Nonlinear dependence of actin-activated Mg2+-ATPase activity on the extent of phosphorylation of gizzard myosin and H-meromyosin

Structure of the shutdown state of myosin-2

Myosin-2 is essential for processes as diverse as cell division and muscle contraction. Dephosphorylation of its regulatory light chain (RLC) promotes an inactive, ‘shutdown’ state with the filament-forming tail folded onto the two heads¹, preventing filament formation and inactivating the motors². The mechanism by which this happens is obscure. Here we report a cryo-electron microscopy structure of shutdown smooth muscle myosin, with a resolution of 6 Å in the head region. A pseudo-atomic model, obtained by flexible fitting of crystal structures into the density and molecular dynamics simulations, describes interaction interfaces at the atomic level. The N-terminal extension of one RLC interacts with the tail and the other with the partner head, revealing how the RLCs stabilise the shutdown state in different ways and how their phosphorylation would allow myosin activation. Additional interactions between the three segments of the coiled coil, the motor domains and LCs stabilise the shutdown molecule. The structure of the lever in each head is competent to generate force upon activation. This shutdown structure is relevant to all myosin-2 isoforms and provides a framework for understanding their disease-causing mutations.

Modeling smooth muscle myosin's two heads: long-lived enzymatic roles and phosphorylation-dependent equilibria

Smooth muscle myosin has two heads, each capable of interacting with actin to generate force and/or motion as it hydrolyzes ATP. These heads are inhibited when their associated regulatory light chain is unphosphorylated (0P), becoming active and hydrolyzing ATP maximally when phosphorylated (2P). Interestingly, with only one of the two regulatory light chains phosphorylated (1P), smooth muscle myosin is active but its ATPase rate is <2P. To explain published 1P single ATP turnover and steady-state ATPase activities, we propose a kinetic model in which 1P myosin exists in an equilibrium between being fully active (2P) and inhibited (0P). Based on the single ATP turnover data, we also propose that each 2P head adopts a hydrolytic role distinct from its partner at any point in time, i.e., one head strongly binds actin and hydrolyzes ATP at its actin-activated rate while the other weakly binds actin. Surprisingly, the heads switch roles slowly (<0.1 s(-1)), suggesting that their activities are not independent. The phosphorylation-dependent equilibrium between active and inhibited states and the hydrolytic role that each head adopts during its interaction with actin may have implications for understanding regulation and mechanical performance of other members of the myosin family of molecular motors.

Smooth muscle heavy meromyosin phosphorylated on one of its two heads supports force and motion

Smooth muscle myosin is activated by regulatory light chain (RLC) phosphorylation. In the unphosphorylated state the activity of both heads is suppressed due to an asymmetric, intramolecular interaction between the heads. The properties of myosin with only one of its two RLCs phosphorylated, a state likely to be present both during the activation and the relaxation phase of smooth muscle, is less certain despite much investigation. Here we further characterize the mechanical properties of an expressed heavy meromyosin (HMM) construct with only one of its RLCs phosphorylated (HMM-1P). This construct was previously shown to have more than 50% of the ATPase activity of fully phosphorylated myosin (HMM-2P) and to move actin at the same speed in a motility assay as HMM-2P (Rovner, A. S., Fagnant, P. M., and Trybus, K. M. (2006) Biochemistry 45, 5280-5289). Here we show that the unitary step size and attachment time to actin of HMM-1P is indistinguishable from that of HMM-2P. Force-velocity measurements on small ensembles show that HMM-1P can generate approximately half the force of HMM-2P, which may relate to the observed duty ratio of HMM-1P being approximately half that of HMM-2P. Therefore, single-phosphorylated smooth muscle HMM molecules are active species, and the head associated with the unphosphorylated RLC is mechanically competent, allowing it to make a substantial contribution to both motion and force generation during smooth muscle contraction.

