Ca2+, calmodulin and cyclic AMP-dependent modulation of actin-myosin interactions in aorta

Incubation of bovine aortic native actomyosin with cyclic AMP and bovine aortic cyclic AMP-dependent protein kinase produced a rightward shift in the relation between free Ca2+ and both superprecipitation and actomyosin ATPase activity. The relation between free Ca2+ and phosphorylation of myosin light chains was also shifted to the right. The concentration of free Ca2+ required for half-maximal activation of both ATPase activity and myosin light chain phosphorylation was approximately 1.0 microM for control actomyosin and 2.5 microM for actomyosin incubated with cyclic AMP-protein kinase. Neither basal nor maximal activities were significantly affected by incubation with cyclic AMP-protein kinase. Addition of e microM calmodulin to cyclic AMP-protein kinase-treated actomyosin relieved inhibition of both superprecipitation and myosin light chain phosphorylation. These findings suggest that cyclic AMP-protein kinase-mediated inhibition of actin-myosin interactions in vascular smooth muscle involve a shift in the Ca2+ sensitivity of the system. This shift probably involves Ca2+-calmodulin interactions and the control of phosphorylation of the myosin light chains.     

Phosphorylation of uterine smooth muscle myosin permits actin-activation.

Myosin was purified from ovine uterine smooth muscle. The 20,000 dalton myosin light chain was phosphorylated to varying degrees by an endogenous Ca2+ dependent kinase. The kinase and endogenous phosphatases were then removed via column chromatography. In the absence of actin neither the size of the initial phosphate burst nor the steady state Mg2+-dependent ATPase activity were affected by phosphorylation. However, phosphorylation was required for actin to increase the Mg2+-dependent ATPase activity and for the myosin to superprecipitate with actin. Ca2+ did not affect the Mg2+-dependent ATPase activity in the presence or absence of action or the rate or extent of superprecipitation with actin once phosphorylation was obtained. These data indicate that: 1) phosphorylation of the 20,000 dalton myosin light chain controls the uterine smooth muscle actomyosin interaction, 2) in the absence of actin, phosphorylation does not affect either the ATPase of myosin or the size of the initial burst of phosphate and, 3) Ca2+ is important in controlling the light chain kinase but not the actomyosin interaction.     

Regulation of embryonic smooth muscle myosin by protein kinase C

Phosphorylation of the 20-kDa light chain regulates adult smooth muscle myosin; phosphorylation by the Ca2+/calmodulin-dependent enzyme myosin light chain kinase stimulates the actomyosin ATPase activity of adult smooth muscle myosin; the simultaneous phosphorylation of a separate site on the 20-kDa light chain by the Ca2+/phospholipid-dependent enzyme protein kinase C attenuates the myosin light chain kinase-induced increase in the actomyosin ATPase activity of adult myosin. Fetal smooth muscle myosin, purified from 12-day-old fertilized chicken eggs, is structurally different from adult smooth muscle myosin. Nevertheless, phosphorylation of a single site on the 20-kDa light chain of fetal myosin by myosin light chain kinase results in stimulation of the actomyosin ATPase activity of this myosin. Protein kinase C, in contrast, phosphorylates three sites on the fetal myosin 20-kDa light chain including a serine or threonine residue on the same peptide phosphorylated by myosin light chain kinase. Interestingly, phosphorylation by protein kinase C stimulates the actomyosin ATPase activity of fetal myosin. Moreover, unlike adult myosin, there is no attenuation of the actomyosin ATPase activity when fetal myosin is simultaneously phosphorylated by myosin light chain kinase and protein kinase C. These data demonstrate, for the first time, the in vitro activation of a smooth muscle myosin by another enzyme besides myosin light chain kinase and raise the possibility of alternate pathways for regulating smooth muscle myosin in vivo.     

