甲壳动物横纹肌的收缩蛋白质与调节机制

甲壳动物横纹肌肌原纤维的肌丝陈列,收缩蛋白质和收缩的Ca²⁺依赖性调节机制与脊椎动物横纹肌有不少差异.脊椎动物横纹肌、甲壳动物快肌与慢肌的粗丝与细丝的数量比依次为1:2,1:3和1:6,肌丝阵列各异.甲壳动物粗肌丝由肌球蛋白和副肌球蛋白组成,其分子装配与脊椎动物不同.细肌丝含有肌动蛋白、原肌球蛋白和肌钙蛋白,肌钙蛋白-T分子量较高,肌钙蛋白-C仅1个Ca²⁺结合位点.甲壳动物横纹肌兼有细肌丝调节与粗肌丝调节.     

Signal transduction and protein phosphorylation in smooth muscle contraction

Smooth muscles are important constituents of vertebrate organisms that provide for contractile activity of internal organs and blood vessels. Basic molecular mechanism of both smooth and striated muscle contractility is the force-producing ATP-dependent interaction of the major contractile proteins, actin and myosin II molecular motor, activated upon elevation of the free intracellular Ca²⁺ concentration ([Ca²⁺]_i). However, whereas striated muscles display a proportionality of generated force to the [Ca²⁺]_i level, smooth muscles feature molecular mechanisms that modulate sensitivity of contractile machinery to [Ca²⁺]_i. Phosphorylation of proteins that regulate functional activity of actomyosin plays an essential role in these modulatory mechanisms. This provides an ability for smooth muscle to contract and maintain tension within a broad range of [Ca²⁺]_i and with a low energy cost, unavailable to a striated muscle. Detailed exploration of these mechanisms is required to understand the molecular organization and functioning of vertebrate contractile systems and for development of novel advances for treating cardiovascular and many other disorders. This review summarizes the currently known and hypothetical mechanisms involved in regulation of smooth muscle Ca²⁺-sensitivity with a special reference to phosphorylation of regulatory proteins of the contractile machinery as a means to modulate their activity.     

Seasonal changes in the isoform composition of the myosin heavy chains in skeletal muscles of hibernating ground squirrels Spermophilus undulatus

Seasonal changes of the isoform composition of myosin heavy chains in skeletal muscles (m. triceps, m. longissimus dorsi, m. soleus, m. gastrocnemius, m. vastus lateralis) of hibernating ground squirrels Spermophilus undulatus were studied. Functional properties of myosin (the actin-activated ATPase activity and its Ca²⁺-sensitivity in vitro) were also examined. It was observed that the content of slow myosin heavy chain I isoform increased and the content of fast IIx/d isoform decreased in muscles of torpid ground squirrels and animals which are active in autumn and winter. In muscles of these animals the content of N2A-titin isoform decreased although the relative content of NT-titin isoform, observed in striated muscles of mammals in our previous experimental works, increased. Actin-activated ATPase activity and Ca²⁺-sensitivity of myosin isolated from skeletal muscles of torpid and interbout ground squirrels were found to reduce. The changes observed are discussed in the context of adaptation of skeletal muscles of ground squirrels to hibernation conditions.     

Evidence from insect fibrillar muscle about the elementary contractile process

Bundles of myofibrils prepared from the dorsal longitudinal flight muscles of giant water bugs show oscillatory contractile activity in solutions of low ionic strength containing ATP and 10^-8-10^-7 M Ca²⁺. This is due to delay between changes of length and changes of tension under activating conditions. The peculiarities of insect fibrillar muscle which give rise to this behavior are (1) the high elasticity of relaxed myofibrils, (2) a smaller degree of Ca²⁺ activation of ATPase activity in unstretched myofibrils and extracted actomyosin, and (3) a direct effect of stretch on ATPase activity. It is shown that the cross-bridges of striated muscle are probably formed from the heads of three myosin molecules and that in insect fibrillar muscle the cycles of mechanochemical energy conversion in the cross-bridges can be synchronized by imposed changes of length. This material is more suitable than vertebrate striated muscle for a study of the nature of the elementary contractile process.     

