Phosphorylation by cyclic adenosine 3':5'-monophosphate-dependent protein kinase regulates myosin light chain kinase

Smooth muscle myosin light chain kinase, purified to homogeneity, has a molecular weight of 130,000 +/- 5,000 in sodium dodecyl sulfate polyacrylamide gel electrophoresis. The purified enzyme has a specific activity under maximal conditions of 30 mumol Pi transferred to myosin light chain/mg kinase/min at 24 C and is totally dependent on calmodulin and calcium for activity. Incubation of myosin kinase with the catalytic subunit of cyclic adenosine 3':5'-monophosphate-dependent protein kinase results in the covalent incorporation of up to one mol of phosphate per mol of myosin kinase in the absence of bound calmodulin. Limited tryptic digestion of the radioactively labeled kinase indicates that all of the label has been incorporated into a single tryptic peptide (mol wt approximately 22,000), suggesting that a single site is being phosphorylated. Phosphorylation of myosin kinase lowers the rate at which the kinase phosphorylates myosin light chain. The lower rate of light chain phosphorylation is due to a weaker binding of calmodulin to the phosphorylated kinase than to the unphosphorylated kinase. Cyclic adenosine 3':5'-monophosphate-dependent phosphorylation of the kinase actin-myosin interaction represents a possible link between hormonal binding to smooth muscle receptors and muscle relaxation. A scheme for this sequence of events is presented.     

Myosin light chain kinase phosphorylation in tracheal smooth muscle

Purified myosin light chain kinase from smooth muscle is phosphorylated by cyclic AMP-dependent protein kinase, protein kinase C, and the multifunctional calmodulin-dependent protein kinase II. Because phosphorylation in a specific site (site A) by any one of these kinases desensitizes myosin light chain kinase to activation by Ca2+/calmodulin, kinase phosphorylation could play an important role in regulating smooth muscle contractility. This possibility was investigated in 32P-labeled bovine tracheal smooth muscle. Treatment of tissues with carbachol, KCl, isoproterenol, or phorbol 12,13-dibutyrate increased the extent of kinase phosphorylation. Six primary phosphopeptides (A-F) of myosin light chain kinase were identified. Site A was phosphorylated to an appreciable extent only with carbachol or KCl, agents which contract tracheal smooth muscle. The extent of site A phosphorylation correlated to increases in the concentration of Ca2+/calmodulin required for activation. These results show that cyclic AMP-dependent protein kinase and protein kinase C do not affect smooth muscle contractility by phosphorylating site A in myosin light chain kinase. It is proposed that phosphorylation of myosin light chain kinase in site A in contracting tracheal smooth muscle may play a role in the reported desensitization of contractile elements to activation by Ca2+.     

Phosphorylation of mammalian myosin light chain kinases by the catalytic subunit of cyclic AMP-dependent protein kinase and by cyclic GMP-dependent protein kinase

The phosphorylation of the calmodulin-dependent enzyme myosin light chain kinase, purified from bovine tracheal smooth muscle and human blood platelets, by the catalytic subunit of cAMP-dependent protein kinase and by cGMP-dependent protein kinase was investigated. When myosin light chain kinase which has calmodulin bound is phosphorylated by the catalytic subunit of cAMP-dependent protein kinase, 1 mol of phosphate is incorporated per mol of tracheal myosin light chain kinase or platelet myosin light chain kinase, with no effect on the catalytic activity. Phosphorylation when calmodulin is not bound results in the incorporation of 2 mol of phosphate and significantly decreases the activity. The decrease in myosin light chain kinase activity is due to a 5 to 7-fold increase in the amount of calmodulin required for half-maximal activation of both tracheal and platelet myosin light chain kinase. In contrast to the results with the catalytic subunit of cAMP-dependent protein kinase, cGMP-dependent protein kinase cannot phosphorylate tracheal myosin light chain kinase in the presence of bound calmodulin. When calmodulin is not bound to tracheal myosin light chain kinase, cGMP-dependent protein kinase phosphorylates only one site, and this phosphorylation has no effect on myosin light chain kinase activity. On the other hand, cGMP-dependent protein kinase incorporates phosphate into two sites in platelet myosin light chain kinase when calmodulin is not bound. The sites phosphorylated by the two cyclic nucleotide-dependent protein kinases were compared by two-dimensional peptide mapping following extensive tryptic digestion of the phosphorylated myosin light chain kinases. With respect to the tracheal myosin light chain kinase, the single site phosphorylated by cGMP-dependent protein kinase when calmodulin is not bound appears to be the same site phosphorylated in the tracheal enzyme by the catalytic subunit of cAMP-dependent protein kinase when calmodulin is bound. With respect to the platelet myosin light chain kinase, the additional site that was phosphorylated by cGMP-dependent protein kinase when calmodulin was not bound was different from that phosphorylated by the catalytic subunit of cAMP-dependent protein kinase.     

