Autophosphorylation of smooth muscle myosin light chain kinase at its regulatory domain

A temporal Ca2+-desensitization of myosin light chain kinase in phasic smooth muscles induced by CaMKKß/PP2A pathways

Cell signaling pathways regulating myosin regulatory light chain (LC20) phosphorylation contribute to determining contractile responses in smooth muscles. Following excitation and contraction, phasic smooth muscles, such as digestive tract and urinary bladder, undergo a relaxation due to a decline of cellular [Ca²⁺] and a decreased Ca²⁺ sensitivity of LC20 phosphorylation, named Ca²⁺ desensitization. Here, we determined mechanisms underlying the temporal Ca²⁺ desensitization of LC20 phosphorylation in phasic smooth muscles using permeabilized strips of mouse ileum and urinary bladder. Upon the stimulation with pCa6.0 at 20°C, the contraction and the LC20 phosphorylation peaked within 30 sec and then declined to about 50% of the peak force at 2 min after stimulation. During the relaxation phase after the contraction, the LC20 kinase (MLCK) was inactivated, but no fluctuation in the LC20 phosphatase activity occurred, suggesting that the MLCK inactivation is a cause of the Ca²⁺-induced Ca²⁺-desensitization of LC20 phosphorylation. The MLCK inactivation was associated with phosphorylation at the calmodulin binding domain of the kinase. Treatment with antagonists for CaMKKß (STO-609 and TIM-063) attenuated both the phasic response of the contraction and MLCK phosphorylation, whereas neither CaMKII, AMPK nor PAK induced the MLCK inactivation in phasic smooth muscles. Conversely, PP2A inhibition amplified the phasic response. Signaling pathways through CaMKKß and PP2A may contribute to regulating the Ca²⁺ sensitivity of MLCK and the contractile response of phasic smooth muscles.

A novel phosphorylation site at Ser130 adjacent to the pseudosubstrate domain contributes to the activation of protein kinase C-δ

Protein kinase C-δ (PKCδ) is a signalling kinase that regulates many cellular responses. Although most studies focus on allosteric mechanisms that activate PKCδ at membranes, PKCδ also is controlled via multi-site phosphorylation [Gong et al. (2015) Mol. Cell. Biol. 35: , 1727-1740]. The present study uses MS-based methods to identify PKCδ phosphorylation at Thr(50) and Ser(645) (in resting and PMA-treated cardiomyocytes) as well as Thr(37), Thr(38), Ser(130), Thr(164), Thr(211), Thr(215), Ser(218), Thr(295), Ser(299) and Thr(656) (as sites that increase with PMA). We focused on the consequences of phosphorylation at Ser(130) and Thr(141) (sites just N-terminal to the pseudosubstrate domain). We show that S130D and T141E substitutions co-operate to increase PKCδ's basal lipid-independent activity and that Ser(130)/Thr(141) di-phosphorylation influences PKCδ's substrate specificity. We recently reported that PKCδ preferentially phosphorylates substrates with a phosphoacceptor serine residue and that this is due to constitutive phosphorylation at Ser(357), an ATP-positioning G-loop site that limits PKCδ's threonine kinase activity [Gong et al. (2015) Mol. Cell. Biol. 35: , 1727-1740]. The present study shows that S130D and T141E substitutions increase PKCδ's threonine kinase activity indirectly by decreasing G loop phosphorylation at Ser(357). A S130F substitution [that mimics a S130F single-nt polymorphism (SNP) identified in some human populations] also increases PKCδ's maximal lipid-dependent catalytic activity and confers threonine kinase activity. Finally, we show that Ser(130)/Thr(141) phosphorylations relieve auto-inhibitory constraints that limit PKCδ's activity and substrate specificity in a cell-based context. Since phosphorylation sites map to similar positions relative to the pseudosubstrate domains of other PKCs, our results suggest that phosphorylation in this region of the enzyme may constitute a general mechanism to control PKC isoform activity.

Abl tyrosine kinase phosphorylates nonmuscle Myosin light chain kinase to regulate endothelial barrier function

This study identified multiple novel c-Abl–mediated nmMLCK phosphorylation sites by mass spectroscopy and examined their influence on nmMLCK function and human lung endothelial barrier regulation. The data indicate an essential role for Abl kinase in vascular barrier regulation via phosphorylation of nmMLCK and the actin-binding protein cortactin.