Role of regulatory proteins (troponin-tropomyosin) in pathologic states

In vertebrate striated muscle, troponon-tropomyosin is responsible, in part, not only for transducing the effect of calcium on contractile protein activation, but also for inhibiting actin and myosin interaction when calcium is absent. The regulatory troponin (Tn) complex displays several molecular and calcium binding variations in cardiac muscles of different species and undergoes genetic changes with development and in various pathologic states.Extensive reviews on the role of tropomyosin (Tm) and Tn in the regulation of striated muscle contraction have been published describing the molecular mechanisms involved in contractile protein regulation. In our studies, we have found an increase in Mg²⁺ ATPase activity in cardiac myofibrils from dystrophic hamsters and in rats with chronic coronary artery narrowing. The abnormalities in myofibrillar ATPase activity from cardiomyopathic hamsters were largely corrected by recombining the preparations with a TnTm, complex isolated from normal hamsters indicating that the TnTm, may play a major role in altered myocardial function. We have also observed down regulation of Ca²⁺ Mg²⁺ ATPase of myofibrils from hypertrophic guinea pig hearts, myocardial infarcted rats and diabetic-hypertensive rat hearts. In myosin from diabetic rats, this abnormality was substantially corrected by adding troponin-tropomyosin complex from control hearts. All of these disease models are associated with decreased ATPase activities of pure myosin and in the case of rat and hamster models, shifts of myosin, heavy chain from alpha to beta predominate.In summary, there are three main troponin subunit components which might alter myofibrillar function however, very few direct links of molecular alterations in the regulatory proteins to physiologic and pathologic function have been demonstrated so far.     

Characterization of homologous divalent metal ion binding sites of vertebrate and molluscan myosins using electron paramagnetic resonance spectroscopy.

The binding of Ca²⁺, Mg²⁺ and Mn²⁺ to myosins from rabbit skeletal muscle, scallop striated adductor muscle and clam adductor muscle has been investigated. All three myosins bind two moles of divalent metal ion non-specifically and with high affinity (Mn²⁺ > Ca²⁺ > Mg²⁺). In addition, the molluscan myosins bind about a further two moles of Ca²⁺ specifically. Although rabbit myosin binds some Ca²⁺ in the presence of an excess of free Mg²⁺, this binding occurs at the nonspecific sites and should not be taken as evidence for a myosin-linked regulatory system of the type found in molluscan muscles. If such a system exists in vertebrate skeletal muscle, the homologous Ca²⁺-specific sites must be lost during the early stages of the myosin preparation.The characteristic electron paramagnetic resonance spectrum of the bound Mn²⁺ was utilized to confirm the homology of the non-specific sites in vertebrate and molluscan myosins. The sites are located on the “regulatory” class of light chain. Mn²⁺ bound to scallop myosin has a broad electron paramagnetic resonance spectrum, in contrast to the well-resolved spectra that it gives when bound to many other myosin species. This situation was exploited to identify homologous nonspecific, divalent metal-ion sites on the regulatory light chains from a variety of muscle types, including frog skeletal, rabbit cardiac, chicken gizzard and molluscan adductor muscles. When these light chains are combined with desensitized scallop myofibrils the electron paramagnetic resonance spectra of Mn²⁺ bound to the resultant hybrids are dominated by the signal from the non-specific site of the foreign regulatory light chain.     

Comparison of Orientation and Rotational Motion of Skeletal Muscle Cross-bridges Containing Phosphorylated and Dephosphorylated Myosin Regulatory Light Chain

Calcium binding to thin filaments is a major element controlling active force generation in striated muscles. Recent evidence suggests that processes other than Ca²⁺ binding, such as phosphorylation of myosin regulatory light chain (RLC) also controls contraction of vertebrate striated muscle (Cooke, R. (2011) Biophys. Rev. 3, 33–45). Electron paramagnetic resonance (EPR) studies using nucleotide analog spin label probes showed that dephosphorylated myosin heads are highly ordered in the relaxed fibers and have very low ATPase activity. This ordered structure of myosin cross-bridges disappears with the phosphorylation of RLC (Stewart, M. (2010) Proc. Natl. Acad. Sci. U.S.A. 107, 430–435). The slower ATPase activity in the dephosporylated moiety has been defined as a new super-relaxed state (SRX). It can be observed in both skeletal and cardiac muscle fibers (Hooijman, P., Stewart, M. A., and Cooke, R. (2011) Biophys. J. 100, 1969–1976). Given the importance of the finding that suggests a novel pathway of regulation of skeletal muscle, we aim to examine the effects of phosphorylation on cross-bridge orientation and rotational motion. We find that: (i) relaxed cross-bridges, but not active ones, are statistically better ordered in muscle where the RLC is dephosporylated compared with phosphorylated RLC; (ii) relaxed phosphorylated and dephosphorylated cross-bridges rotate equally slowly; and (iii) active phosphorylated cross-bridges rotate considerably faster than dephosphorylated ones during isometric contraction but the duty cycle remained the same, suggesting that both phosphorylated and dephosphorylated muscles develop the same isometric tension at full Ca²⁺ saturation. A simple theory was developed to account for this fact.     