Phosphorylation of a single head of smooth muscle myosin activates the whole molecule

Regulatory light chain (RLC) phosphorylation activates smooth and non-muscle myosin II, but it has not been established if phosphorylation of one head turns on the whole molecule. Baculovirus expression and affinity chromatography were used to isolate heavy meromyosin (HMM) containing one phosphorylated and one dephosphorylated RLC (1-P HMM). Motility and steady-state ATPase assays indicated that 1-P HMM is nearly as active as HMM with two phosphorylated heads (2-P HMM). Single-turnover experiments further showed that both the dephosphorylated and phosphorylated heads of 1-P HMM can be activated by actin. Singly phosphorylated full-length myosin was also an active species with two cycling heads. Our results suggest that phosphorylation of one RLC abolishes the asymmetric inhibited state formed by dephosphorylated myosin [Liu, J., et al. (2003) J. Mol. Biol. 329, 963-972], allowing activation of both the phosphorylated and dephosphorylated heads. These findings help explain how smooth muscles are able to generate high levels of stress with low phosphorylation levels.

Kinetics of smooth muscle heavy meromyosin with one thiophosphorylated head

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

Role of the N-terminal region of the regulatory light chain in the dephosphorylation of myosin by myosin light chain phosphatase

Myosin regulatory light chain (RLC) is phosphorylated at various sites at its N-terminal region, and heterotrimeric myosin light chain phosphatase (MLCP) has been assigned as a physiological phosphatase that dephosphorylates myosin in vivo. Specificity of MLCP toward the various phosphorylation sites of RLC was studied, as well as the role of the N-terminal region of RLC in the dephosphorylation of myosin by MLCP. MLCP dephosphorylated phosphoserine 19, phosphothreonine 18, and phosphothreonine 9 efficiently with almost identical rates, whereas it failed to dephosphorylate phosphorylated serine 1/serine 2. Deletion of the N-terminal seven amino acid residues of RLC markedly decreased the dephosphorylation rate of phosphoserine 19 of RLC incorporated in the myosin molecule, whereas this deletion did not significantly affect the dephosphorylation rate of isolated RLC. On the other hand, deletion of only four N-terminal amino acid residues showed no effect on dephosphorylation of phosphoserine 19 of incorporated RLC. The inhibition of dephosphorylation by deletion of the seven N-terminal residues was also found with the catalytic subunit of MLCP. Phosphorylation at serine 1/serine 2 and threonine 9 did not influence the dephosphorylation rate of serine 19 and threonine 18 by MLCP. These results suggest that the N-terminal region of RLC plays an important role in substrate recognition of MLCP.

Two heads are required for phosphorylation-dependent regulation of smooth muscle myosin

Recent structural evidence (Rayment, I., Holden, H. M., Whittaker, M., Yohn, C. B., Lorenz, M., Holmes, K. C., and Milligan, R. A. (1993) Science 261, 58-65) suggests that the two heads of skeletal muscle myosin interact when the protein is bound to filamentous actin. Direct chemical cross-linking experiments show that the two heads of smooth muscle myosin interact in the presence of filamentous actin and the absence of ATP (Onishi, H., Maita, T., Matsuda, G., and Fujiwara, K. (1992) Biochemistry 31, 1201-1210). Head-head interactions may be important in the mechanism of phosphorylation-dependent regulation of smooth muscle myosin. To explore the structural elements essential for phosphorylation-dependent regulation, we purified a proteolytic fragment of chicken gizzard myosin containing only one head attached to an intact tail. This molecule contained a partially digested regulatory light chain, which was replaced with exogenously added intact light chain in either the thiophosphorylated or the unphosphorylated state. Control experiments showed that this replacement was nearly quantitative and did not alter the actin-activated ATPase of this myosin. Electron micrographs confirmed that the single-headed preparation contained an intact form of single-headed myosin. The unphosphorylated single-headed myosin hydrolyzed ATP rapidly and moved actin filaments in an in vitro motility assay. Phosphorylation had minimal effects upon these properties. Therefore, we conclude that phosphorylation-dependent regulation in this myosin requires two heads. These findings may have important implications in studies of other regulated motor proteins that contain two motor domains.

