1)S-1 and actin. In both the presence and absence of smooth muscle tropomyosin, caldesmon caused a twofold decrease in the kcat and more than a 12-fold change in the KATPase. Therefore, competition of binding of myosin to actin by caldesmon contributes to the reduction in ATPase activity in both the presence and the absence of tropomyosin.

Characterization of a caldesmon fragment that competes with myosin-ATP binding to actin

The protein caldesmon inhibits actin-activated ATP hydrolysis of myosin and inhibits the binding of myosin.ATP to actin. A fragment isolated from a chymotryptic digest of caldesmon contains features of the intact molecule that make it useful as a selective inhibitor of the binding of myosin.ATP complexes to actin without having the complexity of binding to myosin. The COOH-terminal 20 kDa region of caldesmon binds to actin with one-sixth the affinity of caldesmon with a stoichiometry of binding of one fragment per two actin monomers. This contrasts with the 1:6-9 stoichiometry of intact caldesmon. The binding of the 20 kDa fragments to actin is totally reversed by Ca(2+)-calmodulin and, like intact caldesmon, the 20 kDa fragments are competitive with the binding of myosin subfragments to actin. This competition with myosin binding is largely responsible for the inhibition of ATP hydrolysis, although both the fragments and intact caldesmon also reverse the potentiation of ATPase activity caused by tropomyosin. These polypeptides are useful both in defining the function of caldesmon and in studying the role of weakly bound cross-bridges in muscle.

Reversal of caldesmon binding to myosin with calcium-calmodulin or by phosphorylating caldesmon

Caldesmon, an actin-binding protein from smooth muscle and non-muscle cells, has previously been shown to bind stoichiometrically to smooth muscle myosin in an ATP-dependent manner. We now show quantitatively the effects of Ca(2+)-calmodulin and phosphorylation on the binding of caldesmon to myosin. Ca(2+)-calmodulin reduces the binding of caldesmon to myosin with the same effectiveness as it does the binding of caldesmon to actin. However, Ca(2+)-calmodulin is ineffective in antagonizing the binding of the purified myosin-binding region of caldesmon to myosin. These and other results suggest that Ca(2+)-calmodulin binding to the COOH-terminal region of caldesmon is responsible for reversal of binding to myosin. Phosphorylation of the NH2-terminal region of caldesmon by the co-purifying kinase, calmodulin-dependent protein kinase II, weakens but does not eliminate the binding of caldesmon to smooth muscle myosin. Finally, phosphorylation of smooth muscle myosin by smooth muscle myosin light chain kinase has no effect on the binding of caldesmon to myosin. Since Ca(2+)-calmodulin and phosphorylation of caldesmon weaken the binding of caldesmon to both actin and myosin, these events may be coordinately regulated.

Caldesmon and a 20-kDa actin-binding fragment of caldesmon inhibit tension development in skinned gizzard muscle fiber bundles

Caldesmon is known to inhibit actin-activated myosin ATPase activity in solution, to inhibit force production when added to skeletal muscle fibers, and to alter actin movement in the in vitro cell motility assay. It is less clear that caldesmon can inhibit contraction in smooth muscle cells in which caldesmon is abundant. We now show that caldesmon and its 20-kDa actin-binding fragment are able to inhibit force in chemically skinned gizzard fiber bundles, which are activated by a constitutively active myosin light-chain kinase in the presence and absence of okadaic acid. This inhibitory effect is reversed by high concentrations of Ca2+ and calmodulin. Therefore, caldesmon may act by increasing the level of myosin phosphorylation required to obtain full activation. Our results also suggest that caldesmon does not act to maintain force in smooth muscle by cross-linking myosin with actin since competition of binding of caldesmon with myosin does not cause a reduction in tension.

Purification and partial characterization of relaxin and relaxin precursors from the hamster placenta

Previous immunological studies have indicated that the molecular structure of hamster relaxin is quite different from that of porcine relaxin. In the present study, hamster relaxin was purified from placentas and characterized in order to investigate its biochemical properties. Placentas from Days 14 and 15 of gestation were homogenized in 0.26 N HCl-62.5% acetone containing protease inhibitors. After centrifugation, soluble proteins were acetone precipitated. Soluble proteins were applied to a carboxymethyl cellulose ion-exchange column and bound proteins were eluted with 0.1 and 0.3 M NaCl. Western blot analysis detected 16.5-, 18.7-, and 36.0-kDa relaxin-immunoreactive (IR) proteins within the 0.1 M NaCl eluant and detected a 5.6-kDa relaxin-IR protein within the 0.3 M NaCl eluant. The 5.6-kDa protein was purified to homogeneity by gel filtration (Sephadex G-50), ion-exchange HPLC, and C18-HPLC. Reduction of the 5.6-kDa protein prior to electrophoresis resulted in a single band of lower molecular mass, suggesting that hamster relaxin consists of two chains of approximately equal molecular mass. Isoelectric point of the 5.6-kDa protein was 7.78. The 16.5- and 18.7-kDa IR proteins were copurified by gel filtration and ion-exchange HPLC. At least five isoelectric point variants were observed for the 16.5- and 18.7-kDa proteins. The N-terminal amino acid for the 5.6 and 18.7 relaxin-IR proteins was arginine, and subsequent cycles indicated an identical partial sequence that was consistent with that for relaxins from other species.

