Properties of myosin light chain kinase prepared from rabbit skeletal muscle by an improved method

Differential roles of regulatory light chain and myosin binding protein-C phosphorylations in the modulation of cardiac force development

Phosphorylation of myosin regulatory light chain (RLC) by myosin light chain kinase (MLCK) and myosin binding protein-C (cMyBP-C) by protein kinase A (PKA) independently accelerate the kinetics of force development in ventricular myocardium. However, while MLCK treatment has been shown to increase the Ca(2+) sensitivity of force (pCa(50)), PKA treatment has been shown to decrease pCa(50), presumably due to cardiac troponin I phosphorylation. Further, MLCK treatment increases Ca(2+)-independent force and maximum Ca(2+)-activated force, whereas PKA treatment has no effect on either force. To investigate the structural basis underlying the kinase-specific differential effects on steady-state force, we used synchrotron low-angle X-ray diffraction to compare equatorial intensity ratios (I(1,1)/I(1,0)) to assess the proximity of myosin cross-bridge mass relative to actin and to compare lattice spacings (d(1,0)) to assess the inter-thick filament spacing in skinned myocardium following treatment with either MLCK or PKA. As we showed previously, PKA phosphorylation of cMyBP-C increases I(1,1)/I(1,0) and, as hypothesized, treatment with MLCK also increased I(1,1)/I(1,0), which can explain the accelerated rates of force development during activation. Importantly, interfilament spacing was reduced by 2 nm (3.5%) with MLCK treatment, but did not change with PKA treatment. Thus, RLC or cMyBP-C phosphorylation increases the proximity of cross-bridges to actin, but only RLC phosphorylation affects lattice spacing, which suggests that RLC and cMyBP-C modulate the kinetics of force development by similar structural mechanisms; however, the effect of RLC phosphorylation to increase the Ca(2+) sensitivity of force is mediated by a distinct mechanism, most probably involving changes in interfilament spacing.

Acceleration of stretch activation in murine myocardium due to phosphorylation of myosin regulatory light chain

The regulatory light chains (RLCs) of vertebrate muscle myosins bind to the neck region of the heavy chain domain and are thought to play important structural roles in force transmission between the cross-bridge head and thick filament backbone. In vertebrate striated muscles, the RLCs are reversibly phosphorylated by a specific myosin light chain kinase (MLCK), and while phosphorylation has been shown to accelerate the kinetics of force development in skeletal muscle, the effects of RLC phosphorylation in cardiac muscle are not well understood. Here, we assessed the effects of RLC phosphorylation on force, and the kinetics of force development in myocardium was isolated in the presence of 2,3-butanedione monoxime (BDM) to dephosphorylate RLC, subsequently skinned, and then treated with MLCK to phosphorylate RLC. Since RLC phosphorylation may be an important determinant of stretch activation in myocardium, we recorded the force responses of skinned myocardium to sudden stretches of 1% of muscle length both before and after treatment with MLCK. MLCK increased RLC phosphorylation, increased the Ca(2+) sensitivity of isometric force, reduced the steepness of the force-pCa relationship, and increased both Ca(2+)-activated and Ca(2+)-independent force. Sudden stretch of myocardium during an otherwise isometric contraction resulted in a concomitant increase in force that quickly decayed to a minimum and was followed by a delayed redevelopment of force, i.e., stretch activation, to levels greater than pre-stretch force. MLCK had profound effects on the stretch activation responses during maximal and submaximal activations: the amplitude and rate of force decay after stretch were significantly reduced, and the rate of delayed force recovery was accelerated and its amplitude reduced. These data show that RLC phosphorylation increases force and the rate of cross-bridge recruitment in murine myocardium, which would increase power generation in vivo and thereby enhance systolic function.