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.     

Protein kinase C modulates in vitro phosphorylation of the smooth muscle heavy meromyosin by myosin light chain kinase

Protein kinase C phosphorylates different sites on the 20,000-Da light chain of smooth muscle heavy meromyosin (HMM) than did myosin light chain kinase (Nishikawa, M., Hidaka, H., and Adelstein, R. S. (1983) J. Biol. Chem. 258, 14069-14072). Although protein kinase C incorporates 1 mol of phosphate into 1 mol of 20,000-Da light chain when either HMM or the whole myosin molecule is used as a substrate, it catalyzes the incorporation of up to 3 mol of phosphate/mol of 20,000-Da light chain when the isolated light chains are used as a substrate. Threonine is the major phosphoamino acid resulting from phosphorylation of HMM by protein kinase C. Prephosphorylation of HMM by protein kinase C decreases the rate of phosphorylation of HMM by myosin light chain kinase due to a 9-fold increase of the Km for prephosphorylated HMM compared to that of unphosphorylated HMM. Prephosphorylation of HMM by myosin light chain kinase also results in a decrease of the rate of phosphorylation by protein kinase C due to a 2-fold increase of the Km for HMM. Both prephosphorylations have little or no effect on the maximum rate of phosphorylation. The sequential phosphorylation of HMM by myosin light chain kinase and protein kinase C results in a decrease in actin-activated MgATPase activity due to a 7-fold increase of the Km for actin over that observed with phosphorylated HMM by myosin light chain kinase but has little effect on the maximum rate of the actin-activated MgATPase activity. The decrease of the actin-activated MgATPase activity correlates well with the extent of the additional phosphorylation of HMM by protein kinase C following initial phosphorylation by myosin light chain kinase.     

Sites phosphorylated in myosin light chain in contracting smooth muscle

Purified smooth muscle myosin light chain can be phosphorylated at multiple sites by myosin light chain kinase and protein kinase C. We have determined the sites phosphorylated on myosin light chain in intact bovine tracheal smooth muscle. Stimulation with 10 microM carbachol resulted in 66 +/- 5% monophosphorylated and 11 +/- 2% diphosphorylated myosin light chain after 1 min, and 47 +/- 4% monophosphorylated and 5 +/- 2% diphosphorylated myosin light chain after 30 min. Myosin heavy chain contained 0.06 +/- 0.01 mol of phosphate/mol of protein which did not change with carbachol. At both 1 and 30 min the monophosphorylated myosin light chain contained only phosphoserine whereas the diphosphorylated myosin light chain contained both phosphoserine and phosphothreonine. Two-dimensional peptide mapping of tryptic digests of monophosphorylated and diphosphorylated myosin light chain obtained from carbachol-stimulated tissue was similar to the peptide maps of purified light chain monophosphorylated and diphosphorylated, respectively, by myosin light chain kinase; these maps were distinct from the map obtained with tracheal light chain phosphorylated by protein kinase C. Phosphorylation of tracheal smooth muscle myosin light chain by myosin light chain kinase yields the tryptic phosphopeptide ATSNVFAMFDQSQIQEFK with S the phosphoserine in the monophosphorylated myosin light chain and TS the phosphotreonine and phosphoserine in the diphosphorylated myosin light chain. Thus, stimulation of tracheal smooth muscle with a high concentration of carbachol results in formation of both monophosphorylated and diphosphorylated myosin light chain although the amount of diphosphorylated light chain is substantially less than monophosphorylated light chain. In the intact muscle, myosin light chain is phosphorylated at sites corresponding to myosin light chain kinase phosphorylation.     