Nonmuscle myosin light chain kinase (nmMLCK), a multi-functional cytoskeletal protein critical to vascular homeostasis, is highly regulated by tyrosine phosphorylation. We identified multiple novel c-Abl–mediated nmMLCK phosphorylation sites by mass spectroscopy analysis (including Y²³¹, Y⁴⁶⁴, Y⁵⁵⁶, Y⁸⁴⁶) and examined their influence on nmMLCK function and human lung endothelial cell (EC) barrier regulation. Tyrosine phosphorylation of nmMLCK increased kinase activity, reversed nmMLCK-mediated inhibition of Arp2/3-mediated actin polymerization, and enhanced binding to the critical actin-binding phosphotyrosine protein, cortactin. EC challenge with sphingosine 1-phosphate (S1P), a potent barrier-enhancing agonist, resulted in c-Abl and phosphorylated nmMLCK recruitment into caveolin-enriched microdomains, rapid increases in Abl kinase activity, and spatial targeting of c-Abl to barrier-promoting cortical actin structures. Conversely, reduced c-Abl expression in EC (siRNA) markedly attenuated S1P-mediated cortical actin formation, reduced the EC modulus of elasticity (assessed by atomic force microscopy), reduced nmMLCK and cortactin tyrosine phosphorylation, and attenuated S1P-mediated barrier enhancement. These studies indicate an essential role for Abl kinase in vascular barrier regulation via posttranslational modification of nmMLCK and strongly support c-Abl-cortactin-nmMLCK interaction as a novel determinant of cortical actin-based cytoskeletal rearrangement critical to S1P-mediated EC barrier enhancement.

Kinase-related protein/telokin inhibits Ca2+-independent contraction in Triton-skinned guinea pig taenia coli

KRP (kinase-related protein), also known as telokin, has been proposed to inhibit smooth muscle contractility by inhibiting the phosphorylation of the rMLC (regulatory myosin light chain) by the Ca2+-activated MLCK (myosin light chain kinase). Using the phosphatase inhibitor microcystin, we show in the present study that KRP also inhibits Ca2+-independent rMLC phosphorylation and smooth muscle contraction mediated by novel Ca2+-independent rMLC kinases. Incubating KRP-depleted Triton-skinned taenia coli with microcystin at pCa>8 induced a slow contraction reaching 90% of maximal force (Fmax) at pCa 4.5 after approximately 25 min. Loading the fibres with KRP significantly slowed down the force development, i.e. the time to reach 50% of Fmax was increased from 8 min to 35 min. KRP similarly inhibited rMLC phosphorylation of HMM (heavy meromyosin) in vitro by MLCK or by the constitutively active MLCK fragment (61K-MLCK) lacking the myosin-docking KRP domain. A C-terminally truncated KRP defective in myosin binding inhibited neither force nor HMM phosphorylation. Phosphorylated KRP inhibited the rMLC phosphorylation of HMM in vitro and Ca2+-insensitive contractions in fibres similar to unphosphorylated KRP, whereby the phosphorylation state of KRP was not altered in the fibres. We conclude that (i) KRP inhibits not only MLCK-induced contractions, but also those elicited by Ca2+-independent rMLC kinases; (ii) phosphorylation of KRP does not modulate this effect; (iii) binding of KRP to myosin is essential for this inhibition; and (iv) KRP inhibition of rMLC phosphorylation is most probably due to the shielding of the phosphorylation site on the rMLC.

Identification of cardiac-specific myosin light chain kinase

Two myosin light chain (MLC) kinase (MLCK) proteins, smooth muscle (encoded by mylk1 gene) and skeletal (encoded by mylk2 gene) MLCK, have been shown to be expressed in mammals. Even though phosphorylation of its putative substrate, MLC2, is recognized as a key regulator of cardiac contraction, a MLCK that is preferentially expressed in cardiac muscle has not yet been identified. In this study, we characterized a new kinase encoded by a gene homologous to mylk1 and -2, named cardiac MLCK, which is specifically expressed in the heart in both atrium and ventricle. In fact, expression of cardiac MLCK is highly regulated by the cardiac homeobox protein Nkx2-5 in neonatal cardiomyocytes. The overall structure of cardiac MLCK protein is conserved with skeletal and smooth muscle MLCK; however, the amino terminus is quite unique, without significant homology to other known proteins, and its catalytic activity does not appear to be regulated by Ca(2+)/calmodulin in vitro. Cardiac MLCK is phosphorylated and the level of phosphorylation is increased by phenylephrine stimulation accompanied by increased level of MLC2v phosphorylation. Both overexpression and knockdown of cardiac MLCK in cultured cardiomyocytes revealed that cardiac MLCK is likely a new regulator of MLC2 phosphorylation, sarcomere organization, and cardiomyocyte contraction.

Effects of ML-7 and Y-27632 on carbachol- and endothelin-1-induced contraction of bovine trabecular meshwork

The trabecular meshwork is considered a smooth muscle like tissue contributing to aqueous outflow regulation and thus to regulation of intraocular pressure. An elevation in intraocular pressure is one of the greatest risk factors for most forms of glaucoma. We assume that contraction of trabecular meshwork reduces aqueous humor outflow and thus enhances intraocular pressure, whereas relaxation exerts the opposite effect. The present paper supports the hypothesis of the trabecular meshwork being a smooth muscle-like tissue. We perform measurements of isometric force in isolated bovine trabecular meshwork strips. Contractility of this tissue is induced by carbachol or endothelin-1. The contractile force is successfully inhibited by ML-7, a highly specific inhibitor of myosin light chain kinase. The contraction is also reduced in the presence of the RhoA kinase inhibitor Y-27632. We further describe the protein expression of smooth muscle myosin and its regulatory kinase, the myosin light chain kinase, in human and bovine trabecular meshwork cells. Additionally, the serine phosphorylation of myosin light chain kinase is shown. These data indicate that the trabecular meshwork expresses major contractility regulating proteins which are involved in tissue function. Inhibition of the signaling pathways which lead to myosin phosphorylation causes inhibition of contractile force in trabecular meshwork. According to our concept of aqueous humor outflow regulation, trabecular meshwork relaxing substances appear to be ideal antiglaucomatous drugs, leading to increased outflow facility.