Deoxyguanosine-resistant leukemia L1210 cells. Loss of specific deoxyribonucleoside kinase activity

A mouse leukemia L1210 cell line was selected for resistance to deoxyguanosine. The deoxyguanosine-resistant cells (dGuo-R) were 126-fold less sensitive to deoxyguanosine than the wild-type cells. The IC50 values for araC and araG were increased, but only 10-12-fold in the dGuo-R cells when compared with the wild-type cells. The dGuo-R cell line showed an increased level of resistance to 2-fluoro-2'-deoxyadenosine and 2-fluoroadenine arabinoside (11-14-fold), but essentially no increase in resistance to deoxyadenosine or adenine arabinoside. Deoxyribonucleoside kinase activity was decreased only slightly (19%) when deoxycytidine was utilized as substrate; when cytosine arabinoside or deoxyguanosine was used as the substrate, the kinase activity in the extracts from the dGuo-R cells was only 10% of the enzyme activity in the extracts from the wild-type cells. The determination of the kinetic parameters, Km and Vmax, indicated that there were marked decreases in the Vmax values for deoxyguanosine and cytosine arabinoside as substrates, but not for deoxycytidine as substrate; the Km values for deoxycytidine and cytosine arabinoside were increased in the extracts from the dGuo-R cells. By use of high-performance liquid chromatography, the kinase activities in the extracts from the wild-type and resistant cells could be resolved. There was the specific loss of kinase activity toward cytosine arabinoside and deoxyguanosine as substrates. These data indicate that the dGuo-R cells have decreased levels of a specific deoxyribonucleoside kinase activity.

Phosphorylation-contraction coupling in smooth muscle: role of caldesmon

In intact smooth muscle strips from chicken gizzard, carbachol elicited brief, phasic contractions which were associated with a very rapid, transient phosphorylation of the 20 kDa myosin light chains. Phosphorylation was not significantly different from basal levels after 30 s while force still amounted to 50% of the peak value. The rate of tension decline could be increased by addition of atropine, even at apparently basal phosphorylation levels suggesting a phosphorylation independent regulation. The force, at a given level of phosphorylation, could also be modulated by addition of the actin binding, putative regulatory protein, caldesmon. Caldesmon, inhibits phosphorylation dependent force in skinned fiber bundles of chicken gizzard without affecting myosin light chain phosphorylation. This suggests that caldesmon might modulate contraction in smooth muscle. Moreover our results suggest that caldesmon does not function to maintain passive tension.

A mosaic multiple-binding model for the binding of caldesmon and myosin subfragment-1 to actin

Binding of caldesmon to actin causes a decrease in the quantity of bound myosin and results in a reduction in the rate of actin-activated adenosine triphosphate hydrolysis. It is generally assumed that the binding of caldesmon and myosin to actin is a pure competitive interaction. However, recent binding studies of enzyme digested caldesmon subfragments directed at mapping the actin binding site of caldesmon have shown that a small 8-kD fragment around the COOH-terminal can compete directly with the myosin subfragment 1 (S-1) binding to actin; at least one other fragment that binds to actin does not inhibit the actin-activated adenosine triphosphate activity of myosin. That is, only a part of the caldesmon sequence may be responsible for directly blocking the binding of S-1 to actin. This prompts us to question the actual mode of binding of intact caldesmon and myosin S-1 to actin: whether the entire intact caldesmon molecule is competing with S-1 binding (pure competitive model) or just a small part of it (mosaic multiple-binding model). To answer this question, we measured the amount of myosin S-1 and caldesmon bound per actin monomer as a function of the total concentration of S-1 added to the system at constant concentrations of actin and caldesmon. A formalism for calculating the titration data based on the pure competitive model and a mosaic multiple-binding model was then developed. When compared with theoretical calculations, it is found that the binding of caldesmon and S-1 to actin cannot be pure competitive if no cooperativity exists between S-1 and caldesmon. In contrast, the mosaic multiple-binding model can fit the binding data rather well regardless of the existence of cooperativity between S-1 and caldesmon.