A comparative study of the rat heart sarcolemmal Ca2+-dependent ATPase and myosin ATPase

In order to gain some information regarding Ca²⁺-dependent ATPase, the enzyme was purified from cardiac sarcolemma and its properties were compared with Ca²⁺-ATPase activity of myosin purified from rat heart. Both Ca²⁺-dependent ATPase and myosin ATPase were stimulated by Ca²⁺ but the maximal activation of Ca²⁺-dependent ATPase required 4 mM Ca²⁺ whereas that of myosin ATPase required 10 mM Ca²⁺. These ATPases were also activated by other divalent cations in the order of Ca²⁺ > Mn²⁺ > Sr²⁺ > Br²⁺ > Mg²⁺; however, there was a marked difference in the pattern of their activation by these cations. Unlike the myosin ATPase, the ATP hydrolysis by Ca²⁺-dependent ATPase was not activated by actin. The pH optima of Ca²⁺-dependent ATPase and myosin ATPase were 9.5 and 6.5 respectively. Na⁺ markedly inhibited Ca²⁺-dependent ATPase but had no effect on the myosin ATPase activity. N-ethylmaleimide inhibited Ca²⁺-dependent ATPase more than myosin ATPase whereas the inhibitory effect of vanadate was more on myosin ATPase than Ca²⁺-dependent ATPase. Both Ca²⁺-dependent ATPase and myosin ATPase were stimulated by K-EDTA and NH₄-EDTA. When myofibrils were treated with trypsin and passed through columns similar to those used for purifying Ca²⁺-ATPase from sarcolemma, an enzyme with ATPase activity was obtained. This myofibrillar ATPase was maximally activated at 3–4 mM Ca²⁺ and 3 to 4 mM ATP like sarcolemmal Ca²⁺-dependent ATPase. K⁺ stimulated both ATPase activities in the absence of Ca²⁺ and inhibited in the presence of Ca²⁺. Both enzymes were inhibited by Na⁺, Mg²⁺, La³⁺, and azide similarly. However, Ca²⁺ ATPase from myofibrils showed three peptide bands in SDS polyacrylamide gel electrophoresis whereas Ca²⁺ ATPase from sarcolemma contained only two bands. Sarcolemmal Ca²⁺-ATPase had two affinity sites for ATP (0.012 mM and 0.23 mM) while myofibrillar Ca²⁺-ATPase had only one affinity site (0.34 mM). Myofibrillar Ca²⁺-ATPase was more sensitive to maleic anhydride and iodoacetamide than sarcolemmal Ca²⁺-ATPase. These observations suggest that Ca²⁺-dependent ATPase may be a myosin like protein in the heart sarcolemma and is unlikely to be a tryptic fragment of myosin present in the myofibrils.     

Evidence that myosin light chain phosphorylation regulates contraction in the body wall muscles of the sea cucumber

The Ca²⁺ activation mechanism of the longitudinal body wall muscles of Parastichopus californicus (sea cucumber) was studied using skinned muscle fiber bundles. Reversible phosphorylation of the myosin light chains correlated with Ca²⁺-activated tension and relaxation. Pretreatment of the skinned fibers with ATPγS and high Ca²⁺ (10^-5M) resulted in irreversible thiophosphorylation of the myosin light chains and activation of a Ca²⁺ insensitive tension. In contrast, pretreatment with low Ca²⁺ (10^-8M) and ATPγS results in no thiophosphorylation of the myosin light chains or irreversible activation of tension. These results are consistent with a Ca²⁺-sensitive myosin light chain kinase/phosphatase system being responsible for the activation of the muscle. Other agents known to have an effect upon the Ca²⁺-activated tension in skinned vertebrate smooth muscle fibers (trifluoperazine, catalytic subunit of the cyclic AMP-dependent protein kinase, and calmodulin) did not have an effect on myosin light chain phosphorylation or Ca²⁺-activated tension. These results suggest a different type of myosin light chain kinase than is found in vertebrate smooth muscle is responsible for the activation of parastichopus longitudinal body wall muscle.     