Determination of human myosin III as a motor protein having a protein kinase activity

The class III myosin is the most divergent member of the myosin superfamily, having a domain with homology to a protein kinase. However, the function of class III myosin at a molecular level is not known at all, and it has been questioned whether it is actually an actin-based motor molecule. Here, we showed that human myosin III has an ATPase activity that is significantly activated by actin (20-fold) with Kactin of 112 microm and Vmax of 0.34 s-1, indicating the mechanoenzymatic activity of myosin III. Furthermore, we found that human myosin III has actin translocating activity (0.11 +/- 0.05 microm/s) using an in vitro actin gliding assay, and it moves toward the plus end of actin filaments. Myosin III containing calmodulin as the light chain subunit showed a protein kinase activity and underwent autophosphorylation. The autophosphorylation was the intramolecular process, and the sites were at the C-terminal end of the motor domain. Autophosphorylation significantly activated the kinase activity, although it did not affect the ATPase activity. The present study is the first report that clearly demonstrates that the class III myosin is an actin-based motor protein having a protein kinase activity.

The pro-apoptotic function of death-associated protein kinase is controlled by a unique inhibitory autophosphorylation-based mechanism

Death-associated protein kinase is a calcium/calmodulin serine/threonine kinase, which positively mediates programmed cell death in a variety of systems. Here we addressed its mode of regulation and identified a mechanism that restrains its apoptotic function in growing cells and enables its activation during cell death. It involves autophosphorylation of Ser(308) within the calmodulin (CaM)-regulatory domain, which occurs at basal state, in the absence of Ca(2+)/CaM, and is inversely correlated with substrate phosphorylation. This type of phosphorylation takes place in growing cells and is strongly reduced upon their exposure to the apoptotic stimulus of C(6)-ceramide. The substitution of Ser(308) to alanine, which mimics the ceramide-induced dephosphorylation at this site, increases Ca(2+)/CaM-independent substrate phosphorylation as well as binding and overall sensitivity of the kinase to CaM. At the cellular level, it strongly enhances the death-promoting activity of the kinase. Conversely, mutation to aspartic acid reduces the binding of the protein to CaM and abrogates almost completely the death-promoting function of the protein. These results are consistent with a molecular model in which phosphorylation on Ser(308) stabilizes a locked conformation of the CaM-regulatory domain within the catalytic cleft and simultaneously also interferes with CaM binding. We propose that this unique mechanism of auto-inhibition evolved to impose a locking device, which keeps death-associated protein kinase silent in healthy cells and ensures its activation only in response to apoptotic signals.

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.

Differential regulation of alternatively spliced endothelial cell myosin light chain kinase isoforms by p60(Src)

The Ca(2+)/calmodulin-dependent endothelial cell myosin light chain kinase (MLCK) triggers actomyosin contraction essential for vascular barrier regulation and leukocyte diapedesis. Two high molecular weight MLCK splice variants, EC MLCK-1 and EC MLCK-2 (210-214 kDa), in human endothelium are identical except for a deleted single exon in MLCK-2 encoding a 69-amino acid stretch (amino acids 436-505) that contains potentially important consensus sites for phosphorylation by p60(Src) kinase (Lazar, V., and Garcia, J. G. (1999) Genomics 57, 256-267). We have now found that both recombinant EC MLCK splice variants exhibit comparable enzymatic activities but a 2-fold reduction of V(max), and a 2-fold increase in K(0.5 CaM) when compared with the SM MLCK isoform, whereas K(m) was similar in the three isoforms. However, only EC MLCK-1 is readily phosphorylated by purified p60(Src) in vitro, resulting in a 2- to 3-fold increase in EC MLCK-1 enzymatic activity (compared with EC MLCK-2 and SM MLCK). This increased activity of phospho-MLCK-1 was observed over a broad range of submaximal [Ca(2+)] levels with comparable EC(50) [Ca(2+)] for both phosphorylated and unphosphorylated EC MLCK-1. The sites of tyrosine phosphorylation catalyzed by p60(Src) are Tyr(464) and Tyr(471) within the 69-residue stretch deleted in the MLCK-2 splice variant. These results demonstrate for the first time that p60(Src)-mediated tyrosine phosphorylation represents an important mechanism for splice variant-specific regulation of nonmuscle MLCK and vascular cell function.