平滑肌肌球蛋白B的超沉淀与肌球蛋白轻链磷酸化

本文报导了牛胃肌球蛋白B(天然肌动球蛋白)的超沉淀性质。当钙离子、钙调蛋白和ATP存在时,肌球蛋白B出现超沉淀,在pH6.8和7.5处,有两个峰值。Ca~(2+)(PCa值8-4)对超沉淀影响的浓度-反应曲线呈典型的S形,表明当Ca~(2+)浓度处于微摩尔水平时产生超沉淀。伴随超沉淀发生了肌球蛋白调节轻链磷酸化。这说明肌球蛋白轻链的Ca~(2+)-CaM依赖性磷酸化可能包含在脊椎动物平滑肌收缩活动的调节机制中。     

Ca2+ regulation not associated with phosphorylation of myosin light chain in aortic intima smooth muscle. II. Effects of regulatory proteins on the actin-myosin interaction

Contractile and regulatory proteins were prepared from bovine aortic intima, and actin from bovine stomach smooth and rabbit skeletal muscles. In the desensitized and reconstituted actomyosin system, the superprecipitation activity was measured by the turbidity method. Superprecipitation of each system was not exhibited even in the presence of Ca ions, but was observable only in the presence of tropomyosin and Ca ions, while 20,000-dalton light chain of myosin remained dephosphorylated during the reaction. Addition of tropomyosin to the reconstituted acto-myosin digest system (trypsin-digested myosin was devoid of 20,000-dalton light chain) also restored the Ca2+-sensitivity. These results indicate that the phosphorylation of myosin light chain is not a crucial step in the contraction of aortic intima smooth muscle. For full activation of the actin-myosin-ATP interaction, additional factors other than the myosin light chain kinase are required, although some contribution of the kinase to the full activation cannot be ruled out.     

Ca-linked phosphorylation of a light chain of vertebrate smooth-muscle myosin.

In vertebrate smooth muscle actomyosin and myofibrils a myosin light chain of molecular weight about 20,000 becomes phosphorylated at the same Ca2+ concentration as required to stimulate the actin-activated ATPase activity of myosin. Further, the degree of phosphorylation in the preparations as well as in various reconstituted actomyosins is proportional to their measured Ca2+ sensitivity. The phosphorylation process is very rapid and is essentially completed before the rise in ATPase activity. The enzyme responsible for the observed myosin phosphoylation is a specific myosin light chain kinase which is routinely co-purified with myosin. This kinase is normally present in actomyosin and its removal together with tropomyosin leads to a complete loss of the actin-activated ATPase activity. It is suggested that the Ca-dependent phosphorylation of the light chain via the light chain kinase represents the initial step in the activation of myosin that leads to contraction. Relaxation is probably effected by an as yet uncharacterised light chain phosphatase.     

Substrate based inhibitors of smooth muscle myosin light chain kinase.

Activation of myosin light chain kinase is a prerequisite for smooth muscle activation. In this study, short peptide analogs of the phosphorylation site of the myosin light chain were studied for their effects on several contractile protein systems. The peptides inhibited phosphorylation of isolated ventricular and smooth muscle myosin light chains by smooth muscle myosin light chain kinase, but they were only weak inhibitors of phosphorylation of intact myosin and actomyosin. The peptides were also unable to block force development or myosin light chain phosphorylation in glycerol permeabilized fibers of swine carotid media. Apparently, the association of the myosin light chain with myosin changes its conformation such that substrate analogs which are potent inhibitors of the phosphorylation of isolated myosin light chains by myosin light chain kinase are ineffective at blocking phosphorylation of the intact molecule.     

Phosphorylation and actin activation of brain myosin

A method is described for obtaining brain myosin that shows significant actin activation, after phosphorylation with chicken gizzard myosin light chain kinase. Myosin with this activity could be obtained only via the initial purification of brain actomyosin. The latter complex, isolated by a method similar to that used for smooth muscle, contained actin, myosin, tropomyosin of the non-muscle type and another actin-binding protein of approximately 100,000 daltons. From the presence of a specific myosin light chain kinase and phosphatase in brain tissue it is suggested that the regulation of actin-myosin interaction operates via phosphorylation and dephosphorylation of myosin.     