Role of actin C-terminus in regulation of striated muscle thin filament

In striated muscle, regulation of actin-myosin interactions depends on a series of conformational changes within the thin filament that result in a shifting of the tropomyosin-troponin complex between distinct locations on actin. The major factors activating the filament are Ca²⁺ and strongly bound myosin heads. Many lines of evidence also point to an active role of actin in the regulation. Involvement of the actin C-terminus in binding of tropomyosin-troponin in different activation states and the regulation of actin-myosin interactions were examined using actin modified by proteolytic removal of three C-terminal amino acids. Actin C-terminal modification has no effect on the binding of tropomyosin or tropomyosin-troponin + Ca²⁺, but it reduces tropomyosin-troponin affinity in the absence of Ca²⁺. In contrast, myosin S1 induces binding of tropomyosin to truncated actin more readily than to native actin. The rate of actin-activated myosin S1 ATPase activity is reduced by actin truncation both in the absence and presence of tropomyosin. The Ca²⁺-dependent regulation of the ATPase activity is preserved. Without Ca²⁺ the ATPase activity is fully inhibited, but in the presence of Ca²⁺ the activation does not reach the level observed for native actin. The results suggest that through long-range allosteric interactions the actin C-terminus participates in the thin filament regulation.     

Mg-ATPase and CA2+ activated myosin ATPase activity in ventricular myofibrils from non-failing and diseased human hearts – effects of calcium sensitizing agents MCI-154, DPI 201–106, and caffeine

We investigated the effects of two purported calcium sensitizing agents, MCI-154 and DPI 201–106, and a known calcium sensitizer caffeine on Mg-ATPase (myofibrillar ATPase) and myosin ATPase activity of left ventricular myofibrils isolated from non-failing, idiopathic (IDCM) and ischemic cardiomyopathic (ISCM) human hearts (i.e. failing hearts). The myofibrillar ATPase activity of non-failing myofibrils was higher than that of diseased myofibrils. MCI-154 increased myofibrillar ATPase Ca²⁺ sensitivity in myofibrils from non-failing and failing human hearts. Effects of caffeine similarly increased Ca²⁺ sensitivity. Effects of DPI 201–106 were, however, different. Only at the 10^–6 M concentration was a significant increase in myofibrillar ATPase calcium sensitivity seen in myofibrils from non-failing human hearts. In contrast, in myofibrils from failing hearts, DPI 201–106 caused a concentration-dependent increase in myofibrillar ATPase Ca²⁺ sensitivity. Myosin ATPase activity in failing myocardium was also decreased. In the presence of MCI-154, myosin ATPase activity increased by 11, 19, and 24% for non-failing, IDCM, and ISCM hearts, respectively. DPI 201–106 caused an increase in the enzymatic activity of less than 5% for all preparations, and caffeine induced an increase of 4, 11, and 10% in non-failing, IDCM and ISCM hearts, respectively. The mechanism of restoring the myofibrillar Ca²⁺ sensitivity and myosin enzymatic activity in diseased human hearts is most likely due to enhancement of the Ca²⁺ activation of the contractile apparatus induced by these agents. We propose that myosin light chain-related regulation may play a complementary role to the troponin-related regulation of myocardial contractility.     

Preserved Close Linkage Between the Genes Encoding Troponin I and Troponin T, Reflecting an Evolution of Adapter Proteins Coupling the Ca2+ Signaling of Contractility

Ca²⁺-regulated motility is essential to numerous cellular functions, including muscle contraction. Systems with troponin C, myosin light chain, or calmodulin as the Ca²⁺ receptor have evolved in striated muscle and other types of cells to transduce the cytoplasm Ca²⁺ signals into allosteric conformational changes of contractile proteins. While these Ca²⁺ receptors are homologous proteins, their coupling to the responding elements is quite different in various cell types. The Ca²⁺ regulatory system in vertebrate striated muscle represents a highly specialized such signal transduction pathway consisting of the troponin complex and tropomyosin associated with the actin filament. To understand the molecular mechanism in the Ca²⁺ regulation of muscle contraction and cell motility, we have revealed a preserved ancestral close linkage between the genes encoding two of the troponin subunits, troponin I and troponin T, in the genome of mouse. The data suggest that the troponin I and troponin T genes may have originated from a single locus and evolved in parallel to encode a striated muscle-specific adapter to couple the Ca²⁺ receptor, troponin C, to the actin–myosin contractile machinery. This hypothesis views the three troponin subunits as two structure–function domains: the Ca²⁺ receptor and the signal transducing adapter. This model may help to further our understanding of the Ca²⁺ regulation of muscle contraction and the structure–function relationship of other potential adapter proteins which are converged to constitute the Ca²⁺ signal transduction pathways governing nonmuscle cell motility. Received: 15 April 1999 / Accepted: 15 July 1999     

The contractile proteins of smooth muscle. Properties and components of a Ca2+-sensitive actomyosin from chicken gizzard.