Autophosphorylation restrains the apoptotic activity of DRP-1 kinase by controlling dimerization and calmodulin binding

DRP-1 is a pro-apoptotic Ca2+/calmodulin (CaM)-regulated serine/threonine kinase, recently isolated as a novel member of the DAP-kinase family of proteins. It contains a short extra-catalytic tail required for homodimerization. Here we identify a novel regulatory mechanism that controls its pro-apoptotic functions. It comprises a single autophosphorylation event mapped to Ser308 within the CaM regulatory domain. A negative charge at this site reduces both the binding to CaM and the formation of DRP-1 homodimers. Conversely, the dephosphorylation of Ser308, which takes place in response to activated Fas or tumour necrosis factor-alpha death receptors, increases the formation of DRP-1 dimers, facilitates the binding to CaM and activates the pro-apoptotic effects of the protein. Thus, the process of enzyme activation is controlled by two unlocking steps that must work in concert, i.e. dephosphorylation, which probably weakens the electrostatic interactions between the CaM regulatory domain and the catalytic cleft, and homodimerization. This mechanism of negative autophosphorylation provides a safety barrier that restrains the killing effects of DRP-1, and a target for efficient activation of the kinase by various apoptotic stimuli.

An oncogenic epidermal growth factor receptor signals via a p21-activated kinase-caldesmon-myosin phosphotyrosine complex

Many ligand-independent receptor tyrosine kinases are tumorigenic. The biochemical signals that mediate ligand-independent transformation of cells by these transmembrane receptors are poorly defined. In this report, we demonstrate that a constitutively activated mutant epidermal growth factor receptor (v-ErbB) induces the formation of a transformation-specific signaling module that complexes with myosin II. The components of this signaling complex include the signal adapter proteins Shc, Grb2, and Nck, and tyrosine-phosphorylated forms of p21-activated kinase (Pak), caldesmon, and myosin light chain kinase. Transformation-specific, tyrosine phosphorylation of Pak enhances the catalytic activity of this serine/threonine kinase. Furthermore, the tyrosine phosphorylation of Pak is Rho-, but not Ras-, Rac-, or Cdc42-dependent. These results demonstrate that a ligand-independent epidermal growth factor receptor mutant can transduce oncogenic signals that are distinct from ligand-dependent, mitogenic signals. In addition, these data provide evidence for the coupling of oncogenic receptor tyrosine kinases with the actomyosin molecular motor. This myosin-associated signaling module may mediate some of the biochemical changes of myosin found in v-ErbB- transformed fibroblasts, thereby contributing to the regulation of the mechanical forces governing cellular adhesion, cytoskeletal tension, and, hence, anchorage-independent cell growth.

Regulation of Ca2+-independent smooth muscle contraction by alternative staurosporine-sensitive kinase

It is well known that inhibition of myosin phosphatase induces smooth muscle contraction in the absence of Ca2+. We characterized the kinase(s) which plays a role in Ca2+-independent, microcystin-LR-induced contraction in permeabilized smooth muscle of the rabbit portal vein. Assessments of various protein kinase inhibitors revealed this kinase(s) (1) was sensitive to staurosporine (1 microM), but resistant to other agents including wortmannin (10 microM), Y-27632 ((R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide+ ++, 100 microM). HA1077 (1-(5-isoquinolinylsulfonyl)-homopiperazine, 100 microM), H-7 (1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, 100 microM), and calphostin C (100 microM), and (2) induced phosphorylation of 20 kDa myosin light chain at serine-19. We concluded that other kinases exist which phosphorylate myosin light chain at serine-19 and induce Ca2+-independent smooth muscle contraction, distinct from Rho-associated kinase, myosin light chain kinase, and protein kinase C.

Ca2+-independent phosphorylation of myosin in rat caudal artery and chicken gizzard myofilaments

1. Smooth muscle contraction is activated primarily by the Ca2+-calmodulin (CaM)-dependent phosphorylation of the 20 kDa light chains (LC20) of myosin. Activation can also occur in some instances without a change in intracellular free [Ca2+] or indeed in a Ca2+-independent manner. These signalling pathways often involve inhibition of myosin light chain phosphatase and unmasking of basal kinase activity leading to LC20 phosphorylation and contraction. 2. We have used demembranated rat caudal arterial smooth muscle strips and isolated chicken gizzard myofilaments in conjunction with the phosphatase inhibitor microcystin-LR to investigate the mechanism of Ca2+-independent phosphorylation of LC20 and contraction. 3. Treatment of Triton X-100-demembranated rat caudal arterial smooth muscle strips with microcystin at pCa 9 triggered a concentration-dependent contraction that was slower than that induced by pCa 4.5 or 6 but reached comparable steady-state levels of tension. 4. This Ca2+-independent, microcystin-induced contraction correlated with phosphorylation of LC20 at serine-19 and threonine-18. 5. Whereas Ca2+-dependent LC20 phosphorylation and contraction were inhibited by a synthetic peptide (AV25) based on the autoinhibitory domain of myosin light chain kinase (MLCK), Ca2+-independent, microcystin-induced LC20 phosphorylation and contraction were resistant to AV25. 6. Ca2+-independent LC20 kinase activity was also detected in chicken gizzard smooth muscle myofilaments and catalysed phosphorylation of endogenous myosin LC20 at serine-19 and/or threonine-18. This is in contrast to MLCK which phosphorylates threonine-18 only after prior phosphorylation of serine-19. 7. Gizzard Ca2+-independent LC20 kinase could be separated from MLCK by differential extraction from myofilaments and by CaM affinity chromatography. Its activity was resistant to AV25. 8. We conclude that inhibition of smooth muscle myosin light chain phosphatase (MLCP) unmasks the activity of a Ca2+-independent LC20 kinase associated with the myofilaments and distinct from MLCK. This kinase, therefore, probably plays a role in Ca2+ sensitization and Ca2+-independent contraction of smooth muscle in response to stimuli that act via Ca2+-independent pathways, leading to inhibition of MLCP.

