Definition of the inhibitory domain of smooth muscle myosin light chain kinase by site-directed mutagenesis

Regulation of myosin light-chain phosphorylation and its roles in cardiovascular physiology and pathophysiology

The regulation of muscle contraction is a critical function in the cardiovascular system, and abnormalities may be life-threatening or cause illness. The common basic mechanism in muscle contraction is the interaction between the protein filaments myosin and actin. Although this interaction is primarily regulated by intracellular Ca²⁺, the primary targets and intracellular signaling pathways differ in vascular smooth muscle and cardiac muscle. Phosphorylation of the myosin regulatory light chain (RLC) is a primary molecular switch for smooth muscle contraction. The equilibrium between phosphorylated and unphosphorylated RLC is dynamically achieved through two enzymes, myosin light chain kinase, a Ca²⁺-dependent enzyme, and myosin phosphatase, which modifies the Ca²⁺ sensitivity of contractions. In cardiac muscle, the primary target protein for Ca²⁺ is troponin C on thin filaments; however, RLC phosphorylation also plays a modulatory role in contraction. This review summarizes recent advances in our understanding of the regulation, physiological function, and pathophysiological involvement of RLC phosphorylation in smooth and cardiac muscles.

The role of molecular regulation and targeting in regulating calcium/calmodulin stimulated protein kinases

Calcium/calmodulin-stimulated protein kinases can be classified as one of two types - restricted or multifunctional. This family of kinases contains several structural similarities: all possess a calmodulin binding motif and an autoinhibitory region. In addition, all of the calcium/calmodulin-stimulated protein kinases examined in this chapter are regulated by phosphorylation, which either activates or inhibits their kinase activity. However, as the multifunctional calcium/calmodulin-stimulated protein kinases are ubiquitously expressed, yet regulate a broad range of cellular functions, additional levels of regulation that control these cell-specific functions must exist. These additional layers of control include gene expression, signaling pathways, and expression of binding proteins and molecular targeting. All of the multifunctional calcium/calmodulin-stimulated protein kinases examined in this chapter appear to be regulated by these additional layers of control, however, this does not appear to be the case for the restricted kinases.

Myosin light chain kinase in microvascular endothelial barrier function

Microvascular barrier dysfunction is implicated in the initiation and progression of inflammation, posttraumatic complications, sepsis, ischaemia-reperfusion injury, atherosclerosis, and diabetes. Under physiological conditions, a precise equilibrium between endothelial cell-cell adhesion and actin-myosin-based centripetal tension tightly controls the semi-permeability of microvascular barriers. Myosin light chain kinase (MLCK) plays an important role in maintaining the equilibrium by phosphorylating myosin light chain (MLC), thereby inducing actomyosin contractility and weakening endothelial cell-cell adhesion. MLCK is activated by numerous physiological factors and inflammatory or angiogenic mediators, causing vascular hyperpermeability. In this review, we discuss experimental evidence supporting the crucial role of MLCK in the hyperpermeability response to key cell signalling events during inflammation. At the cellular level, in vitro studies of cultured endothelial monolayers treated with MLCK inhibitors or transfected with specific inhibiting peptides have demonstrated that induction of endothelial MLCK activity is necessary for hyperpermeability. Ex vivo studies of live microvessels, enabled by development of the isolated, perfused venule method, support the importance of MLCK in endothelial permeability regulation in an environment that more closely resembles in vivo tissues. Finally, the role of MLCK in vascular hyperpermeability has been confirmed with in vivo studies of animal disease models and the use of transgenic MLCK210 knockout mice. These approaches provide a more complete view of the role of MLCK in vascular barrier dysfunction.