The preparation and characterization of a Ca²⁺-sensitive actomyosin from chicken gizzard is described. The pH curve of the Mg²⁺ ATPase activity of the actomyosin was dominated by the activity of the myosin component, and this gave rise to the acid and alkaline optima. Skeletal muscle myosin showed a similar curve. Both the activation of myosin ATPase by actin, and the Ca²⁺ sensitivity were confined to the neutral pH region. The subunit composition of the Ca²⁺-sensitive actomyosin was interesting in that no components corresponding to skeletal muscle troponin were obvious. It is suggested that the activity of gizzard actomyosin is regulated by a protein on the thin filaments with a subunit weight of ~130,000.     

Activation by actin of ATPase activity of chemically modified gizzard myosin without phosphorylation.

The ATPase activity of myosin from chicken gizzard measured in the presence of either Mg²⁺ or Ca²⁺ is increased in the absence of dithiothreitol or upon reaction with Cu²⁺, o-iodosobenzoate, or N-ethylmaleimide. Iodosobenzoate or Cu²⁺ produce no change in K⁺(EDTA)-ATPase while N-ethylmaleimide produces a decrease. These treatments also make the actin-activated ATPase insensitive to Ca²⁺ when assayed in the presence of tropomyosin and a partially purified myosin light chain kinase. Phosphorylation of N-ethylmaleimide modified myosin remains dependent on Ca²⁺ and therefore appears not to be required for activation by actin of the ATPase activity of modified myosin.     

Structural Basis for the Activation of Muscle Contraction by Troponin and Tropomyosin

The molecular regulation of striated muscle contraction couples the binding and dissociation of Ca²⁺ on troponin (Tn) to the movement of tropomyosin on actin filaments. In turn, this process exposes or blocks myosin binding sites on actin, thereby controlling myosin crossbridge dynamics and consequently muscle contraction. Using 3D electron microscopy, we recently provided structural evidence that a C-terminal extension of TnI is anchored on actin at low Ca²⁺ and competes with tropomyosin for a common site to drive tropomyosin to the B-state location, a constrained, relaxing position on actin that inhibits myosin-crossbridge association. Here, we show that release of this constraint at high Ca²⁺ allows a second segment of troponin, probably representing parts of TnT or the troponin core domain, to promote tropomyosin movement on actin to the Ca²⁺-induced C-state location. With tropomyosin stabilized in this position, myosin binding interactions can begin. Tropomyosin appears to oscillate to a higher degree between respective B- and C-state positions on troponin-free filaments than on fully regulated filaments, suggesting that tropomyosin positioning in both states is troponin-dependent. By biasing tropomyosin to either of these two positions, troponin appears to have two distinct structural functions; in relaxed muscles at low Ca²⁺, troponin operates as an inhibitor, while in activated muscles at high Ca²⁺, it acts as a promoter to initiate contraction.     

ATPase Activity of Myosin Correlated with Speed of Muscle Shortening

Myosin was isolated from 14 different muscles (mammals, lower vertebrates, and invertebrates) of known maximal speed of shortening. These myosin preparations were homogeneous in the analytical ultracentrifuge or, in a few cases, showed, in addition to the main myosin peak, part of the myosin in aggregated form. Actin- and Ca⁺⁺-activated ATPase activities of the myosins were generally proportional to the speed of shortening of their respective muscles; i.e. the greater the intrinsic speed, the higher the ATPase activity. This relation was found when the speed of shortening ranged from 0.1 to 24 muscle lengths/sec. The temperature coefficient of the Ca⁺⁺-activated myosin ATPase was the same as that of the speed of shortening, Q₁₀ about 2. Higher Q₁₀ values were found for the actin-activated myosin ATPase, especially below 10°C. By using myofibrils instead of reconstituted actomyosin, Q₁₀ values close to 2 could be obtained for the Mg⁺⁺-activated myofibrillar ATPase at ionic strength of 0.014. In another series of experiments, myosin was isolated from 11 different muscles of known isometric twitch contraction time. The ATPase activity of these myosins was inversely proportional to the contraction time of the muscles. These results suggest a role for the ATPase activity of myosin in determining the speed of muscle contraction. In contrast to the ATPase activity of myosin, which varied according to the speed of contraction, the F-actin-binding ability of myosin from various muscles was rather constant.     