An aortic spontaneously active phosphatase dephosphorylates myosin and inhibits actin-myosin interaction

Actin-myosin interaction in aortic actomyosin reportedly requires phosphorylation of the 20,000 dalton myosin light chains. A spontaneously active phosphatase which dephosphorylates phosphorylase $\text{a}$ and isolated phosphorylated cardiac myosin light chains was extracted from bovine aortic smooth muscle. This enzyme, when added to aortic native actomyosin (a) significantly suppressed phosphorylation of the light chains of the native hexameric smooth muscle myosin, (b) accelerated the rate and increased the magnitude of myosin light chain dephosphorylation in actomyosin that had been prephosphorylated, and (c) markedly attenuated the rate of actin-myosin interaction. These results support the hypothesis that myosin phosphorylation and subsequent actin-myosin interactions (contractility) in vascular smooth muscle may be modulated by spontaneously active aortic phosphatase.     

Inhibition of smooth muscle actin-activated myosin Mg2+-ATPase activity by caldesmon

Caldesmon, a major calmodulin- and actin-binding protein of smooth muscle (Sobue, K., Muramoto, Y., Fujita, M., and Kakiuchi, S. (1981) Proc. Natl. Acad. Sci. U. S. A. 78, 5652-5655), has been obtained in highly purified form from chicken gizzard by a modification of a previously published procedure (Ngai, P. K., Carruthers, C. A., and Walsh, M. P. (1984) Biochem. J. 218, 863-870) and was found to cause a significant inhibition of both superprecipitation and actin-activated myosin Mg2+-ATPase activity in a system reconstituted from the purified contractile and regulatory proteins without influencing the phosphorylation state of myosin. This inhibitory effect was seen both in the presence and absence of tropomyosin. A Ca2+-and calmodulin-dependent kinase which catalyzed phosphorylation of caldesmon was identified in chicken gizzard; this kinase is distinct from myosin light-chain kinase. Caldesmon prepared by calmodulin-Sepharose affinity chromatography was contaminated with caldesmon kinase activity and was unable to inhibit actomyosin ATPase activity or superprecipitation. Phosphatase activity capable of dephosphorylating caldesmon was also identified in smooth muscle. These results indicate that caldesmon can inhibit smooth muscle actomyosin ATPase activity in vitro, and this function may itself be subject to regulation by reversible phosphorylation of caldesmon.     

Role of myosin light chain kinase in muscle contraction

In resting striated muscles of the rabbit muscle in vivo, the phosphorylatable light chain is partially phosphorylated. Tetanic stimulation increased the level of phosphorylation more rapidly in fast twitch than in slow twitch muscle. In both types of muscle the rate of dephosphorylation was relatively slow. In rabbit fast twitch muscles, phosphorylation levels persisted significantly above the resting value for some time after posttetanic potentiation had disappeared. The role of myosin light chain kinase in modulating contractile response in striated muscle is uncertain. In vertebrate smooth muscle the role of myosin phosphorylation appears to be different from that in striated muscle despite the general similarity of the actomyosin system in both tissues. Although phosphorylation in vitro increases the Mg2+ -ATPase of actomyosin, a number of features imply that a somewhat complex relationship exists between the level of phosphorylation and the actin activation of the Mg2+ -ATPase in vertebrate smooth muscle. Contrary to many earlier reports, preparations of smooth muscle actomyosin can be obtained with Mg2+ -ATPase activities comparable to those of actomyosin from skeletal muscle. Preliminary evidence is presented that suggests that phosphorylation changes the Ca2+ sensitivity of the Mg2+ -ATPase of smooth muscle actomyosin.     

Site-specific dephosphorylation of smooth muscle myosin light chain kinase by protein phosphatases 1 and 2A.