Myosin light chain phosphorylation regulates barrier function by remodeling tight junction structure

Epithelial tight junctions form a barrier against passive paracellular flux. This barrier is regulated by complex physiologic and pathophysiologic signals that acutely fine-tune tight junction permeability. Although actomyosin contraction and myosin light chain phosphorylation are clearly involved in some forms of tight junction regulation, the contributions of other signaling events and the role of myosin light chain phosphorylation in this response are poorly understood. Here we ask if activation of myosin light chain kinase alone is sufficient to induce downstream tight junction regulation. We use a confluent polarized intestinal epithelial cell model system in which constitutively active myosin light chain kinase, tMLCK, is expressed using an inducible promoter. tMLCK expression increases myosin light chain phosphorylation, reorganizes perijunctional F-actin, and increases tight junction permeability. TJ proteins ZO-1 and occludin are markedly redistributed, morphologically and biochemically, but effects on claudin-1 and claudin-2 are limited. tMLCK inhibition prevents changes in barrier function and tight junction organization induced by tMLCK expression, suggesting that these events both require myosin light chain phosphorylation. We conclude that myosin light chain phosphorylation alone is sufficient to induce tight junction regulation and provide new insights into the molecular mechanisms that mediate this regulation.

Ca2+-calmodulin-dependent phosphodiesterase (PDE1): Current perspectives

Ca2+-calmodulin-dependent phosphodiesterases (PDE1), like Ca2+-sensitive adenylyl cyclases (AC), are key enzymes that play a pivotal role in mediating the cross-talk between cAMP and Ca2+ signalling. Our understanding of how ACs respond to Ca2+ has advanced greatly, with significant breakthroughs at both the molecular and functional level. By contrast, little is known of the mechanisms that might underlie the regulation of PDE1 by Ca2+ in the intact cell. In living cells, Ca2+ signals are complex and diverse, exhibiting different spatial and temporal properties. The potential therefore exists for dynamic changes in the subcellular distribution and activation of PDE1 in relation to intracellular Ca2+ dynamics. PDE1s are a large family of multiply-spliced gene products. Therefore, it is possible that a cell-type specific response to elevation in [Ca2+]i can occur, depending on the isoform of PDE1 expressed. In this article, we summarize current knowledge on Ca2+ regulation of PDE1 in the intact cell and discuss approaches that might be undertaken to delineate the responses of this important group of enzymes to changes in [Ca2+]i.

RhoA activation in vascular smooth muscle cells from stroke-prone spontaneously hypertensive rats

RhoA is commonly activated in the aorta in various hypertensive models, indicating that RhoA seems to be a molecular switch in hypertension. The molecular mechanisms for RhoA activation in stroke-prone spontaneously hypertensive rats (SHRSP) were here investigated using cultured aortic smooth muscle cells (VSMC). The level of the active form of RhoA was higher in VSMC from SHRSP than in those from Wistar-Kyoto rats (WKY). The phosphorylation level of myosin phosphatase target subunit 1 (MYPT1) at the inhibitory site was also significantly higher in SHRSP, and the phosphorylation levels in both VSMCs were strongly inhibited to a similar extent by treatment with Y-27632, a Rho-kinase inhibitor. The expression levels of RhoA/Rho-kinase related molecules, namely RhoA, Rho-kinase, MYPT1, CPI-17 (inhibitory phosphoprotein for myosin phosphatase) and myosin light chain kinase, were not different between SHRSP and WKY. Valsartan, an angiotensin II (Ang II)- type 1 receptor antagonist, selectively and significantly reduced the RhoA activation in VSMC from SHRSP. The expression levels of the Rho GDP-dissociation inhibitor (RhoGDI) and leukemia-associated Rho-specific guanine nucleotide exchange factor (RhoGEF) did not differ between SHRSP and WKY. In cyclic nucleotide signaling, cyclic GMP (cGMP)-dependent protein kinase Ialpha (cGKIalpha) was significantly downregulated in SHRSP cells, although there were no changes in the expression levels of guanylate cyclase beta and cyclic AMP (cAMP)-dependent protein kinase or the intracellular contents of cGMP and cAMP between the two rat models. These results suggest that the possible mechanisms underlying RhoA activation in VSMC from SHRSP are autocrine/paracrine regulation by Ang II and/or cGKIalpha downregulation.