Pharmacomechanical coupling: the role of calcium, G-proteins, kinases and phosphatases

The concept of pharmacomechanical coupling, introduced 30 years ago to account for physiological mechanisms that can regulate contraction of smooth muscle independently of the membrane potential, has since been transformed from a definition into what we now recognize as a complex of well-defined, molecular mechanisms. The release of Ca2+ from the SR by a chemical messenger, InsP3, is well known to be initiated not by depolarization, but by agonist-receptor interaction. Furthermore, this G-protein-coupled phosphatidylinositol cascade, one of many processes covered by the umbrella of pharmacomechanical coupling, is part of complex and general signal transduction mechanisms also operating in many non-muscle cells of diverse organisms. It is also clear that, although the major contractile regulatory mechanism of smooth muscle, phosphorylation/dephosphorylation of MLC20, is [Ca2+]-dependent, the activity of both the kinase and the phosphatase can also be modulated independently of [Ca2+]i. Sensitization to Ca2+ is attributed to inhibition of SMPP-1M, a process most likely dominated by activation of the monomeric GTP-binding protein RhoA that, in turn, activates Rho-kinase that phosphorylates the regulatory subunit of SMPP-1M and inhibits its myosin phosphatase activity. It is likely that the tonic phase of contraction activated by a variety of excitatory agonists is, at least in part, mediated by this Ca(2+)-sensitizing mechanism. Desensitization to Ca2+ can occur either through inhibitory phosphorylation of MLCK by other kinases or autophosphorylation and by activation of SMPP-1M by cyclic nucleotide-activated kinases, probably involving phosphorylation of a phosphatase activator. Based on our current understanding of the complexity of the many cross-talking signal transduction mechanisms that operate in cells, it is likely that, in the future, our current concepts will be refined, additional mechanisms of pharmacomechanical coupling will be recognized, and those contributing to the pathologenesis diseases, such as hypertension and asthma, will be identified.

From pharmacomechanical coupling to G-proteins and myosin phosphatase

A brief summary of recent studies of pharmacomechanical coupling is presented, with emphasis on the role of GTP-binding proteins and Ca(2+)-independent regulation of contraction (Ca(2+)-sensitization/desensitization) through regulatory myosin light chain (MLC20) phosphorylation and dephosphorylation. Pharmacomechanical regulation of cytosolic [Ca2+] is largely, though not solely, controlled by the phosphatidylinositol cascade and Ca(2+)-pumps of the plasma membrane and the sarcoplasmic reticulum. The monomeric GTPase, RhoA, is a major upstream component of Ca(2+)-sensitization. Its crystal structure and apparently obligatory translocation to the plasma membrane for activation of its downstream effectors are described. Inhibition of RhoA activity by a membrane-permeant ADP-ribosylating bacterial exoenzyme, DC3B, causes severe depression of the tonic component of agonist-induced contraction, suggesting that this component is largely due to Ca(2+)-sensitization. A relatively specific inhibitor (Y27632) of Rho-kinase, a downstream effector of Ca(2+)-sensitization (Uehata et al 1997), also inhibits oxytoxin-induced Ca(2+)-sensitization of myometrium. The major mechanism of physiological, G-protein-coupled Ca(2+)-sensitization is through inhibition of smooth muscle myosin phosphatase (SMPP-1M), whereas conventional or novel protein kinase Cs play very little or no role in this process. Mechanisms of Ca(2+)-desensitization include inhibition of myosin light chain kinase and activation of SMPP-1M. Activation of SMPP-1M in phasic smooth muscle can be attributed, at least in part, to the synergistic phosphatase activating activities of a cyclic nucleotide-dependent kinase and its major substrate, telokin.

The unimportance of being (protein kinase C) epsilon

The purpose of our study was to determine the mechanism through which phorbol esters and smooth muscle myosin phosphatase inhibitors can induce contraction of smooth muscle in the absence of Ca2+. Protein kinase C-epsilon (PKC-epsilon) was previously implicated in this process based largely on its supposed absence in the ferret portal vein, and a correlation was drawn between the presence of this isoform and the ability of smooth muscle to contract independently of Ca2+ and phosphorylation of the 20 kDa regulatory light chains of myosin (MLC20). We demonstrate here, with two antibodies, one to the NH2 terminus and the other to the COOH terminus of PKC-epsilon, that epsilon is present in both ferret portal vein and rabbit portal vein smooth muscle, neither of which exhibits phorbol ester-induced contraction in the absence of Ca2+. However, in the presence of clamped submaximal Ca2+, phorbol es ter increased MLC20 phosphorylation from 17.7+/-1.7% to 46.4+/-3.6% in ferret portal vein smooth muscle and evoked an increase in force. Prolonged (48 h) incubation of ferret portal vein with phorbol esters completely down-regulated PKC-epsilon, as shown by Western blots, and abolished the phorbol ester-evoked contraction at submaximal Ca2+, but not Ca2+-independent, contractions induced by the phosphatase inhibitor microcystin. Contractions induced by microcystin in Ca2+-free solution were associated with increased phosphorylation of myosin light chain kinase (MLCK). Activation of MLCK by autophosphorylation in the absence of Ca2+ occurs in vitro (1). We conclude that PKC-epsilon is neither necessary nor sufficient for Ca2+-independent regulation of myosin II in smooth muscle, but contractions induced by agents that inhibit smooth muscle myosin phosphatase in the absence of Ca2+ may be mediated by MLCK autophosphorylated or activated by another Ca2+-independent kinase.

