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The effect of adrenaline on the phosphorylation of the P light chain of myosin and troponin I in the perfused rabbit heart.

ABSTRACT: 1. Two-dimensional electrophoresis has been used to study the extent of phosphorylation of the P light chain of myosin and troponin I in the rabbit beating heart. 2. A procedure has been developed that eliminates endogenous protein phosphatase activity during homogenization and sample preparation for electrophoresis. 3. Evidence has been obtained for two unphosphorylated forms of the P light chain in myosin from the ventricle of the rabbit, guinea pig and cow. 4. In vivo and in the rabbit perfused beating heart about 25% of the P light-chain fraction is in the phosphorylated form. 5. Intervention with adrenaline produced a slight increase in the extent of phosphorylation that reached a maximum after the peak in inotropic response. A similar increase was obtained with ischaemia in the absence of adrenaline. 6. The changes in phosphorylation of the major forms of troponin I identified by electrophoresis occurred after the peak of response to adrenaline and were compatible with previous results.

SUBMITTER: Westwood SA

PROVIDER: S-EPMC1163069 | biostudies-other | 1981 Jul

REPOSITORIES: biostudies-other

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The site of phosphorylation of troponin I in the perfused rabbit heart. The effect of adrenaline.

Project description:1. On treatment of the perfused rabbit heart with adrenaline, the total covalently bound phosphate of troponin I increased from 1.14 mol of phosphate/mol to 1.86 mol of phosphate/mol. 2. Covalently bound phosphate could be identified only in the region of the molecule of cardiac troponin I consisting of residues 1--48. 3. When 32P-labelled orthophosphate was present in the perfusion medium the phosphate at serine-20 became radioactively labelled. This residue was the only significant site of phosphorylation that could be identified. 4. The addition of adrenaline caused a 4--5-fold increase in covalently bound [32P]phosphate. Virtually all of the 32P was located at serine-20. 5. It was concluded from these studies that the extent of phosphorylation of serine-20 of cardiac troponin I increased from 30--40% in the control perfused heart to about 100% in the presence of adrenaline.

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Phosphorylation of myosin light chains in perfused rat heart. Effect of adrenaline and increased cytoplasmic calcium ions.

Project description:1. A method was developed for the isolation of essentially pure myosin light chains from perfused rat heart. The phosphorylation of the P-light chains was estimated by hydrolysis and measurement of phosphate released, by electrophoresis in 8 M-urea and by 32P incorporation in perfusion with [32P]Pi. 2. In control perfusions there was 0.5-0.6 mol of phosphate/mol of P-light chain. This was not changed by perfusion with 5 microM-adrenaline for 10-40s. Perfusion for 1 min with medium containing 7.5 mM-CaCl2, or for 30s with medium containing 118 mM-KCl, also did not change the phosphorylation of P-light chains. 3. It is concluded that phosphorylation of P-light chains is not important in mediating the action of inotropic agents in the heart.

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Phosphorylation of rabbit cardiac-muscle troponin I by phosphorylase kinase. The effect of adrenaline.

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Cardiac myosin regulatory light chain kinase modulates cardiac contractility by phosphorylating both myosin regulatory light chain and troponin I.

Project description:Heart muscle contractility and performance are controlled by posttranslational modifications of sarcomeric proteins. Although myosin regulatory light chain (RLC) phosphorylation has been studied extensively in vitro and in vivo, the precise role of cardiac myosin light chain kinase (cMLCK), the primary kinase acting upon RLC, in the regulation of cardiomyocyte contractility remains poorly understood. In this study, using recombinantly expressed and purified proteins, various analytical methods, in vitro and in situ kinase assays, and mechanical measurements in isolated ventricular trabeculae, we demonstrate that human cMLCK is not a dedicated kinase for RLC but can phosphorylate other sarcomeric proteins with well-characterized regulatory functions. We show that cMLCK specifically monophosphorylates Ser23 of human cardiac troponin I (cTnI) in isolation and in the trimeric troponin complex in vitro and in situ in the native environment of the muscle myofilament lattice. Moreover, we observed that human cMLCK phosphorylates rodent cTnI to a much smaller extent in vitro and in situ, suggesting species-specific adaptation of cMLCK. Although cMLCK treatment of ventricular trabeculae exchanged with rat or human troponin increased their cross-bridge kinetics, the increase in sensitivity of myofilaments to calcium was significantly blunted by human TnI, suggesting that human cTnI phosphorylation by cMLCK modifies the functional consequences of RLC phosphorylation. We propose that cMLCK-mediated phosphorylation of TnI is functionally significant and represents a critical signaling pathway that coordinates the regulatory states of thick and thin filaments in both physiological and potentially pathophysiological conditions of the heart.