Activation of RhoA and inhibition of myosin phosphatase as important components in hypertension in vascular smooth muscle

Two mechanisms are proposed to account for the inhibition of myosin phosphatase (MP) involved in Ca2+ sensitization of vascular muscle, ie, phosphorylation of either MYPT1, a target subunit of MP or CPI-17, an inhibitory phosphoprotein. In cultured vascular aorta smooth muscle cells (VSMCs), stimulation with angiotensin II activated RhoA, and this was blocked by pretreatment with 8-bromo-cGMP. VSMCs stimulated by angiotensin II, endothelin-1, or U-46619 significantly increased the phosphorylation levels of both MYPT1 (at Thr696) and CPI-17 (at Thr38). The angiotensin II-induced phosphorylation of MYPT1 was completely blocked by 8-bromo-cGMP or Y-27632 (a Rho-kinase inhibitor), but not by GF109203X (a PKC inhibitor). In contrast, phosphorylation of CPI-17 was inhibited only by GF109203X. Y-27632 dramatically corrected the hypertension in N(omega)-nitro-L-arginine methyl ester (L-NAME)-treated rats, and this hypertension also was sensitive to isosorbide mononitrate. The level of the active form of RhoA was significantly higher in aortas from L-NAME-treated rats. Expression of RhoA, Rho-kinase, MYPT1, CPI-17, and myosin light chain kinase were not significantly different in aortas from L-NAME-treated and control rats. Activation of RhoA without changes in levels of other signaling molecules were observed in three other rat models of hypertension, ie, stroke-prone spontaneously hypertensive rats, renal hypertensive rats, and DOCA-salt rats. These results suggest that independent of the cause of hypertension, a common point in downstream signaling and a critical component of hypertension is activation of RhoA and subsequent activation of Rho-kinase.

Caspase-dependent cleavage of myosin light chain kinase (MLCK) is involved in TNF-alpha-mediated bovine pulmonary endothelial cell apoptosis

Cytoskeletal proteins are key participants in the cellular progression to apoptosis. Our previous work demonstrated the critical dependence of actomyosin rearrangement and MLC phosphorylation in TNF-alpha-induced endothelial cell apoptosis. As these events reflect the activation of the multifunctional endothelial cell (EC) MLCK isoform, we assessed the direct role of EC MLCK in the regulation of TNF-alpha-induced apoptosis. Bovine pulmonary artery endothelial cells expressing either an adenovirus encoding antisense MLCK cDNA (Ad.GFP-AS MLCK) or a dominant/negative EC MLCK construct (EC MLCK-ATPdel) resulted in marked reductions in MLCK activity and TNF-alpha-mediated apoptosis. In contrast, a constitutively active EC MLCK lacking the carboxyl-terminal autoinhibitory domains (EC MLCK-1745) markedly enhanced the apoptotic response to TNF-alpha. Immunostaining in GFP-EC MLCK-expressing cells revealed colocalization of caspase 8 and EC MLCK along actin stress fibers after TNF-alpha. TNF-alpha induced the caspase-dependent cleavage of EC MLCK-1745 in transfected endothelial cells, which was confirmed by mass spectroscopy with in vitro cleavage by caspase 3 at LKKD (D1703). The resulting MLCK fragments displayed significant calmodulin-independent kinase activity. These studies convincingly demonstrate that novel interactions between the apoptotic machinery and EC MLCK exist that regulate the endothelial contractile apparatus in TNF-alpha-induced apoptosis.

Constitutively active myosin light chain kinase alters axon guidance decisions in Drosophila embryos