Kinase-related protein (telokin) is phosphorylated by smooth-muscle myosin light-chain kinase and modulates the kinase activity

Telokin is an abundant smooth-muscle protein with an amino acid sequence identical with that of the C-terminal region of smooth-muscle myosin light-chain kinase (MLCK), although it is expressed as a separate protein [Gallagher and Herring (1991) J. Biol. Chem. 266, 23945-23952]. Here we demonstrate that telokin is also similar to smooth-muscle myosin regulatory light chain (ReLC) not only in its gross physical properties but also as an MLCK substrate. Telokin was slowly phosphorylated by MLCK in the presence of Ca2+ and calmodulin and could be readily dephosphorylated by myosin light-chain phosphatase. A threonine residue was phosphorylated with up to 0.25 mol/mol stoichiometry. This low stoichiometry, together with the observed dimerization of telokin [Sobieszek and Nieznanski (1997) Biochem. J. 322, 65-71], indicates that the telokin dimer was acting as the substrate with a single protomer being phosphorylated. Our enzyme kinetic analysis of the phosphorylation reaction confirms this interpretation. Because telokin phosphorylation also required micromolar concentrations of MLCK, which also facilitates the formation of kinase oligomers, we concluded that the oligomers are interacting with telokin. Thus it seems that telokin modulates the phosphorylation rate of myosin filaments by a mechanism that includes the direct or indirect inhibition of the kinase active site by the telokin dimer, and that removal of the inhibition is controlled by slow phosphorylation of the telokin dimer, which results in MLCK dimerization.

alpha1-Adrenoceptor-mediated phosphorylation of myosin in rat-tail arterial smooth muscle

The mechanism of alpha1-adrenoceptor-mediated contraction was investigated in helical strips of the rat-tail artery. Muscle strips with the endothelium removed contracted in response to the alpha1-adrenoceptor agonist cirazoline, with half-maximal contraction at 0.23 microM. The contractile response to a submaximal concentration of cirazoline (0.3 microM) was biphasic, with a rapid phasic component peaking at approx. 30 s, followed by sustained tonic contraction. Phosphorylation of the 20 kDa light chain of myosin (LC20) in response to 0.3 microM cirazoline was also biphasic and closely matched the time-course of contraction. Resting LC20 phosphorylation levels were 0.22+/-0.06 mol of Pi/mol of LC20 (n=3) and reached a maximum of 0.58+/-0.08 mol of Pi/mol of LC20 (n=3). Phosphopeptide mapping and phosphoamino acid analysis revealed that LC20 phosphorylation occurred exclusively at serine-19. The sustained phase of contraction was eliminated by removal of extracellular Ca2+ and the phasic response was eliminated by depletion of endogenous Ca2+ stores. Both phases of the contractile response were restored by re-addition of Ca2+ to the bathing medium. LC20 phosphorylation and both phases of the contractile response to 0.3 microM cirazoline were inhibited by the myosin light-chain kinase inhibitor ML-9 (30 microM). Resting LC20 phosphorylation, however, was unaffected by ML-9. Finally, both phasic and tonic responses to 0.3 microM cirazoline were partially inhibited by chloroethylclonidine (50 microM), suggesting the involvement of both alpha1A and alpha1B adrenoceptors in these contractile responses.

Oligomerization of smooth muscle myosin light chain kinase and its modifications by melittin and calmodulin

The catalytic activity of smooth muscle myosin light chain kinase (MLCKase) requires the presence of calcium and calmodulin [CM; J. T. Stull et al. (1993) Molecular and Cellular Biochemistry, Vols. 127/128, pp. 229-237] and can also be modified through its own oligomerization [E. B. Babiychuk et al. (1995) Biochemistry, Vol. 34, pp. 6366-6372]. In the present report we demonstrate that melittin, one of the most potent CM antagonists, interacted reversibly with the MLCKase apoenzyme with affinities comparable to those of CM and influenced the oligomeric state of the kinase. At low melittin to kinase ratios the kinase formed insoluble oligomers (aggregates) while at higher melittin concentrations it existed predominantly as soluble oligomers revealed by cross-linking as octamers and hexamers. The kinase alone exhibited similar biphasic solubility within a 5-30 microM range and its solubility was strongly influenced by the ionic strength of the medium. Melittin was also found to promote both the aggregation of the purified 24-kDa C-terminal fragment of the kinase and its analogue telokin, as well as of myosin light chains, but had no effect on the solubility of bovine serum albumin, caldesmon, or calmodulin. These data and our cross-linkage experiments indicate that the insoluble kinase oligomers arose via melittin-induced aggregation of the kinase dimers in which the relokin-like domain played a main role. The soluble oligomers, in turn, were formed after saturation of the kinase with melittin, which resulted in a weakening of the interaction between the protomers with an increase of the long-range order within the oligomers. This interpretation is consistent with the observed effects of melittin on MLCKase catalytic and autocatalytic activities. At concentrations of melittin required to produce soluble oligomers, the binding of the kinase to myosin filaments was considerably enhanced. A plausible mechanism for the formation of the soluble oligomers and aggregates is suggested and its relation to the possible MLCKase assemblies discussed in terms of a model.