Basal myosin light chain phosphorylation is a determinant of Ca2+ sensitivity of force and activation dependence of the kinetics of myocardial force development

It is generally recognized that ventricular myosin regulatory light chains (RLC) are approximately 40% phosphorylated under basal conditions, and there is little change in RLC phosphorylation with agonist stimulation of myocardium or altered stimulation frequency. To establish the functional consequences of basal RLC phosphorylation in the heart, we measured mechanical properties of rat skinned trabeculae in which approximately 7% or approximately 58% of total RLC was phosphorylated. The protocol for achieving approximately 7% phosphorylation of RLC involved isolating trabeculae in the presence of 2,3-butanedione monoxime (BDM) to dephosphorylate RLC from its baseline level. Subsequent phosphorylation to approximately 58% of total was achieved by incubating BDM-treated trabeculae in solution containing smooth muscle myosin light chain kinase, calmodulin, and Ca2+ (i.e., MLCK treatment). After MLCK treatment, Ca2+ sensitivity of force increased by 0.06 pCa units and maximum force increased by 5%. The rate constant of force development (ktr) increased as a function of Ca2+ concentration in the range between pCa 5.8 and pCa 4.5. When expressed versus pCa, the activation dependence of ktr appeared to be unaffected by MLCK treatment; however, when activation was expressed in terms of isometric force-generating capability (as a fraction of maximum), MLCK treatment slowed ktr at submaximal activations. These results suggest that basal phosphorylation of RLC plays a role in setting the kinetics of force development and Ca2+ sensitivity of force in cardiac muscle. Our results also argue that changes in RLC phosphorylation in the range examined here influence actin-myosin interaction kinetics differently in heart muscle than was previously reported for skeletal muscle.

Skeletal muscle myosin monomer in equilibrium with filaments forms a folded conformation

Rabbit skeletal myosin forms stable filaments under physiological conditions, and only a small amount stays as a monomer in equilibrium with filaments. The myosin monomers were observed in two conformational states, as extended and folded forms upon electron microscopy and gel filtration high performance liquid chromatography. The fraction of monomers in the folded conformation increased with a decrease in the concentration of NaCl below 0.2 M, and the conformational state was affected neither by the presence of ATP nor by the phosphorylation of regulatory light chain. In most of the folded monomers, the tail bent back toward the heads at one region, 45 nm apart from the head-tail junction, and the remaining tail portion containing the C-terminal tip appeared to interact with the head-tail junction. Only a small percentage of the folded monomers was in a more compact conformation close to the 10 S conformation of vertebrate smooth muscle and non-muscle myosins. The folded monomers, however, may not trap the products of ATP hydrolysis as assessed by single turnover experiments. The percentage of monomers in the 10 S-like conformation was increased by the exchange of a regulatory light chain with the smooth muscle light chain, indicating the participation of head-tail junction, including the regulatory light chain in the formation of folded conformation. The folded conformation may be common to various myosin IIs, suggestive of common roles for the folded monomers.

Phosphorylation of myosin regulatory light chain eliminates force-dependent changes in relaxation rates in skeletal muscle

The rate of relaxation from steady-state force in rabbit psoas fiber bundles was examined before and after phosphorylation of myosin regulatory light chain (RLC). Relaxation was initiated using diazo-2, a photolabile Ca2+ chelator that has low Ca2+ binding affinity (K(Ca) = 4.5 x 10(5) M(-1)) before photolysis and high affinity (K(Ca) = 1.3 x 10(7) M(-1)) after photolysis. Before phosphorylating RLC, the half-times for relaxation initiated from 0.27 +/- 0.02, 0.51 +/- 0.03, and 0.61 +/- 0.03 Po were 90 +/- 6, 140 +/- 6, and 182 +/- 9 ms, respectively. After phosphorylation of RLC, the half-times for relaxation from 0.36 +/- 0.03 Po, 0.59 +/- 0.03 Po, and 0.65 +/- 0.02 Po were 197 +/- 35 ms, 184 +/- 35 ms, and 179 +/- 22 ms. This slowing of relaxation rates from steady-state forces less than 0.50 Po was also observed when bundles of fibers were bathed with N-ethylmaleimide-modified myosin S-1, a strongly binding cross-bridge derivative of S1. These results suggest that phosphorylation of RLC slows relaxation, most likely by slowing the apparent rate of transition of cross-bridges from strongly bound (force-generating) to weakly bound (non-force-generating) states, and reduces or eliminates Ca2+ and cross-bridge activation-dependent changes in relaxation rates.