Conventional myosin II activity provides the motile force for axon outgrowth, but to achieve directional movement during axon pathway formation, myosin activity should be regulated by the attractive and repulsive guidance cues that guide an axon to its target. Here, evidence for this regulation is obtained by using a constitutively active Myosin Light Chain Kinase (ctMLCK) to selectively elevate myosin II activity in Drosophila CNS neurons. Expression of ctMLCK pan-neurally or in primarily pCC/MP2 neurons causes these axons to cross the midline incorrectly. This occurs without altering cell fates and is sensitive to mutations in the regulatory light chains. These results confirm the importance of regulating myosin II activity during axon pathway formation. Mutations in the midline repulsive ligand Slit, or its receptor Roundabout, enhance the number of ctMLCK-induced crossovers, but ctMLCK expression also partially rescues commissure formation in commissureless mutants, where repulsive signals remain high. Overexpression of Frazzled, the receptor for midline attractive Netrins, enhances ctMLCK-dependent crossovers, but crossovers are suppressed when Frazzled activity is reduced by using loss-of-function mutations. These results confirm that proper pathway formation requires careful regulation of MLCK and/or myosin II activity and suggest that regulation occurs in direct response to attractive and repulsive cues.

Role of the short isoform of myosin light chain kinase in the contraction of cultured smooth muscle cells as examined by its down-regulation

GbaSM-4 cells, smooth muscle cells derived from brain basilar artery, which express both 210-kDa long and 130-kDa short isoforms of myosin light chain kinase (MLCK), were infected with an adenovirus vector carrying a 1.4-kb catalytic portion of MLCK-cDNA in an antisense orientation. Western blot analysis showed that the expression of short MLCK was depressed without affecting long MLCK expression. The contraction of the down-regulated cells was measured by the cell-populated collagen-fiber method. The tension development after stimulation with norepinephrine or was depressed. The additional infection of the down-regulated cells with the adenovirus construct containing the same insert in a sense direction rescued not only the short MLCK expression but also contraction, confirming the physiological role of short MLCK in the contraction. To examine the role of long MLCK in the residual contraction persisting in the short MLCK-deficient cells, long MLCK was further down-regulated by increasing the multiplicity of infection of the antisense construct. The additional down-regulation of long MLCK expression, however, did not alter the residual contraction, ruling out the involvement of long MLCK in the contractile activity. Further, in the cells where short MLCK was down-regulated specifically, the extent of phosphorylation of 20-kDa myosin light chain (MLC20) after the agonist stimulation was not affected. This finding suggests that there are additional factors to MLC20 phosphorylation that contribute to regulate smooth muscle contraction.

'Putting the squeeze' on the tight junction: understanding cytoskeletal regulation

The apical perijunctional actomyosin ring of epithelia is structurally associated with the tight junction. The functional association between the tight junction and the perijunctional actomyosin ring was initially described in studies using pharmacological agents that disrupt microfilaments. More recently, this interaction has been studied in physiological, pathophysiological, and molecular models of tight junction regulation. These studies have demonstrated the central role of actomyosin contraction in tight junction regulation. With the identification of novel tight junction proteins and characterization of their protein:protein interactions comes the promise of detailed understanding of the molecular interactions that mediate tight junction regulation.

Show me the pathway! Regulation of paracellular permeability by Na(+)-glucose cotransport

The physiological impact of Na(+)-nutrient cotransport-dependent regulation of intestinal tight junction permeability has been controversial. Nonetheless, increased permeability of small intestinal mucosae and enterocyte tight junctions as a consequence of Na(+)-nutrient cotransport has been documented by a significant number of in vivo and in vitro studies. Some details of the intracellular signaling events that regulate this process have been described recently. The aims of this article are to: (i) review studies of tight junction regulation and paracellular nutrient absorption in mammalian intestine, (ii) identify potential applications of tight junction regulation, and (iii) summarize recent progress in defining molecular mechanisms that lead to altered tight junction permeability.

Stable transfectants of smooth muscle cell line lacking the expression of myosin light chain kinase and their characterization with respect to the actomyosin system