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Domain characterization of rabbit skeletal muscle myosin light chain kinase.

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Myosin light chain kinase (MLCK) regulates cell migration in a myosin regulatory light chain phosphorylation-independent mechanism.

Project description:Myosin light chain kinase (MLCK) has long been implicated in the myosin phosphorylation and force generation required for cell migration. Here, we surprisingly found that the deletion of MLCK resulted in fast cell migration, enhanced protrusion formation, and no alteration of myosin light chain phosphorylation. The mutant cells showed reduced membrane tether force and fewer membrane F-actin filaments. This phenotype was rescued by either kinase-dead MLCK or five-DFRXXL motif, a MLCK fragment with potent F-actin-binding activity. Pull-down and co-immunoprecipitation assays showed that the absence of MLCK led to attenuated formation of transmembrane complexes, including myosin II, integrins and fibronectin. We suggest that MLCK is not required for myosin phosphorylation in a migrating cell. A critical role of MLCK in cell migration involves regulating the cell membrane tension and protrusion necessary for migration, thereby stabilizing the membrane skeleton through F-actin-binding activity. This finding sheds light on a novel regulatory mechanism of protrusion during cell migration.

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Myosin regulatory light chain phosphorylation attenuates cardiac hypertrophy.

Project description:Hyperphosphorylation of myosin regulatory light chain (RLC) in cardiac muscle is proposed to cause compensatory hypertrophy. We therefore investigated potential mechanisms in genetically modified mice. Transgenic (TG) mice were generated to overexpress Ca2+/calmodulin-dependent myosin light chain kinase specifically in cardiomyocytes. Phosphorylation of sarcomeric cardiac RLC and cytoplasmic nonmuscle RLC increased markedly in hearts from TG mice compared with hearts from wild-type (WT) mice. Quantitative measures of RLC phosphorylation revealed no spatial gradients. No significant hypertrophy or structural abnormalities were observed up to 6 months of age in hearts of TG mice compared with WT animals. Hearts and cardiomyocytes from WT animals subjected to voluntary running exercise and isoproterenol treatment showed hypertrophic cardiac responses, but the responses for TG mice were attenuated. Additional biochemical measurements indicated that overexpression of the Ca2+/calmodulin-binding kinase did not perturb other Ca2+/calmodulin-dependent processes involving Ca2+/calmodulin-dependent protein kinase II or the protein phosphatase calcineurin. Thus, increased myosin RLC phosphorylation per se does not cause cardiac hypertrophy and probably inhibits physiological and pathophysiological hypertrophy by contributing to enhanced contractile performance and efficiency.

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The turnover of phosphate bound to myosin light chain-2 in perfused rat heart.

Project description:The rate of exchange of phosphate bound to ventricular myosin light chain-2 (LC2-P) was measured in rat hearts perfused with [32P]Pi at various levels of perfusate Ca2+. Computer simulations of the light-chain labelling suggested the presence of two isotopically distinct pools of LC2-P, one large pool comprising 90% of the total and a small pool consisting of the remaining 10%. At control levels of perfusate Ca2+ the phosphate of the large pool turned over very slowly (t 1/2 congruent to 250 min), whereas that of the small pool turned over much more rapidly (t 1/2 congruent to 1 min). At high levels of perfusate free Ca2+ (5mM) the turnover of the phosphate of the small pool decreased markedly, whereas that of the large pool remained little changed. Conversely, at low perfusate free Ca2+ (0.2 mM), the turnover of the large pool decreased, whereas that of the small pool remained unchanged. The possible identity of these two pools is discussed. The total myosin-light-chain kinase activity of rat ventricle was found to be only 2-3-fold higher than the kinase activity expressed in the heart under control conditions. This, coupled with the very low turnover of most of the LC2-bound phosphate, implies that, in heart, there is insufficient myosin-light-chain kinase activity to cause a rapid rise in the overall level of light-chain phosphorylation, even under conditions of increased cytoplasmic Ca2+.