Smooth muscle myosin light chain kinase is phosphorylated at two sites (A and B) by different protein kinases. Phosphorylation at site A increases the concentration of Ca2+/calmodulin required for kinase activation. Diphosphorylated myosin light chain kinase was used to determine the site-specificity of several forms of protein serine/threonine phosphatase. These phosphatases readily dephosphorylated myosin light chain kinase in vitro and displayed differing specificities for the two phosphorylation sites. Type 2A protein phosphatase specifically dephosphorylated site A, and binding of Ca2+/calmodulin to the kinase had no effect on dephosphorylation. The purified catalytic subunit of type 1 protein phosphatase dephosphorylated both sites in the absence of Ca2+/calmodulin but only dephosphorylated site A in the presence of Ca2+/calmodulin. A protein phosphatase fraction was prepared from smooth muscle actomyosin by extraction with 80 mM MgCl2. On the basis of sensitivity to okadaic acid and inhibitor 2, this activity was composed of multiple protein phosphatases including type 1 activity. This phosphatase fraction dephosphorylated both sites in the absence of Ca2+/calmodulin. However, dephosphorylation of both sites A and B was completely blocked in the presence of Ca2+/calmodulin. These results indicate that two phosphorylation sites of myosin light chain kinase are dephosphorylated by multiple protein serine/threonine phosphatases with unique catalytic specificities.     

The effects of phosphorylation and dephosphorylation of brain myosin on its actin-activated Mg2+-ATPase and contractile activities

Purified bovine brain myosin contained approximately 1 and 3 mol of protein-bound phosphate/mol myosin in the light chains and heavy chains, respectively. Large portions of this light chain- and heavy chain-bound phosphate (about 0.8 and 2.4 mol, respectively) were removed by incubation with a brain phosphoprotein phosphatase and potato acid phosphatase, respectively. Upon phosphorylation of the dephosphorylated brain myosin with myosin light chain kinase and casein kinase II, about 1.6 and 3.0 mol of phosphate was incorporated into the light chains and heavy chains, respectively, while much lower levels of phosphate were incorporated into the non-dephosphorylated brain myosin under the same conditions. The actin-activated Mg2+-ATPase activity of brain myosin rephosphorylated with myosin light chain kinase was about twice as high as that of dephosphorylated brain myosin (about 30 and 15 nmol phosphate/mg/min, respectively). On the other hand, whereas the rephosphorylated brain myosin superprecipitated rapidly with F-actin, the rate of superprecipitation of the dephosphorylated brain myosin was extremely low. Under appropriate conditions, a loose network of tiny superprecipitates, which formed initially throughout the solution, contracted to form eventually a large and dense particle. These results indicate that phosphorylation of the light chains of brain myosin is a prerequisite for the contraction of brain actomyosin. The role of phosphorylation of the heavy chains by casein kinase II remains to be elucidated.     

Myosin Phosphatase Target Subunit 1 (MYPT1) Regulates the Contraction and Relaxation of Vascular Smooth Muscle and Maintains Blood Pressure

Myosin light chain phosphatase with its regulatory subunit, myosin phosphatase target subunit 1 (MYPT1) modulates Ca²⁺-dependent phosphorylation of myosin light chain by myosin light chain kinase, which is essential for smooth muscle contraction. The role of MYPT1 in vascular smooth muscle was investigated in adult MYPT1 smooth muscle specific knock-out mice. MYPT1 deletion enhanced phosphorylation of myosin regulatory light chain and contractile force in isolated mesenteric arteries treated with KCl and various vascular agonists. The contractile responses of arteries from knock-out mice to norepinephrine were inhibited by Rho-associated kinase (ROCK) and protein kinase C inhibitors and were associated with inhibition of phosphorylation of the myosin light chain phosphatase inhibitor CPI-17. Additionally, stimulation of the NO/cGMP/protein kinase G (PKG) signaling pathway still resulted in relaxation of MYPT1-deficient mesenteric arteries, indicating phosphorylation of MYPT1 by PKG is not a major contributor to the relaxation response. Thus, MYPT1 enhances myosin light chain phosphatase activity sufficient for blood pressure maintenance. Rho-associated kinase phosphorylation of CPI-17 plays a significant role in enhancing vascular contractile responses, whereas phosphorylation of MYPT1 in the NO/cGMP/PKG signaling module is not necessary for relaxation.     