We constructed a plasmid vector having a 1.4-kilobase pair insert of myosin light chain kinase (MLCK) cDNA in an antisense direction to express antisense mRNA. The construct was then transfected to SM3, a cell line from vascular smooth muscle cells, producing a few stable transfectants. The down-regulation of MLCK expression in the transfectants was confirmed by both Northern and Western blots. The control SM3 showed chemotaxic motility to platelet-derived growth factor-BB, which was supported by lamellipodia. However, the transfectants showed neither chemotaxic motility nor developed lamellipodia, indicating the essential role of MLCK in the motility. The specificity for the targeting was assessed by a few tests including the rescue experiment. Despite this importance of MLCK, platelet-derived growth factor-BB failed to induce MLC20 phosphorylation in not only the transfectants but also in SM3. The mode in which MLCK was involved in the development of membrane ruffling is discussed with special reference to the novel property of MLCK that stimulates the ATPase activity of smooth muscle myosin without phosphorylating its light chain (Ye, L.-H., Kishi, H., Nakamura, A., Okagaki, T., Tanaka, T., Oiwa, K., and Kohama, K. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 6666-6671).

A conserved negative regulatory region in alphaPAK: inhibition of PAK kinases reveals their morphological roles downstream of Cdc42 and Rac1

AlphaPAK in a constitutively active form can exert morphological effects (E. Manser, H.-Y. Huang, T.-H. Loo, X.-Q. Chen, J.-M. Dong, T. Leung, and L. Lim, Mol. Cell. Biol. 17:1129-1143, 1997) resembling those of Cdc42G12V. PAK family kinases, conserved from yeasts to humans, are directly activated by Cdc42 or Rac1 through interaction with a conserved N-terminal motif (corresponding to residues 71 to 137 in alphaPAK). alphaPAK mutants with substitutions in this motif that resulted in severely reduced Cdc42 binding can be recruited normally to Cdc42G12V-driven focal complexes. Mutation of residues in the C-terminal portion of the motif (residues 101 to 137), though not affecting Cdc42 binding, produced a constitutively active kinase, suggesting this to be a negative regulatory region. Indeed, a 67-residue polypeptide encoding alphaPAK83-149 potently inhibited GTPgammaS-bound Cdc42-mediated kinase activation of both alphaPAK and betaPAK. Coexpression of this PAK inhibitor with Cdc42G12V prevented the formation of peripheral actin microspikes and associated loss of stress fibers normally induced by the p21. Coexpression of PAK inhibitor with Rac1G12V also prevented loss of stress fibers but not ruffling induced by the p21. Coexpression of alphaPAK83-149 completely blocked the phenotypic effects of hyperactive alphaPAKL107F in promoting dissolution of focal adhesions and actin stress fibers. These results, coupled with previous observations with constitutively active PAK, demonstrate that these kinases play an important role downstream of Cdc42 and Rac1 in cytoskeletal reorganization.

Smooth muscle myosin light chain kinase, supramolecular organization, modulation of activity, and related conformational changes

It has recently been suggested that activation of smooth muscle myosin light chain kinase (MLCK) can be modulated by formation of supramolecular structures (Sobieszek, A. 1991. Regulation of smooth muscle myosin light chain kinase. Allosteric effects and co-operative activation by CaM. J. Mol. Biol. 220:947-957). The present light scattering data demonstrate that the inactive (calmodulin-free) MLCK apoenzyme exists in solution as a mixture of oligomeric (2% by weight), dimeric (53%), and monomeric (45%) species at physiological ionic strength (160 mM salt). These long-living assemblies, the lifetime of which was measured by minutes, were in equilibrium with each other. The most likely form of the oligomer was a spiral-like hexamer, the dimensions of which fit very well the helical structure of self-assembled myosin filaments (Sobieszek, A. 1972. Cross-bridges on self-assembled smooth muscle myosin filaments. J. Mol. Biol. 70:741-744). After activation of the kinase by calmodulin (CaM) we could not detect any appreciable changes in the distribution of the kinase species either when the kinase was saturated with CaM or when its molar concentration exceeded that of CaM. Our fluorescent measurements suggest that the earlier observed inhibition of kinase at substoichiometric amounts of CaM (Sobieszek, A., A. Strobl, B. Ortner, and E. Babiychuk. 1993. Ca2+-calmodulin-dependent modification of smooth-muscle myosin light chain kinase leading to its co-operative activation by calmodulin. Biochem. J. 295:405-411) is associated with slow conformational change(s) of the activated (CaM-bound) kinase molecules. Such conformational rearrangements also took place with equimolar kinase to CaM; however, in this case there was no decrease in MLCK activity. The nature of these conformational changes, which are accompanied by reduction of the kinase for CaM affinity, is discussed.