Purification and characterization of a smooth muscle myosin light chain kinase-phosphatase complex

We show that a myofibrillar form of smooth muscle myosin light chain phosphatase (MLCPase) forms a multienzyme complex with myosin light chain kinase (MLCKase). The stability of the complex was indicated by the copurification of MLCKase and MLCPase activities through multiple steps that included myofibril preparation, gel filtration chromatography, cation (SP-Sepharose BB) and anion (Q-Sepharose FF) exchange chromatography, and affinity purification on calmodulin and on thiophosphorylated regulatory light chain columns. In addition, the purified complex eluted as a single peak from a final gel filtration column in the presence of calmodulin (CaM). Because a similar MLCPase is present in varying amounts in standard preparations of both MLCKase and myosin filaments, we have named it a kinase- and myosin-associated protein phosphatase (KAMPPase). The KAMPPase multienzyme complex was composed of a 37-kDa catalytic (PC) subunit, a 67-kDa targeting (PT) subunit, and MLCKase with or without CaM. The approximate molar ratio of the PC and PT subunits was 1:2 with a variable and usually higher molar content of MLCKase. The targeting role of the PT subunit was directly demonstrated in binding experiments in which the PT subunit bound to both the kinase and to CaM. Its binding to CaM was, however, Ca2+-independent. MLCKase and the PT subunit potentiated activity of the PC subunit when intact myosin was used as the substrate. These data indicated that there is a Ca2+-independent interaction among the MLCPase, MLCKase, and CaM that are involved in the regulation of phosphatase activity.

Analysis of cDNAs encoding Drosophila melanogaster myosin light chain kinase

Myosin light chain kinase regulates the activity of myosin by phosphorylating the myosin regulatory light chain. Here we describe the cloning and characterization of cDNAs encoding Drosophila melanogaster myosin light chain kinase. We amplified a fragment of the Drosophila mlck gene using degenerate primers homologous to a highly conserved region in myosin light chain kinase proteins of vertebrate species. We used the gene fragment to isolate corresponding Drosophila mlck cDNAs. The deduced protein sequence of the cDNAs shows high homology to the catalytic and regulatory domains of vertebrate nonmuscle, smooth muscle and skeletal muscle myosin light chain kinase. Protein motifs I and II, which are present in vertebrate nonmuscle and smooth muscle myosin light chain kinase, but not in skeletal muscle myosin light chain kinase, are also present in Drosophila myosin light chain kinase. Transcript and cDNA analysis shows the gene encodes multiple messages and is expressed in nonmuscle and muscle cells, including the adult indirect flight muscle. Genomic Southern analysis and chromosome hybridization suggest mlck is a single copy gene which maps to chromosome band 52D, and is not haplo-insufficient for flight.

Dual regulation of a chimeric plant serine/threonine kinase by calcium and calcium/calmodulin

A chimeric Ca2+/calmodulin-dependent protein kinase (CCaMK) gene characterized by a catalytic domain, a calmodulin-binding domain, and a neural visinin-like Ca2+-binding domain was recently cloned from plants (Patil, S., Takezawa, D., and Poovaiah, B. W. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 4797-4801). The Escherichia coli-expressed CCaMK phosphorylates various protein and peptide substrates in a Ca2+/calmodulin-dependent manner. The calmodulin-binding region of CCaMK has similarity to the calmodulin-binding region of the alpha-subunit of multifunctional Ca2+/calmodulin-dependent protein kinase (CaMKII). CCaMK exhibits basal autophosphorylation at the threonine residue(s) (0.098 mol of 32P/mol) that is stimulated 3.4-fold by Ca2+ (0.339 mol of 32P/mol), while calmodulin inhibits Ca2+-stimulated autophosphorylation to the basal level. A deletion mutant lacking the visinin-like domain did not show Ca2+-stimulated autophosphorylation activity but retained Ca2+/calmodulin-dependent protein kinase activity at a reduced level. Ca2+-dependent mobility shift assays using E. coli-expressed protein from residues 358 520 revealed that Ca2+ binds to the visinin-like domain. Studies with site-directed mutants of the visinin-like domain indicated that EF-hands II and III are crucial for Ca2+-induced conformational changes in the visinin-like domain. Autophosphorylation of CCaMK increases Ca2+/calmodulin-dependent protein kinase activity by about 5-fold, whereas it did not affect its Ca2+-independent activity. This report provides evidence for the existence of a protein kinase in plants that is modulated by Ca2+ and Ca2+/calmodulin. The presence of a visinin-like Ca2+-binding domain in CCaMK adds an additional Ca2+-sensing mechanism not previously known to exist in the Ca2+/calmodulin-mediated signaling cascade in plants.