Phosphorylation of smooth muscle myosin heavy and light chains. Effects of phorbol dibutyrate and agonists

A number of different protein kinases phosphorylate purified heavy chains or the 20-kDa light chain of smooth muscle myosin. The physiological significance of these phosphorylation reactions has been examined in intact smooth muscle. Myosin heavy chain was slightly phosphorylated (0.08 mol of phosphate/mol) under control conditions in bovine tracheal tissue. Treatment with carbachol, isoproterenol, or phorbol 12,13-dibutyrate resulted in no significant change. In contrast, heavy chain was phosphorylated to 0.30 mol of phosphate/mol of heavy chain in tracheal smooth muscle cells in culture. This value increased significantly with ionomycin treatment. In control tissues, 9% of the light chain was monophosphorylated with 32P in the serine site phosphorylated by myosin light chain kinase. Carbachol (0.1 microM) alone resulted in contraction and 42% monophosphorylated light chain with 32P only in the serine site phosphorylated by myosin light chain kinase. Similarly, stimulation with histamine, 5-hydroxytryptamine, or KCl resulted in 32P incorporation into only the myosin light chain kinase serine site. Phorbol 12,13-dibutyrate (1 microM) alone resulted in 22% monophosphorylated light chain. However, only 25% of the 32P was in the myosin light chain kinase serine site, whereas 75% was in a serine site phosphorylated by protein kinase C. Phorbol 12,13-dibutyrate plus carbachol resulted in 27% monophosphorylated light chain; 75% of the 32P was in the myosin light chain kinase serine site, with the remainder in the protein kinase C serine site. These results indicate that phorbol esters act to increase phosphorylation of myosin light chain by protein kinase C. However, receptor-mediated stimulation or depolarization leading to tracheal smooth muscle contraction results in phosphorylation of myosin light chain by myosin light chain kinase alone.     

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.     

Characterization of chicken skeletal muscle myosin light chain kinase. Evidence for muscle-specific isozymes

Myosin light chain kinase purified from chicken white skeletal muscle (Mr = 150,000) was significantly larger than both rabbit skeletal (Mr = 87,000) and chicken gizzard smooth (Mr = 130,000) muscle myosin light chain kinases, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Km and Vmax values with rabbit or chicken skeletal, bovine cardiac, and chicken gizzard smooth muscle myosin P-light chains were very similar for the chicken and rabbit skeletal muscle myosin light chain kinases. In contrast, comparable Km and Vmax data for the chicken gizzard smooth muscle myosin light chain kinase showed that this enzyme was catalytically very different from the two skeletal muscle kinases. Affinity-purified antibodies to rabbit skeletal muscle myosin light chain kinase cross-reacted with chicken skeletal muscle myosin light chain kinase, but the titer of cross-reacting antibodies was approximately 20-fold less than the anti-rabbit skeletal muscle myosin light chain kinase titer. There was no detectable antibody cross-reactivity against chicken gizzard myosin light chain kinase. Proteolytic digestion followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis or high performance liquid chromatography showed that these enzymes are structurally very different with few, if any, overlapping peptides. These data suggest that, although chicken skeletal muscle myosin light chain kinase is catalytically very similar to rabbit skeletal muscle myosin light chain kinase, the two enzymes have different primary sequences. The two skeletal muscle myosin light chain kinases appear to be more similar to each other than either is to chicken gizzard smooth muscle myosin light chain kinase.     

The Ca2+ -activated protease (calpain) modulates Ca2+/calmodulin dependent activity of smooth muscle myosin light chain kinase

The Ca2+ -activated neutral protease can proteolyze both Ca2+ -dependent cyclic nucleotide phosphodiesterase and smooth muscle myosin light chain kinase. Ca2+ -dependent cyclic nucleotide phosphodiesterase from rat brain was converted to the Ca2+ -independent active form by Ca2+ -activated protease. The proteolytic effects on myosin light chain kinase of Ca2+-activated protease differed in the presence and absence of the Ca2+-calmodulin (CaM) complex. In the presence of bound CaM, myosin light chain kinase (130k dalton) was degradated to a major fragment of 62 kDa, which had Ca2+/CaM-dependent enzyme and CaM-binding activity. When digestion occurred in the absence of bound CaM, myosin light chain kinase cleaved to a fragment of 60 kDa. This peptide had no enzymatic activity in the presence or absence of the Ca2+-CaM complex. Available evidence suggests that the Ca2+-activated proteases may recognize the conformational change of smooth muscle myosin light chain kinase induced by Ca2+-CaM complex.     