Expression of the catalytic domain of myosin light chain kinase increases paracellular permeability

Contractile events resulting from phosphorylation of the 20-kDa myosin light chain (MLC20) have been implicated in the regulation of epithelial tight junction permeability. To address this question, Madin-Darby canine kidney cells were transfected with a murine leukemia retroviral vector containing DNA encoding either the catalytic domain of myosin light chain kinase (tMK) or the beta-galactosidase gene (beta-gal). Autoradiograms of sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of myosin immunoprecipitated from 32Pi-labeled transfected cells demonstrated that MLC20 phosphorylation was increased 3.1 +/- 0.9-fold in cells expressing tMK compared with cells expressing beta-gal. Phosphopeptide mapping confirmed that myosin light chain kinase was responsible for the increased MLC20 phosphorylation. Transepithelial electrical resistance, a measurement of barrier function, of tMK cell monolayers was consistently < 10% (123 +/- 20 omega.cm2) of that of monolayers comprised of wild-type cells (1,456 +/- 178 omega.cm2) or cells expressing beta-gal (1,452 +/- 174 omega.cm2). Dual 22Na+ and [3H]mannitol flux studies indicated that the decrease in resistance in tMK cells was attributable to increased paracellular flow. These data support the idea that MLC20 phosphorylation by myosin light chain kinase is involved in regulating epithelial tight junction permeability.

Identification of an autoinhibitory region in the activation loop of the Mos protein kinase

The Mos protein is a serine/threonine protein kinase which acts to regulate progression through meiosis in vertebrate oocytes. Although Mos function is dependent on its ability to act as a protein kinase, little is known about the factors which regulate Mos kinase activity. To understand the mechanism by which Mos kinase activity is regulated, we have used molecular modeling to construct a three-dimensional model of Mos based on the crystallographic coordinates of cyclic AMP-dependent kinase (PKA). This model identified a loop in Mos which is positioned near the active site and appears capable of blocking substrate access to the active site. Mutagenesis was used to construct altered forms of the Mos protein with deletions of parts or all of the loop. In vitro kinase assays showed that Mos proteins with the loop removed had up to a fourfold increase in kinase activity compared with the wild-type protein, indicating that the loop acts in an autoinhibitory manner for Mos kinase activity. Point mutations were also made on individual residues of the loop which were determined from the molecular model to be capable of reaching the active site. Determination of the kinase activities of these mutants showed that individual mutations in the loop region are capable of either increasing or decreasing kinase activity with regard to the wild-type protein. These data suggest that the loop identified in Mos acts as an autoinhibitor of kinase activity.

Role of calmodulin and Spc110p interaction in the proper assembly of spindle pole body compenents

Previously we demonstrated that calmodulin binds to the carboxy terminus of Spc110p, an essential component of the Saccharomyces cerevisiae spindle pole body (SPB), and that this interaction is required for chromosome segregation. Immunoelectron microscopy presented here shows that calmodulin and thus the carboxy terminus of Spc110p localize to the central plaque. We created temperature-sensitive SPC110 mutations by combining PCR mutagenesis with a plasmid shuffle strategy. The temperature-sensitive allele spc110-220 differs from wild type at two sites. The cysteine 911 to arginine mutation resides in the calmodulin-binding site and alone confers a temperature-sensitive phenotype. Calmodulin overproduction suppresses the temperature sensitivity of spc110-220. Furthermore, calmodulin levels at the SPB decrease in the mutant cells at the restrictive temperature. Thus, calmodulin binding to Spc110-220p is defective at the nonpermissive temperature. Synchronized mutant cells incubated at the nonpermissive temperature arrest as large budded cells with a G2 content of DNA and suffer considerable lethality. Immunofluorescent staining demonstrates failure of nuclear DNA segregation and breakage of many spindles. Electron microscopy reveals an aberrant nuclear structure, the intranuclear microtubule organizer (IMO), that differs from a SPB but serves as a center of microtubule organization. The IMO appears during nascent SPB formation and disappears after SPB separation. The IMO contains both the 90-kD and the mutant 110-kD SPB components. Our results suggest that disruption of the calmodulin Spc110p interaction leads to the aberrant assembly of SPB components into the IMO, which in turn perturbs spindle formation.