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

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

Actin-facilitated assembly of smooth muscle myosin induces formation of actomyosin fibrils

To identify regulatory mechanisms potentially involved in formation of actomyosin structures in smooth muscle cells, the influence of F-actin on smooth muscle myosin assembly was examined. In physiologically relevant buffers, AMPPNP binding to myosin caused transition to the soluble 10S myosin conformation due to trapping of nucleotide at the active sites. The resulting 10S myosin-AMPPNP complex was highly stable and thick filament assembly was suppressed. However, upon addition to F-actin, myosin readily assembled to form thick filaments. Furthermore, myosin assembly caused rearrangement of actin filament networks into actomyosin fibers composed of coaligned F-actin and myosin thick filaments. Severin-induced fragmentation of actin in actomyosin fibers resulted in immediate disassembly of myosin thick filaments, demonstrating that actin filaments were indispensable for mediating myosin assembly in the presence of AMPPNP. Actomyosin fibers also formed after addition of F-actin to nonphosphorylated 10S myosin monomers containing the products of ATP hydrolysis trapped at the active site. The resulting fibers were rapidly disassembled after addition of millimolar MgATP and consequent transition of myosin to the soluble 10S state. However, reassembly of myosin filaments in the presence of MgATP and F-actin could be induced by phosphorylation of myosin P-light chains, causing regeneration of actomyosin fiber bundles. The results indicate that actomyosin fibers can be spontaneously formed by F-actin-mediated assembly of smooth muscle myosin. Moreover, induction of actomyosin fibers by myosin light chain phosphorylation in the presence of actin filament networks provides a plausible hypothesis for contractile fiber assembly in situ.

Effects of NA0344, a new smooth muscle relaxant, on the actin-myosin-ATP interaction and myosin light chain phosphorylation in vitro

1. Effects of antibiotic NA0344, a smooth muscle relaxant, on phosphorylation of myosin light chain (MLC, 20 kDa) were compared with those on actin-myosin-ATP interaction using native actomyosin preparation containing MLC kinase activity. 2. MLC kinase was shown to be the site of action of NA0344. 3. NA0344 inhibited the interaction and phosphorylation with IC50 = 7.5 x 10(-6) and 1.6 x 10(-5) M, respectively. 4. The discrepancy between the inhibitory effects is explained that myosin is in an active form when myosin is fully phosphorylated. 5. However, the inhibitory effects of NA0359 and NA0362, analogs of NA0344, on the phosphorylation were similar to or more effective than those on the interaction, which cannot be explained by the active form hypothesis. 6. Plausible explanations for the discrepancies are discussed.

Phosphorylation of smooth muscle myosin by type II Ca2+/calmodulin-dependent protein kinase

Brain type II Ca2+/calmodulin-dependent protein kinase was found to phosphorylate smooth muscle myosin, incorporating maximally approximately 2 mol of phosphoryl per mol of myosin, exclusively on the 20,000 dalton light chain subunit. After maximal phosphorylation of myosin or the isolated 20,000 dalton light chain subunit by myosin light chain kinase, the addition of type II Ca2+/calmodulin-dependent protein kinase led to no further incorporation indicating the two kinases phosphorylated a common site. This conclusion was supported by two dimensional mapping of tryptic digests of myosin phosphorylated by the two kinases. By phosphoamino acid analysis the phosphorylated residue was identified as a serine. The phosphorylation by type II Ca2+/calmodulin-dependent protein kinase of myosin resulted in enhancement of its actin-activated Mg2(+)-ATPase activity. Taken together, these data strongly support the conclusion that type II Ca2+/calmodulin-dependent protein kinase phosphorylates the same amino acid residue on the 20,000 dalton light chain subunit of smooth muscle myosin as is phosphorylated by myosin light chain kinase and suggest an alternative mechanism for the regulation of actin-myosin interaction.