Protein kinase C mediation of Ca(2+)-independent contractions of vascular smooth muscle

Tumour-promoting phorbol esters induce slow, sustained contractions of vascular smooth muscle, suggesting that protein kinase C (PKC) may play a role in the regulation of smooth muscle contractility. In some cases, e.g., ferret aortic smooth muscle, phorbol ester induced contractions occur without a change in [Ca2+]i or myosin phosphorylation. Direct evidence for the involvement of PKC came from the use of single saponin-permeabilized ferret aortic cells. A constitutively active catalytic fragment of PKC induced a slow, sustained contraction similar to that triggered by phenylephrine. Both responses were abolished by a peptide inhibitor of PKC. Contractions of similar magnitude occurred even when the [Ca2+] was reduced to close to zero, implicating a Ca(2+)-independent isoenzyme of PKC. Of the two Ca(2+)-independent PKC isoenzymes, epsilon and zeta, identified in ferret aorta, PKC epsilon is more likely to mediate the contractile response because (i) PKC epsilon, but not PKC zeta, is responsive to phorbol esters; (ii) upon stimulation with phenylephrine, PKC epsilon translocates from the sarcoplasm to the sarcolemma, whereas PKC zeta, translocates from a perinuclear localization to the interior of the nucleus; and (iii) when added to permeabilized single cells of the ferret aorta at pCa 9, PKC epsilon, but not PKC zeta, induced a contractile response similar to that induced by phenylephrine. A possible substrate of PKC epsilon is the smooth muscle specific, thin filament associated protein, calponin. Calponin is phosphorylated in intact smooth muscle strips in response to carbachol, endothelin-1, phorbol esters, or okadaic acid. Phosphorylation of calponin in vitro by PKC (a mixture of alpha, beta, and gamma isoenzymes) dramatically reduces its affinity for F-actin and alleviates its inhibition of the cross-bridge cycling rate. Calponin is phosphorylated in vitro by PKC epsilon but is a very poor substrate of PKC zeta. A signal transduction pathway is proposed to explain Ca(2+)-independent contraction of ferret aorta whereby extracellular signals trigger diacylglycerol production without a Ca2+ transient. The consequent activation of PKC epsilon would result in calponin phosphorylation, its release from the thin filaments, and alleviation of inhibition of cross-bridge cycling. Slow, sustained contraction then results from a slow rate of cross-bridge cycling because of the basal level of myosin light chain phosphorylation (approximately 0.1 mol Pi/mol light chain). We also suggest that signal transduction through PKC epsilon is a component of contractile responses triggered by agonists that activate phosphoinositide turnover; this may explain why smooth muscles often develop more force in response, e.g., to alpha 1-adrenergic agonists than to K+.

The role of MgADP in force maintenance by dephosphorylated cross-bridges in smooth muscle: a flash photolysis study

The effect of [MgADP] on relaxation from isometric tension, initiated by reducing free [Ca2+] through photolysis of the caged photolabile Ca2+ chelator diazo-2, was determined at 20 degrees C in alpha-toxin permeabilized tonic (rabbit femoral artery, Rf) and phasic (rabbit bladder, Rb) smooth muscle. In Rf, the shape of the relaxation curve was clearly biphasic, consisting of a slow "plateau" phase followed by a monotonic exponential decline with rate constant k. The duration of the plateau (d = 44 +/- 4 s, mean +/- SEM, n = 28) was well correlated (R = 0.92) with the total t1/2 of relaxation that was 66 +/- 3 s (n = 28) in the presence of 20 mM creatine phosphate (CP), and was prolonged in the absence of CP (t1/2 = 83 +/- 3 s, n = 7); addition of 100 microM MgADP further slowed relaxation (t1/2 = 132 +/- 7 s, n = 14). In Rb, a plateau was not detectable and t1/2 (= 15 +/- 2 s, n = 6) was not affected by 100 microM MgADP. In Rf the Q10 between 20 degrees C and 30 degrees C was 4.3 +/- 0.4 for d-1 and 2.8 +/- 0.3 for k (n = 8; p = 0.006). The regulatory myosin light chain (MLC20) in Rf was dephosphorylated at 0.07 +/- 0.02 s-1, from 42 +/- 3% before to 20 +/- 2% after photolysis of diazo-2, reaching basal values at a time when force had fallen by only 40%. We conclude that, in the presence of ATP, as during rigor, the affinity of dephosphorylated cross-bridges for MgADP is significantly higher in tonic than in phasic smooth muscle and contributes to the maintenance of force at low levels of phosphorylation. The MgADP dependence of the post-dephosphorylation phase of relaxation is consistent with its being rate-limited by the slow off-rate of ADP from cross-bridges that were dephosphorylated while in force-generating ADP-bound (AM*D) cross-bridge states. The fourfold faster off-rate of ADP from AM*D in the phasic, Rb, compared to tonic, Rf, smooth muscle is a major determinant of the different kinetics of relaxation in the two types of smooth muscle.