Localization and characterization of a 7.3-kDa region of caldesmon which reversibly inhibits actomyosin ATPase activity

Cleavage of caldesmon with chymotrypsin yields a series of fragments which bind both calmodulin and actin and inhibit the binding of myosin subfragments to actin and the subsequent stimulation of ATPase activity. Several of these fragments have been purified by cation exchange chromatography and their amino-terminal sequences determined. The smallest fragment has a molecular mass of about 7.3 kDa and extends from Leu597 to Phe665. This polypeptide inhibits the actin-activated ATPase of myosin S-1; this inhibition is augmented by smooth muscle tropomyosin and relieved by Ca(2+)-calmodulin. The binding of the 7.3-kDa fragment to actin is competitive with the binding of S-1 to actin. Thus, this polypeptide has several of the important features characteristic of intact caldesmon. However, although an intact caldesmon molecule covers between six and nine actin monomers, the 7.3-kDa fragment binds to actin in a 1:1 complex. Comparison of this fragment with others suggests that a small region of caldesmon is responsible for at least part of the interaction with both calmodulin and actin.

Interaction of caldesmon and myosin subfragment 1 with the C-terminus of actin

The interactions of caldesmon and S1 with the C-terminus of actin were examined in co-sedimentation experiments using proteolytically truncated actin. It is shown that removal of 6 residues from the C-terminus of actin reduces the binding of caldesmon by about 50% while improving the binding of S1 to actin. We also show that S1 protects actin's C-terminus from enzymatic cleavage. Both S1 and caldesmon binding to actin are decreased in the presence of an actin C-terminal peptide. These results emphasize the importance of the C-terminus of actin in binding to S1 and caldesmon.

Actin mediated regulation of muscle contraction

Striated and smooth muscles have different mechanisms of regulation of contraction which can be the basis for selective pharmacological alteration of the contractility of these muscle types. The progression in our understanding of the tropomyosin-troponin regulatory system of striated muscle from the early 1970s through the early 1990s is described along with key concepts required for understanding this complex system. This review also examines the recent history of the putative contractile regulatory proteins of smooth muscle, caldesmon and calponin. A contrast is made between the actin linked regulatory systems of striated and smooth muscle.

The interaction of caldesmon with the COOH terminus of actin

Caldesmon interacts with the NH2-terminal region of actin. It is now shown in airfuge centrifugation experiments that modification of the penultimate cysteine residue of actin significantly weakens its binding to caldesmon both in the presence and absence of tropomyosin. Furthermore, as revealed by fluorescence measurements, caldesmon increases the exposure of the COOH-terminal region of actin to the solvent. This effect of caldesmon, like its inhibitory effect on actomyosin ATPase activity, is enhanced in the presence of tropomyosin. Proteolytic removal of the last three COOH-terminal residues of actin, containing the modified cysteine residue, restores the normal binding between caldesmon and actin. These results establish a correlation between the binding of caldesmon to actin and the conformation of the COOH-terminal region of actin and suggest an indirect rather than direct interaction between caldesmon and this part of actin.

Activation of skeletal S-1 ATPase activity by actin-tropomyosin-troponin. Effect of Ca++ on the fluorescence transient

Regulation in striated muscles primarily involves the effect of changes in the free calcium concentration on the interaction of subfragment-1 (S-1) with the actin-tropomyosin-troponin complex (henceforth referred to as [acto]R). At low concentrations of free Ca++ the rate of ATP hydrolysis by (acto)R S-1 can be as much as 20-fold lower than that in the presence of high free Ca++, even though the binding of S-1 to (actin)R in the presence of ATP is virtually independent of the calcium concentration. This implies that the mechanism of regulation involves a kinetic transition between actin-bound states, rather than the result of changes in actin binding. In the current work, we have investigated the fluorescence transient that occurs with the binding and hydrolysis of ATP both at low and high free [Ca++]. The magnitude of this transition at low free [Ca++] is higher than at high free [Ca++]. At low free [Ca++], the rate of the fluorescence transient either stays constant or decreases slightly with increasing free actin concentrations, but at high free [Ca++] the rate increases slightly with increasing free actin concentration. The observed changes in rate are not great enough to be of regulatory importance. The results of the fluorescence transient experiments together with the binding studies performed at steady state also show that neither the binding of M.ATP or M.ADP.Pi to (actin)R is appreciably Ca++ sensitive. These data imply that an additional step (or steps) in the ATPase cycle, i.e., other than the burst transition, must be regulated by calcium.