Phosphorylation of a second site for myosin light chain kinase on platelet myosin

The 20,000-dalton light chain of bovine platelet myosin is phosphorylated at two sites by myosin light chain kinase. The first and second phosphorylation sites are at a serine and a threonine residue, respectively. The location of the phosphorylation sites was determined by using limited proteolysis. The N-terminal sequence of the 17,000-dalton tryptic fragment of platelet myosin 20,000-dalton light chain was found to be identical with that of gizzard 20,000-dalton light chain from Ala-17 to Phe-33. On the basis of these results and the distribution of 32P among the proteolytic fragments, it was concluded that serine-19 and threonine-18 were the two phosphorylation sites. Phosphorylation at the threonine residue markedly increases the actin-activated ATPase activity of myosin. It was found that platelet myosin forms 10S and 6S conformations and its Mg2+-ATPase activity parallels the transition from the 6S to the 10S conformation. The conformational transition was influenced by phosphorylation at both sites, and the phosphorylation at the threonine residue further shifted the equilibrium toward the 6S conformation. The phosphorylation at the threonine residue also induced thick filament formation in the presence of ATP. These results suggest that the phosphorylation at the threonine residue as well as at the serine residue may play an important role in the contractility of nonmuscle cells.

Proteolysis and actin-binding properties of 10S and 6S smooth muscle myosin: identification of a site protected from proteolysis in the 10S conformation and by the binding of actin

It was shown previously [Ikebe, M., & Hartshorne, D. J. (1985) Biochemistry 24, 2380-2387] that the conformation of gizzard myosin, either 10S or 6S, influences proteolysis of myosin at two regions designated sites A and B. The studies reported here are focused on site A, which is located approximately 68,000 daltons from the N-terminus of the myosin heavy chain. With papain, Staphylococcus aureus protease, and actinidin, it is shown that the formation of 10S myosin reduces proteolysis at site A. Binding of actin to 6S myosin also hinders cleavage at site A for each of these proteases. To investigate binding of actin to 6S and 10S myosins, adenosine 5'-(beta,gamma-imidotriphosphate) (AMPPNP) is used as a substitute for ATP. In the presence of AMPPNP, it is shown that the 6S to 10S transition occurs and that 10S myosin binds actin with lower affinity than 6S myosin. For 6S myosin at high salt (0.35 M KCl) the dissociation constant of actin from the actin-myosin-nucleotide complex (K3) is approximately the same for phosphorylated (1.9 mol of P/mol of myosin) and dephosphorylated myosin, i.e., 1.3-2.4 microM, respectively. At lower ionic strength (0.17 M KCl) K3 for dephosphorylated myosin (10S myosin) is 42 microM and K3 for phosphorylated myosin (6S myosin) is 0.3 microM. These data show that the conformation of myosin influences the actin-myosin interaction. The constant (K4) for the dissociation of nucleotide from the actin-myosin-nucleotide complex varies slightly (in the range of 0.2-1.3 mM), but there is no marked change as a result of either a conformational change or phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of myosin in non-muscle and smooth muscle cells. Possible rules and evolutionary trends

Reversible phosphorylation of myosin subunits is observed in almost all eukaryotic cells. The data concerning sites and effects of phosphorylation on actin-activated ATPase activity of myosin and on its filament formation are described. These observations are discussed in terms of possible evolutionary trends and rules which may govern the process of myosin phosphorylation.

The role of myosin phosphorylation in the contraction-relaxation cycle of smooth muscle

Considerable evidence from a variety of experimental procedures indicates that the phosphorylation of myosin is involved in the regulation of contractile activity in smooth muscle. Phosphorylation of the 20,000-dalton myosin light chains is required to initiate crossbridge cycling and this is consistent with the observation that the actin-activated Mg2+ -ATPase activity of myosin is phosphorylation-dependent. In the simplest interpretation of this process it may be proposed that phosphorylation acts as an 'on-off' switch. Clearly this cannot explain the observed complexity of smooth muscle contractile behavior and such may imply either that additional mechanisms are involved or that the role of myosin phosphorylation is not fully appreciated. Recently it has been shown that monomeric smooth muscle myosin can exist in a 'folded' and an 'extended' conformation and that each form is characterized by distinct enzymatic properties. Under appropriate solvent conditions phosphorylation of myosin favors the extended conformation. It is tentatively suggest that this, or an analogous, transition might be involved in the regulation of the smooth muscle contractile apparatus, and this possibility is discussed.