Phosphorylation of smooth muscle myosin light chain kinase by protein kinase C. Comparative study of the phosphorylated sites

Smooth muscle myosin light chain kinase is phosphorylated in vitro by protein kinase C purified from human platelets. When myosin light chain kinase which has calmodulin bound is phosphorylated by protein kinase C, 0.8-1.1 mol of phosphate is incorporated per mol of myosin light chain kinase with no effect on its enzyme activity. Phosphorylation of myosin light chain kinase with no calmodulin bound results in the incorporation of 2-2.4 mol of phosphate and significantly decreases the rate of myosin light chain kinase activity. The decrease in myosin light chain kinase activity is due to a 3.3-fold increase in the concentration of calmodulin necessary for the half-maximal activation of myosin light chain kinase. The sites phosphorylated by protein kinase C and the catalytic subunit of cAMP-dependent protein kinase were compared by two-dimensional peptide mapping following extensive tryptic digestion of phosphorylated myosin light chain kinase. The single site phosphorylated by protein kinase C when calmodulin is bound to myosin light chain kinase (site 3) is different from that phosphorylated by the catalytic subunit of cAMP-dependent protein kinase (site 1). The additional site that is phosphorylated by protein kinase C when calmodulin is not bound appears to be the same site phosphorylated by the catalytic subunit of cAMP-dependent protein kinase (site 2). These studies confirm the important role of site 2 in binding calmodulin to myosin light chain kinase. Sequential studies using both protein kinase C and the catalytic subunit of cAMP-dependent protein kinase suggest that the phosphorylation of site 1 also plays a part in decreasing the affinity of myosin light chain kinase for calmodulin.     

Phosphorylation of myosin light chain kinase: a cellular mechanism for Ca2+ desensitization

Phosphorylation of the regulatory light chain of myosin by the Ca²⁺/calmodulin-dependent myosin light chain kinase plays an important role in smooth muscle contraction, nonmuscle cell shape changes, platelet contraction, secretion, and other cellular processes. Smooth muscle myosin light chain kinase is also phosphorylated, and recent results from experiments designed to satisfy the criteria of Krebs and Beavo for establishing the physiological significance of enzyme phosphorylation have provided insights into the cellular regulation and function of this phosphorylation in smooth muscle. The multifunctional Ca²⁺/calmodulin-dependent protein kinase II phosphorylates myosin light chain kinase at a regulatory site near the calmodulin-binding domain. This phosphorylation increases the concentration of Ca²⁺/calmodulin required for activation and hence increases the Ca²⁺ concentrations required for myosin light chain kinase activity in cells. However, the concentration of cytosolic Ca²⁺ required to effect myosin light chain kinase phosphorylation is greater than that required for myosin light chain phosphorylation. Phosphorylation of myosin light chain kinase is only one of a number of mechanisms used by the cell to down regulate the Ca²⁺ signal in smooth muscle. Since both smooth and nonmuscle cells express the same form of myosin light chain kinase, this phosphorylation may play a regulatory role in cellular processes that are dependent on myosin light chain phosphorylation.     

Selective inhibition of catalytic activity of smooth muscle myosin light chain kinase

Systematically synthesized derivatives of ML-9, 1-(5-chloronaphthalenesulfonyl)-1H-hexahydro-1,4-diazepine, were found to inhibit both Ca2+-calmodulin-dependent and -independent smooth muscle myosin light chain kinases with a similar concentration dependence, and their inhibitions were of the competitive type with respect to ATP. Moreover, ML-9 as well as ATP or ADP exhibited an effective protection to inactivation of smooth muscle myosin light chain kinase by the nucleotide affinity label 5'-p-fluorosulfonylbenzoyladenosine, suggesting that ML-9 binds at or near the ATP-binding site on the kinase molecule. These derivatives, which were structurally unrelated to ATP and exhibited more hydrophobic properties detected by reverse-phase high-performance liquid chromatography, exhibited more potent inhibition toward smooth muscle myosin light chain kinase, indicating that the hydrophobic properties of these derivatives positively correlated well with their potencies of inhibiting the catalytic activity for the enzyme. These findings suggest that the ATP-binding site at the active center of smooth muscle myosin light chain kinase is located in a hydrophobic environment. The potent vaso-relaxing effect of ML-9 on rabbit vascular strips and on saponin-treated skinned smooth muscle cells was discussed in relation to the in vivo inhibition by this drug of smooth muscle myosin light chain kinase.     