Adenosine 3':5'-monophosphate-mediated inhibition of myosin light chain phosphorylation in bovine aortic actomyosin.

Ca2+-dependent phosphorylation of the myosin light chains in bovine aortic native actomyosin is markedly depressed in the presence of cyclic AMP and its dependent protein kinase. This inhibition occurs with either cardiac, skeletal, or aortic protein kinase plus cyclic AMP, while little or no inhibition occurs with either cyclic AMP or protein kinase alone. The extent of inhibition is related to the concentration of protein kinase and approaches a maximum of approximately 50%. Concomitant with the inhibition of myosin light chain phosphorylation is (a) an increased phosphorylation of a 100,000-dalton moiety which possibly corresponds to the myosin light chain kinase present in the native actomyosin preparation and (b) a decrease in the actomyosin Mg2+-ATPase activity. These findings suggest that modulation of actin-myosin interactions by the cAMP system directly at the level of the contractile proteins may represent a mechanism by which beta adrenergic relaxation occurs in mammalian vascular smooth muscle.     

Dephosphorylation of myosin by the catalytic subunit of a type-2 phosphatase produces relaxation of chemically skinned uterine smooth muscle

It is now well-established that phosphorylation of the 20,000-dalton light chain of smooth muscle myosin (LC20) is a prerequisite for muscle contraction. However, the relationship between myosin dephosphorylation and muscle relaxation remains controversial. In the present study, we utilized a highly purified catalytic subunit of a type-2, skeletal muscle phosphoprotein phosphatase (protein phosphatase 2A) and a glycerinated smooth muscle preparation to determine if myosin dephosphorylation, in the presence of saturating calcium and calmodulin, would cause relaxation of contracted uterine smooth muscle. Addition of the phosphatase catalytic subunit (0.28 microM) to the muscle bath produced complete relaxation of the muscle. The phosphatase-induced relaxation could be reversed by adding to the muscle bath either purified, thiophosphorylated, chicken gizzard 20,000-dalton myosin light chains or purified, chicken gizzard myosin light chain kinase. Incubation of skinned muscles with adenosine 5'-O-(thiotriphosphate) prior to the addition of phosphatase resulted in the incorporation of 0.93 mol of PO4/mol of LC20 and prevented phosphatase-induced relaxation. Under all of the above conditions, changes in steady-state isometric force were associated with parallel changes in myosin light chain phosphorylation over a range of phosphorylation extending from 0.01 to 0.97 mol of PO4/mol of LC20. We found no evidence that dephosphorylation of contracted uterine smooth muscles, in the presence of calcium and calmodulin, could produce a latch-state where isometric force was maintained in the absence of myosin light chain phosphorylation. These results show that phosphorylation or dephosphorylation of the 20,000-dalton myosin light chain is adequate for the regulation of contraction or relaxation, respectively, in glycerinated uterine smooth muscle.     

Zipper-interacting protein kinase induces Ca(2+)-free smooth muscle contraction via myosin light chain phosphorylation

The inhibition of myosin phosphatase evokes smooth muscle contraction in the absence of Ca(2+), yet the underlying mechanisms are not understood. To this end, we have cloned smooth muscle zipper-interacting protein (ZIP) kinase cDNA. ZIP kinase is present in various smooth muscle tissues including arteries. Triton X-100 skinning did not diminish ZIP kinase content, suggesting that ZIP kinase associates with the filamentous component in smooth muscle. Smooth muscle ZIP kinase phosphorylated smooth muscle myosin as well as the isolated 20-kDa myosin light chain in a Ca(2+)/calmodulin-independent manner. ZIP kinase phosphorylated myosin light chain at both Ser(19) and Thr(18) residues with the same rate constant. The actin-activated ATPase activity of myosin increased significantly following ZIP kinase-induced phosphorylation. Introduction of ZIP kinase into Triton X-100-permeabilized rabbit mesenteric artery provoked a Ca(2+)-free contraction. A protein phosphatase inhibitor, microcystin LR, also induced contraction in the absence of Ca(2+), which was accompanied by an increase in both mono- and diphosphorylation of myosin light chain. The observed sensitivity of the microcystin-induced contraction to various protein kinase inhibitors was identical to the sensitivity of isolated ZIP kinase to these inhibitors. These results suggest that ZIP kinase is responsible for Ca(2+) independent myosin phosphorylation and contraction in smooth muscle.     