Effects of Ca2+ and Mg2+ on the actomyosin adenosine-5'-triphosphatase of stably phosphorylated gizzard myosin

There are conflicting reports on the effect of Ca2+ on actin activation of myosin adenosine-triphosphatase (ATPase) once the light chain is fully phosphorylated by a calcium calmodulin dependent kinase. Using thiophosphorylated gizzard myosin, Sherry et al. [Sherry, J. M. F., Gorecka, A., Aksoy, M. O., Dabrowska, R., & Hartshorne, D. J. (1978) Biochemistry 17, 4417-4418] observed that the actin activation of ATPase was not inhibited by the removal of Ca2+. Hence, it was suggested that the regulation of actomyosin ATPase activity of gizzard myosin by calcium occurs only via phosphorylation. In the present study, phosphorylated and thiophosphorylated myosins were prepared free of kinase and phosphatase activity; hence, the ATPase activity could be measured at various concentrations of Ca2+ and Mg2+ without affecting the level of phosphorylation. The ATPase activity of myosin was activated either by skeletal muscle or by gizzard actin at various concentrations of Mg2+ and either at pCa 5 or at pCa 8. The activation was sensitive to Ca2+ at low Mg2+ concentrations with both actins. Tropomyosin potentiated the actin-activated ATPase activity at all Mg2+ and Ca2+ concentrations. The calcium sensitivity of phosphorylated and thiophosphorylated myosin reconstituted with actin and tropomyosin was most pronounced at a free Mg2+ concentration of about 3 mM. The binding of 125I-tropomyosin to actin showed that the calcium sensitivity of ATPase observed at low Mg2+ concentration is not due to a calcium-mediated binding of tropomyosin to F-actin. The actin activation of both myosins was insensitive to Ca2+ when the Mg2+ concentration was increased above 5 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of magnesium chloride on smooth muscle actomyosin adenosine-5'-triphosphatase activity, myosin conformation, and tension development in glycerinated smooth muscle fibers

The contractile system of smooth muscle exhibits distinctive responses to varying Mg2+ concentrations in that maximum adenosine-5'-triphosphatase (ATPase) activity of actomyosin requires relatively high concentrations of Mg2+ and also that tension in skinned smooth muscle fibers can be induced in the absence of Ca2+ by high Mg2+ concentrations. We have examined the effects of MgCl2 on actomyosin ATPase activity and on tension development in skinned gizzard fibers and suggest that the MgCl2-induced changes may be correlated to shifts in myosin conformation. At low concentrations of free Mg2+ (less than or equal to 1 mM) the actin-activated ATPase activity of phosphorylated turkey gizzard myosin is reduced and is increased as the Mg2+ concentration is raised. The increase in Mg2+ (over a range of 1-10 mM added MgCl2) induces the conversion of 10S phosphorylated myosin to the 6S form, and it was found that the proportion of myosin as 10S is inversely related to the level of actin-activated ATPase activity. Activation of the actin-activated ATPase activity also occurs with dephosphorylated myosin but at higher MgCl2 concentrations, between 10 and 40 mM added MgCl2. Viscosity and fluorescence measurements indicate that increasing Mg2+ levels over this concentration range favor the formation of the 6S conformation of dephosphorylated myosin, and it is proposed that the 10S to 6S transition is a prerequisite for the observed activation of ATPase activity. With glycerinated chicken gizzard fibers high MgCl2 concentrations (6-20 mM) promote tension in the absence of Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

The variable relation between myosin light-chain phosphorylation and actin-activated ATPase activity in chicken gizzard smooth muscle. Modulation by tropomyosin