| S-EPMC1147394 | biostudies-other

Myosin regulatory light chain phosphorylation enhances cardiac ?-myosin in vitro motility under load.

Project description:Familial hypertrophic cardiomyopathy (HCM) is characterized by left ventricular hypertrophy and myofibrillar disarray, and often results in sudden cardiac death. Two HCM mutations, N47K and R58Q, are located in the myosin regulatory light chain (RLC). The RLC mechanically stabilizes the myosin lever arm, which is crucial to myosin's ability to transmit contractile force. The N47K and R58Q mutations have previously been shown to reduce actin filament velocity under load, stemming from a more compliant lever arm (Greenberg, 2010). In contrast, RLC phosphorylation was shown to impart stiffness to the myosin lever arm (Greenberg, 2009). We hypothesized that phosphorylation of the mutant HCM-RLC may mitigate distinct mutation-induced structural and functional abnormalities. In vitro motility assays were utilized to investigate the effects of RLC phosphorylation on the HCM-RLC mutant phenotype in the presence of an ?-actinin frictional load. Porcine cardiac ?-myosin was depleted of its native RLC and reconstituted with mutant or wild-type human RLC in phosphorylated or non-phosphorylated form. Consistent with previous findings, in the presence of load, myosin bearing the HCM mutations reduced actin sliding velocity compared to WT resulting in 31-41% reductions in force production. Myosin containing phosphorylated RLC (WT or mutant) increased sliding velocity and also restored mutant myosin force production to near WT unphosphorylated values. These results point to RLC phosphorylation as a general mechanism to increase force production of the individual myosin motor and as a potential target to ameliorate the HCM-induced phenotype at the molecular level.

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Phosphorylation of myosin regulatory light chain controls myosin head conformation in cardiac muscle.

Project description:The effect of phosphorylation on the conformation of the regulatory light chain (cRLC) region of myosin in ventricular trabeculae from rat heart was determined by polarized fluorescence from thiophosphorylated cRLCs labelled with bifunctional sulforhodamine (BSR). Less than 5% of cRLCs were endogenously phosphorylated in this preparation, and similarly low values of basal cRLC phosphorylation were measured in fresh intact ventricle from both rat and mouse hearts. BSR-labelled cRLCs were thiophosphorylated by a recombinant fragment of human cardiac myosin light chain kinase, which was shown to phosphorylate cRLCs specifically at serine 15 in a calcium- and calmodulin-dependent manner, both in vitro and in situ. The BSR-cRLCs were exchanged into demembranated trabeculae, and polarized fluorescence intensities measured for each BSR-cRLC in relaxation, active isometric contraction and rigor were combined with RLC crystal structures to calculate the orientation distribution of the C-lobe of the cRLC in each state. Only two of the four C-lobe orientation populations seen during relaxation and active isometric contraction in the unphosphorylated state were present after cRLC phosphorylation. Thus cRLC phosphorylation alters the equilibrium between defined conformations of the cRLC regions of the myosin heads, rather than simply disordering the heads as assumed previously. cRLC phosphorylation also changes the orientation of the cRLC C-lobe in rigor conditions, showing that the orientation of this part of the myosin head is determined by its interaction with the thick filament even when the head is strongly bound to actin. These results suggest that cRLC phosphorylation controls the contractility of the heart by modulating the interaction of the cRLC region of the myosin heads with the thick filament backbone.

| S-EPMC4535163 | biostudies-literature

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