Ca2+-independent smooth muscle contraction. a novel function for integrin-linked kinase

Smooth muscle contraction follows an increase in cytosolic Ca(2+) concentration, activation of myosin light chain kinase, and phosphorylation of the 20-kDa light chain of myosin at Ser(19). Several agonists acting via G protein-coupled receptors elicit a contraction without a change in [Ca(2+)](i) via inhibition of myosin light chain phosphatase and increased myosin phosphorylation. We showed that microcystin (phosphatase inhibitor)-induced contraction of skinned smooth muscle occurred in the absence of Ca(2+) and correlated with phosphorylation of myosin light chain at Ser(19) and Thr(18) by a kinase distinct from myosin light chain kinase. In this study, we identify this kinase as integrin-linked kinase. Chicken gizzard integrin-linked kinase cDNA was cloned, sequenced, expressed in E. coli, and shown to phosphorylate myosin light chain in the absence of Ca(2+) at Ser(19) and Thr(18). Subcellular fractionation revealed two distinct populations of integrin-linked kinase, including a Triton X-100-insoluble component that phosphorylates myosin in a Ca(2+)-independent manner. These results suggest a novel function for integrin-linked kinase in the regulation of smooth muscle contraction via Ca(2+)-independent phosphorylation of myosin, raise the possibility that integrin-linked kinase may also play a role in regulation of nonmuscle motility, and confirm that integrin-linked kinase is indeed a functional protein-serine/threonine kinase.     

Effect of vanadate on force and myosin light chain phosphorylation in skinned aortic smooth muscle.

The effects of vanadate were examined on Ca2+-activated force and phosphorylation of 20-kDa myosin light chain in membrane-permeabilized rabbit aortic smooth muscle strips. Addition of vanadate during maximum contraction reduced the force in a dose-dependent manner, and inhibited it almost completely at 1 mM. Two-dimensional polyacrylamide gel electrophoretic analyses revealed that vanadate also reduced the phosphorylation of 20- kDa myosin light chain in a dose-dependent manner from approximately 50% in the absence of vanadate to approximately 20% in the presence of 1 mM vanadate. The effects of 1 mM vanadate on purified myosin light chain kinase and phosphatase were then examined using purified myosin as substrate, and it was found that vanadate neither inhibited myosin light chain kinase nor activated myosin light chain phosphatase. These results indicate that the reduction in the 20-kDa myosin light chain phosphorylation level by vanadate may be effected through its inhibition of the force generation in skinned smooth muscle strip, as evidenced by the finding that vanadate eliminated the enhancement of myosin light chain kinase activity brought about by the interaction between purified myosin and actin.     

The influence of Mg2+ on the phosphorylation and dephosphorylation of myosin by an actomyosin preparation from vascular smooth muscle

Previous work has shown that Mg²⁺ levels modulate the net level of myosin light chain phosphorylation in bovine aortic smooth muscle actomyosin preparations. The goal of this study was to determine the precise step, i.e. phosphorylation or dephosphorylation, where Mg²⁺ modulates the net phosphorylation reaction. The technique using [γ³⁵S]ATPγS to monitor the phosphorylating step yielded no effect of either Mg²⁺ or Ca²⁺. Unfortunately the lack of Ca²⁺-dependence did not allow conclusions about the influence of Mg²⁺ on myosin light chain kinase activity. The study of the effect of Mg²⁺ on dephosphorylation showed that phosphatase activity in the actomyosin preparation exhibited a Mg²⁺ modulation only when the actomyosin was previously exposed to activating levels (3×10^?5M) of Ca²⁺, suggesting the presence of a Ca²⁺ -regulation system for myosin light chain phosphatase.