Using pure myosin light-chain kinase and myosin from chicken gizzard and rabbit skeletal actin, we have established the relationship between myosin phosphorylation and actin-activated ATPase activity by varying the concentration of Ca2+ or calmodulin (CaM). By increasing Ca2+ or CaM the extent of myosin phosphorylation increased from 10% to about 100%, the requirements for half-maximal activation being 0.17 microM Ca2+ and 1 nM CaM. ATPase was activated only when a phosphorylation 'threshold' of about 60% was surpassed. By increasing phosphorylation from 60% to 100%, ATPase activity was further stimulated, the relationship between myosin phosphorylation and ATPase activity being curvilinear. Addition of 1 microM tropomyosin lowered the Ca2+ requirement for half-maximal ATPase activity in the prescence of 0.05 microM CaM from 0.4 microM to 0.2 microM but did not affect the maximal ATPAse activity. In addition, the CaM requirement of ATPase activity was also lowered by gizzard tropomyosin at 9 microM Ca2+. The Ca2+/CaM requirement for myosin phosphorylation remained unaffected by tropomyosin. Thus, tropomyosin altered the relationship between myosin phosphorylation and ATPase activity: the phosphorylation 'threshold' was lowered from 60% to about 10%, hence causing a leftward shift of the relationship which becomes similar to that in the case of native actomyosin.

Correlation of enzymatic properties and conformation of smooth muscle myosin

In the presence of adenosine 5'-triphosphate (ATP) and 1-10 mM MgCl2, the relative viscosity (eta rel) of dephosphorylated gizzard myosin is reduced markedly over a range of KCl from 0.35 to 0.15 M. Sedimentation patterns show that the decrease in eta rel is due to the conversion of the 6S to 10S forms of myosin. Under similar conditions, eta rel of phosphorylated myosin is not altered, and at 0.2 M KCl, the 10S form is not observed. In 1 and 2 mM MgCl2 and less than 0.2 M KCl, 10S can be formed from both phosphorylated myosin plus ATP and dephosphorylated myosin minus ATP. In the presence of ethylenediaminetetraacetic acid (EDTA), the decrease of eta rel and corresponding change in sedimentation pattern are independent of ATP and show only a dependence on KCl. Therefore, ATP and dephosphorylation are not obligatory for the 6S to 10S transition. In all instances, the 6S-10S transition of monomeric myosin is paralleled by an alteration of adenosine-5'-triphosphatase (ATPase) activity; i.e., the KCl dependence of the two processes is the same. Transition from 6S to 10S causes a decrease in Mg2+-and Ca2+-ATPase activity of myosin and an increase in K+-EDTA-ATPase activity. The relationship between myosin shape and the ATP dependence of Mg2+-ATPase activity also is consistent with this generalization. The phosphorylation dependence of the viscosity transition from 6S to 10S is not linear, and phosphorylation of both heads is required for the complete transition. In contrast, the ATP dependence of the transition is linear, and the binding of 2 mol of ATP/myosin is required for maximum effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of the myofibrillar proteins and the regulation of contractile activity in muscle

Evidence now exists for the phosphorylation of all the major proteins of the myofibril with the exception of troponin C. Although uncertainty exists in most cases about the role of phosphorylation of the myofibrillar proteins, there is substantial evidence that phosphorylation of serine 20 of rabbit cardiac troponin I leads to a lowering of the sensitivity of the actomyosin ATPase to Ca2+. This process is of special importance in the physiological response of the heart to adrenalin. A well defined enzymic system involving a specific kinase and a phosphatase is present in most muscles for the phosphorylation and dephosphorylation of the P light chain (regulatory, L2 or DTNB light chain) of myosin. Myosin light-chain kinase is very active in fast skeletal muscles, and although it is unlikely that phosphorylation followed by dephosphorylation of the P light chain occurs fast enough to be synchronous with the contractile cycle, phosphorylation may have a modulatory role in this tissue. Both post-tetanic potentiation and the reduced actomyosin ATPase turnover rate observed in fast-twitch muscle as a consequence of sustained forceful contraction have been suggested by different investigators to be consequences of P light chain phosphorylation. Nevertheless, unequivocal evidence associating either of these effects with phosphorylation is not yet available. Kinase activity is also high in vertebrate smooth muscle and it has been suggested that phosphorylation of the P light chain is the process that activates the actomyosin ATPase in this tissue. Evidence from a number of studies indicates, however, that regulation of smooth muscle actomyosin ATPase may not be a simple phosphorylation-dephosphorylation process.