A multifunctional cyclic nucleotide- and Ca2+-independent protein kinase from rabbit skeletal muscle

A cyclic nucleotide- and Ca²⁺-independent protein kinase, initially identified as a glycogen synthase kinase (Itarte, E. and Huang, K.-P. (1979) J. Biol. Chem. $\text{254}$, 4052–4057), was also found to phosphorylate phosphorylase kinase and troponin from skeletal muscle as well as myosin light chain and myosin light chain kinase from both smooth and skeletal muscles. With the exception of myosin light chain from skeletal muscle, all the above-mentioned proteins are also substrates for the multifunctional cAMP-dependent protein kinase. The results suggest that this cyclic nucleotide- and Ca²⁺-independent protein kinase, like cAMP-dependent protein kinase, may have multiple cellular functions.     

Suppression of superprecipitation of contractile proteins from chicken gizzard muscle by preincubation in the presence of adenosine triphosphate without Ca2+

Superprecipitation of reconstituted actomyosin composed of smooth muscle myosin, skeletal muscle actin and smooth muscle native tropomyosin was studied. When the actomyosin solution was preincubated in the presence of ATP and the absence of Ca2+, or in the relaxed state, superprecipitation was markedly suppressed. The extent of suppression was correlated with the inhibition of the phosphorylation of the 20,000-dalton light chain of smooth muscle myosin. This is consistent with the theory that the interaction of smooth muscle actomyosin is regulated by the phosphorylation of myosin light chain through a system of myosin light chain kinase and phosphatase. However, further studies showed that the myosin light chain kinase and phosphatase system could not explain the present suppression of superprecipitation, even if a cyclic AMP-dependent protein kinase system was also involved. A new regulatory factor should be taken into account in the regulation of smooth muscle actomyosin interaction.     

Phosphorylation of myosin light chain kinase by the multifunctional calmodulin-dependent protein kinase II in smooth muscle cells.

Stimulation of tracheal smooth muscle cells in culture with ionomycin resulted in a rapid increase in cytosolic free Ca2+ concentration ([Ca2+]i) and an increase in both myosin light chain kinase and myosin light chain phosphorylation. These responses were markedly inhibited in the absence of extracellular Ca2+. Pretreatment of cells with 1-[N-O-bis(5-isoquinolinesulfonyl)-N- methyl-L-tyrosyl]-4-phenylpiperazine (KN-62), a specific inhibitor of the multifunctional calmodulin-dependent protein kinase II (CaM kinase II), did not affect the increase in [Ca2+]i but inhibited ionomycin-induced phosphorylation of myosin light chain kinase at the regulatory site near the calmodulin-binding domain. KN-62 inhibited CaM kinase II activity toward purified myosin light chain kinase. Phosphorylation of myosin light chain kinase decreased its sensitivity to activation by Ca2+ in cell lysates. Pretreatment of cells with KN-62 prevented this desensitization to Ca2+ and potentiated myosin light chain phosphorylation. We propose that the Ca(2+)-dependent phosphorylation of myosin light chain kinase by CaM kinase II decreases the Ca2+ sensitivity of myosin light chain phosphorylation in smooth muscle.     

Phosphorylation of caldesmon by protein kinase C

Protein kinase C catalyzes phosphorylation of caldesmon, an F-actin binding protein of smooth muscle, in the presence of Ca2+ and phospholipid. Protein kinase C incorporates about 8 mol of phosphate/mol of chicken gizzard caldesmon. When calmodulin was added in the medium, there was an inhibition of phosphorylation. The fully phosphorylated, but not unphosphorylated, caldesmon inhibited myosin light chain kinase activity. The possibility that protein kinase C plays some role in smooth muscle contractile system through caldesmon, warrants further attention.     

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.     

Phosphorylation of brush border myosin by brush border calmodulin-dependent myosin heavy and light chain kinases

Calmodulin-dependent myosin light chain kinase isolated from chicken intestinal brush border phosphorylates brush border myosin at an apparently single serine identical to that phosphorylated by smooth muscle myosin light chain kinase. Phosphorylation to 1.8 mol phosphate/mol myosin activated the myosin actin-activated ATPase about 10-fold, to about 50 nmol/min per mg. Myosin phosphorylated on its light chains could then be further phosphorylated to a total of 3.2 mol phosphate per mol by brush border calmodulin-dependent heavy chain kinase. Heavy chain phosphorylation did not alter the actin-activated ATPase of either myosin prephosphorylated on its light chains or of unphosphorylated myosin.     

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.