A long helix from the central region of smooth muscle caldesmon

The central region of smooth muscle caldesmon is predicted to form alpha-helices on the basis of its primary structure. We have isolated a fragment (CT54) that contains this region. The hydrodynamic properties and the electron microscopic images suggest that CT54 is an elongated (35 nm), monomeric molecule. The circular dichroic spectrum yields an overall alpha-helical content of 55-58%. These results are consistent with the model that the middle portion of CT54 forms a long stretch of single-stranded alpha-helix. Such a structure, if it in fact exists, is thought to be stabilized by numerous salt bridges between charged residues at positions i and i + 4. The structural characteristics of this fragment not only represent an unusual protein configuration but also provide information about the functional role of caldesmon in smooth muscle contraction.

Parallel inhibition of active force and relaxed fiber stiffness in skeletal muscle by caldesmon: implications for the pathway to force generation

In recent hypotheses on muscle contraction, myosin cross-bridges cycle between two types of actin-bound configuration. These two configurations differ greatly in the stability of their actin-myosin complexes ("weak-binding" vs. "strong-binding"), and force generation or movement is the result of structural changes associated with the transition from the weak-binding (preforce generating) configuration to strong-binding (force producing) configuration [cf. Eisenberg, E. & Hill, T. L. (1985) Science 227, 999-1006]. Specifically, in this concept, the main force-generating states are only accessible after initial cross-bridge attachment in a weak-binding configuration. It has been shown that strong and weak cross-bridge attachment can occur in muscle fibers [Brenner, B., Schoenberg, M., Chalovich, J. M., Greene, L. E. & Eisenberg, E. (1982) Proc. Natl. Acad. Sci. USA 79, 7288-7291]. However, there has been no evidence that attachment in the weak-binding states represents an essential step leading to force generation. It is shown here that caldesmon can be used to selectively inhibit attachment of weak-binding cross-bridges in skeletal muscle. Such inhibition causes a parallel decrease in active force, while the kinetics of cross-bridge turnover are unchanged by this procedure. This suggests that (i) cross-bridge attachment in the weak-binding states is specific and (ii) force production can only occur after cross-bridges have first attached to actin in a weakly bound, nonforce-generating configuration.

Immunochemical evidence for the binding of caldesmon to the NH2-terminal segment of actin

The binding of caldesmon and its actin-binding fragments to actin was studied by using peptide antibodies directed against two actin sites implicated in actomyosin interactions. Antibodies against residues 1-7 on skeletal alpha-actin strongly inhibited the binding of caldesmon to actin and perturbed to a smaller extent the interaction between actin and the actin binding fragments. Carbodiimide coupling of ethylenediamine to the NH2-terminal acidic residues on actin inhibited the binding of caldesmon and its fragments to actin to a similar extent as the (residues 1-7) antibodies. Antibodies against residues 18-28 showed only limited competition with caldesmon for the binding to actin. These results lead to the following conclusions. (i) The NH2-terminal residues on actin play an important role in the binding of caldesmon to actin, (ii) residues 18-28 on actin do not form a major caldesmon interaction site, and (iii) the actin-binding fragments do not contain the full actin-binding interface. These conclusions and other literature data suggest that caldesmon regulates the actomyosin ATPase by competing with myosin.ATP for the NH2-terminal segment on actin.

Characterization of caldesmon binding to myosin

Caldesmon inhibits the binding of skeletal muscle subfragment-1 (S-1).ATP to actin but enhances the binding of smooth muscle heavy meromyosin (HMM).ATP to actin. This effect results from the direct binding of caldesmon to myosin in the order of affinity: smooth muscle HMM greater than skeletal muscle HMM greater than smooth muscle S-1 greater than skeletal muscle S-1 (Hemric, M. E., and Chalovich, J. M. (1988) J. Biol. Chem. 263, 1878-1885). We now show that the difference between skeletal muscle HMM and S-1 is due to the presence of the S-2 region in HMM and is unrelated to light chain composition or to two-headed versus single-headed binding. Differences between the binding of smooth and skeletal muscle myosin subfragments to actin do not result from the lack of light chain 2 in skeletal muscle S-1. In the presence of ATP, caldesmon binds to smooth muscle myosin filaments with a stoichiometry of 1:1 (K = 1 x 10(6) M-1). Similar results were obtained for the binding of caldesmon to smooth muscle rod as well as the binding of the purified myosin-binding fragment of caldesmon to smooth muscle myosin. The binding of caldesmon to intact myosin is ATP sensitive. The interaction of caldesmon with myosin is apparently specific and sensitive to the structure of both proteins.