Chimeras of yeast and chicken calmodulin demonstrate differences in activation mechanisms of target enzymes

Various chimeric proteins were constructed from yeast (Saccharomyces cerevisiae) and chicken calmodulin (CaM), and regions essential for target activation and responsible for the specific features of the yeast CaM were identified. The chimeric CaMs were designed so that each Ca2+ binding site of the yeast CaM was replaced in series from the C-terminus. Resulting CaM proteins showed Ca2+ binding properties inherent to the original Ca2+ binding site. Cooperative Ca2+ binding and a suitable rearrangement of the two EF-hand sites in each half-molecular domain were shown to be important for high-affinity interaction with CaM-dependent cyclic nucleotide phosphodiesterase (PDE). Residues in chicken CaM sequences 129-148 and 88-128, respectively, were required for low values of Kact (the concentration of CaM required for the half-maximal activation) in the activation of PDE and myosin light chain kinase (skMLCK and smMLCK). The difference in the structural requirements indicated different manners of the interaction. While PDE was activated to similar levels by different chimeras, the maximum activity (Vmax) given by chicken CaMs was not achieved by any chimeric CaMs in MLCKs. Residues in chicken CaM sequences 1-50 and 88-129, in addition to Ca2+ binding to the fourth site, were important for high values of Vmax of skMLCK. On the other hand, Met51 and residues in chicken CaM sequence 88-129 were critical for the high Vmax of smMLCK. These residues may work to form the active structure of the catalytic site of each MLCK, while simple binding of CaM seems sufficient to expose the active site of PDE.

Accuracy and precision in the determination of Stokes radii and molecular masses of proteins by gel filtration chromatography

The accuracy and precision of the estimates of hydrodynamic parameters of globular proteins obtained by inverse regression from gel filtration chromatographic data are discussed. The usefulness of gel filtration chromatography as the basis for a rapid and reliable method for the determination of the Stokes radius and the molecular mass is considered. The discussion is supported by an analysis of the models already proposed in the literature, and is based on the precision of the estimates.

Variations in cross-bridge attachment rate and tension with phosphorylation of myosin in mammalian skinned skeletal muscle fibers. Implications for twitch potentiation in intact muscle

The Ca2+ sensitivities of the rate constant of tension redevelopment (ktr; Brenner, B., and E. Eisenberg. 1986. Proceedings of the National Academy of Sciences. 83:3542-3546) and isometric force during steady-state activation were examined as functions of myosin light chain 2 (LC2) phosphorylation in skinned single fibers from rabbit and rat fast-twitch skeletal muscles. To measure ktr the fiber was activated with Ca2+ and steady isometric tension was allowed to develop; subsequently, the fiber was rapidly (less than 1 ms) released to a shorter length and then reextended by approximately 200 nm per half sarcomere. This maneuver resulted in the complete dissociation of cross-bridges from actin, so that the subsequent redevelopment of tension was related to the rate of cross-bridge reattachment. The time course of tension redevelopment, which was recorded under sarcomere length control, was best fit by a first-order exponential equation (i.e., tension = C(1 - e-kt) to obtain the value of ktr. In control fibers, ktr increased sigmoidally with increases in [Ca2+]; maximum values of ktr were obtained at pCa 4.5 and were significantly greater in rat superficial vastus lateralis fibers (26.1 +/- 1.2 s-1 at 15 degrees C) than in rabbit psoas fibers (18.7 +/- 1.0 s-1). Phosphorylation of LC2 was accomplished by repeated Ca2+ activations (pCa 4.5) of the fibers in solutions containing 6 microM calmodulin and 0.5 microM myosin light chain kinase, a protocol that resulted in an increase in LC2 phosphorylation from approximately 10% in the control fibers to greater than 80% after treatment. After phosphorylation, ktr was unchanged at maximum or very low levels of Ca2+ activation. However, at intermediate levels of Ca2+ activation, between pCa 5.5 and 6.2, there was a significant increase in ktr such that this portion of the ktr-pCa relationship was shifted to the left. The steady-state isometric tension-pCa relationship, which in control fibers was left shifted with respect to the ktr-pCa relationship, was further left-shifted after LC2 phosphorylation. Phosphorylation of LC2 had no effect upon steady-state tension during maximum Ca2+ activation. In fibers from which troponin C was partially extracted to disrupt molecular cooperativity within the thin filament (Moss et al. 1985. Journal of General Physiology. 86:585-600), the effect of LC2 phosphorylation to increase the Ca2+ sensitivity of steady-state isometric force was no longer evident, although the effect of phosphorylation to increase ktr was unaffected by this maneuver.(ABSTRACT TRUNCATED AT 400 WORDS)

A simple and rapid preparation of fully phosphorylated and fully dephosphorylated skeletal muscle myosin. Application to the preparation of a phosphorylated LC2-modified artificial isozyme