Calcium regulation of the ATPase activity of Physarum and scallop myosins using hybrid smooth muscle myosin: The role of the essential light chain

To examine the role of two light chains (LCs) of the myosin II on Ca²⁺ regulation, we produced hybrid heavy meromyosin (HMM) having LCs from Physarum and/or scallop myosin using the smooth muscle myosin heavy chain. Ca²⁺ inhibited motility and ATPase activity of hybrid HMMs with LCs from Physarum myosin but activated those of hybrid HMM with LCs from scallop myosin, indicating an active role of LCs. ATPase activity of hybrid HMMs with LCs from different species showed the same effect by Ca²⁺ even though they did not support motility. Our results suggest that communication between the original combinations of LC is important for the motor function.     

Myosin and actomyosin from human skeletal muscle

Human skeletal natural actomyosin contained actin, tropomyosin, troponin and myosin components as judged by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. Purified human myosin contained at least three light chains having molecular weights (±2000) of 25 000, 18 000 and 15 000. Inhibitory and calcium binding components of troponin were identified in an actin-tropomyosin-troponin complex extracted from acetone-dried muscle powder at 37°C. Activation of the Mg-ATPase activity of Ca²⁺-sensitive human natural or reconstituted actomyosin was half maximal at approximately 3.4 μM Ca²⁺ concentration (CaEGTA binding constant = 4.4 · 10⁵ at pH 6.8). Subfragment 1, isolated from the human heavy meromyosin by digestion with papain, appeared as a single peak after DEAE-cellulose chromatography. In the pH 6–9 range, the Ca²⁺-ATPase activity of the subfragment 1 was 1.8-and 4-fold higher that the original heavy meromyosin and myosin, respectively. The ATPase activities of human myosin and its fragments were 6–10 fold lower than those of corresponding proteins from rabbit fast skeletal muscle. Human myosin lost approximately 60% of the Ca²⁺-ATPase activity at pH 9 without a concomitant change in the number of distribution of its light chains. These findings indicate that human skeletal muscle myosin resembles other slow and fast mammalian muscles. Regulation of human skeletal actomyosin by Ca²⁺ is similar to that of rabbit fast or slow muscle     

Cardiac Muscle Activation Blunted by a Mutation to the Regulatory Component,Troponin T

The striated muscle thin filament comprises actin, tropomyosin, and troponin. The Tn complex consists of three subunits, troponin C (TnC), troponin I (TnI), and troponin T (TnT). TnT may serve as a bridge between the Ca²⁺ sensor (TnC) and the actin filament. In the short helix preceding the IT-arm region, H1(T2), there are known dilated cardiomyopathy-linked mutations (among them R205L). Thus we hypothesized that there is an element in this short helix that plays an important role in regulating the muscle contraction, especially in Ca²⁺ activation. We mutated Arg-205 and several other amino acid residues within and near the H1(T2) helix. Utilizing an alanine replacement method to compare the effects of the mutations, the biochemical and mechanical impact on the actomyosin interaction was assessed by solution ATPase activity assay, an in vitro motility assay, and Ca²⁺ binding measurements. Ca²⁺ activation was markedly impaired by a point mutation of the highly conserved basic residue R205A, residing in the short helix H1(T2) of cTnT, whereas the mutations to nearby residues exhibited little effect on function. Interestingly, rigor activation was unchanged between the wild type and R205A TnT. In addition to the reduction in Ca²⁺ sensitivity observed in Ca²⁺ binding to the thin filament, myosin S1-ADP binding to the thin filament was significantly affected by the same mutation, which was also supported by a series of S1 concentration-dependent ATPase assays. These suggest that the R205A mutation alters function through reduction in the nature of cooperative binding of S1.     

Composition of the myosin light chain kinase from chicken gizzard.

The Ca²⁺-dependent protein kinase (ATP:myosin light chain phosphotransferase) from chicken gizzard smooth muscle requires two proteins for enzymatic activity. These have approximate molecular weights of 105,000 and 17,000 daltons. The isolation procedure for each component is described. Neither component alone markedly alters either the actin-moderated ATPase activity or the phosphorylation of myosin. Activation of ATPase activity by a combination of the two components occurred only in the presence of Ca²⁺ and was always accompanied by the phosphorylation of myosin. The simultaneous activation of ATPase activity and myosin phosphorylation establishes a direct correlation between the two events.