Fast skeletal myosin LC2 is phosphorylated on ser-15 by a specific myosin light chain kinase (MLCK) in the presence of Ca2+ and calmodulin, and dephosphorylated by a muscle phosphate in the presence of Mg2+. Fully dephosphorylated myosin is obtained by dialysis of muscle crude extract (0.06 M NaCl, 0.01 M Tris-HCl, pH 7.5, 50 microM EGTA); fully phosphorylated myosin is obtained by addition of Ca2+ (0.2 mM), Mg2+ (10 mM) and ATP (3 mM) and 5 min incubation at 28 degrees C. The following reaction characteristics were noted. The crude extract is a very efficient phosphorylating complex and can be diluted to phosphorylate or dephosphorylate purified myosin. Phosphorylation and dephosphorylation appear monophasic, showing no evidence of negative cooperativity in this particular type of myosin and medium. Phosphorylation is 24 times slower in the presence of 0.45 M KCl, 5 mM pyrophosphate. Thiophosphorylated myosin is slowly dephosphorylated by phosphatase. At the crude myosin stage the dephosphorylation reaction is efficiently inhibited (at 0-4 degrees C) by the presence of 70 mM NaF. Myosin-[(T)-LC2'] (a myosin species in which LC2 has been selectively modified by trypsin) is an interesting species refractory to phosphorylation. The myosin-[(T)-LC2'] isozyme can be obtained fully phosphorylated by phosphorylation of myosin followed by limited tryptic proteolysis as described earlier. Urea-PAGE as used separates LC2, phosphoryl-LC2, LC2' and phosphoryl-LC2' effectively and in this order. Through this procedure the (de)-phosphorylating complex is ipso facto specific to the myosin species considered; the method avoids lengthy preparations of purified proteins and is easy, rapid and efficient.

Steady-state kinetics of skeletal muscle myosin light chain kinase indicate a strong down regulation by products

The kinetic behaviour of myosin light chain kinase isolated from skeletal muscle was studied under steady-state conditions using highly purified phosphorylatable light chains 2 (LC2). Forward reaction, product inhibition, and reverse reaction data indicate a sequential mechanism which can be interpreted best by a rapid-equilibrium random bi-bi reaction model. The forward reaction parameters are KATP = 150 microM, KLC2 = 5.3 microM, and Ki LC2 = 7.6 microM. The enzyme forms a dead-end complex with ADP and light chain 2; Kd, ADP of this complex is 50 microM. The forward reaction is also strongly inhibited by the phosphorylated light chain 2, Ki, LC2P is 1.5 microM. An equilibrium constant Keq of about 70 can be calculated from the kinetic parameters which agrees with the directly measured value of about 60. The role of the two inhibitory mechanisms in the regulation of the enzyme and of the high energy of the light chain phosphate bond as deducible from Keq are discussed.

Identification of the calmodulin-binding domain of skeletal muscle myosin light chain kinase

In the course of determining the primary structure of rabbit skeletal muscle myosin light chain kinase (MLCK; ATP:protein phosphotransferase, EC 2.7.1.37) a peptide fragment was obtained that appears to represent the calmodulin-binding domain of this enzyme. Low concentrations of the peptide inhibited calmodulin activation of MLCK (Ki congruent to 1 nM). The peptide was not associated with a catalytically active, calmodulin-independent form of MLCK that was obtained by limited proteolysis. The peptide is 27 residues in length and represents the carboxyl terminus of MLCK. The sequence of the peptide shows no significant homology with any known protein sequence. The peptide contains one tryptophanyl residue and a high percentage of basic and hydrophobic residues, but no acidic or prolyl residues. Much of the sequence has a high probability of forming alpha helix. A chemically synthesized peptide has been prepared to study the interactions of the peptide and calmodulin in more detail. The intrinsic tryptophan fluorescence of the synthetic peptide shows a significant enhancement (approximately equal to 45%) in the presence of Ca2+ and calmodulin; fluorescence enhancement is maximal at a peptide:calmodulin stoichiometry of 1:1. Calmodulin-Sepharose affinity chromatography in the presence of 2 M urea indicates that the interaction of peptide and calmodulin is Ca2+-dependent. The results of these studies indicate that the catalytic and calmodulin-binding domains of MLCK represent distinct and separable regions of the protein. In addition, the results provide a basis for future studies of the molecular and evolutionary details of calmodulin-dependent enzyme regulation.