Structure of the pseudosubstrate recognition site of chicken smooth muscle myosin light chain kinase

The structure of the chicken smooth muscle myosin light chain kinase pseudosubstrate sequence MLCK(774-807)amide was studied using two-dimensional proton NMR spectroscopy. Resonance assignments were made with the aid of totally correlated and nuclear Overhauser effect spectroscopy. A distance geometry algorithm was used to process the body of NMR distance and angle data and the resulting family of structures was further refined using dynamic simulated annealing. The major structural features determined include two helical segments extending from Asp-777 to Lys-785 and from Arg-790/Met-791 to Trp-800 connected by a turn region from Leu-786 to Asp-789 enabling the helices to interact in solution. The C-terminal helix incorporates the bulk of the pseudosubstrate recognition site which is partially overlapped by the calmodulin binding site while the N-terminal helix forms the bulk of the connecting peptide. The demonstrated turn between the helices may assist in enabling the autoregulatory or pseudosubstrate recognition sequence to be rotated out of the active site of the catalytic core following calmodulin binding.

The regulatory region of calcium/calmodulin-dependent protein kinase I contains closely associated autoinhibitory and calmodulin-binding domains

The mechanism for the regulation of Ca2+/calmodulin-dependent protein kinase I (CaM kinase I) was investigated using a series of COOH-terminal truncated mutants. These mutants were expressed in bacteria as fusion proteins with glutathione S-transferase and purified by affinity chromatography using glutathione Sepharose 4B. A mutant (residues 1-332) showed complete Ca2+/CaM-dependent activity. Truncation mutants (residues 1-321, 1-314, and 1-309) exhibited decreasing affinities for Ca2+/CaM and also exhibited decreasing Ca2+/CaM-dependent activities. Truncation mutants (residues 1-305 or 1-299) were unable to bind Ca2+/CaM and were inactive. In contrast, truncation mutants (residues 1-293 or 1-277) were constitutively active at a slightly higher level (2-fold) than fully active CaM kinase I. These results indicate the location of the Ca2+/CaM-binding domain on CaM kinase I (residues 294-321) and predict the existence of an autoinhibitory domain near, or overlapping, the Ca2+/CaM-binding domain. These conclusions were supported by studies which showed that a synthetic peptide (CaM kinase I (294-321)) corresponding to residues 294-321 of CaM kinase I inhibited the fully active kinase in a manner that was competitive with Ca2+/CaM and also inhibited the constitutively active mutant (residues 1-293) in a manner that was competitive with Syntide-2, a peptide substrate, (Ki = 1.2 microM) but was non-competitive with ATP. Thus, these results suggest that CaM kinase I is regulated through an intrasteric mechanism common to other members of the family of Ca2+/CaM-dependent protein kinases.

Interaction of the ribosomal protein, L5, with protein phosphatase type 1

The two-hybrid system was used to screen for binding proteins of type 1 protein phosphatase. Two plasmids were constructed, one containing the cDNA of the delta isoform of the type 1 catalytic subunit and the other containing a chicken gizzard cDNA library. Yeast (Y190) were transformed with the plasmids and screened for interacting species. 35 positive clones were categorized into 19 gene groups. Most of these were not identified. One clone, however, contained a sequence identical to the C-terminal portion of the chicken ribosomal protein L5 and corresponded to nucleotide residues 606-975. L5 was isolated from rat liver ribosomes as the L5.5 S RNA complex. This activated phosphatase activity of a myosin-bound phosphatase and the isolated type 1 catalytic subunit using phosphorylated myosin light chains and phosphorylase alpha as substrates. In addition, it was found that phosphatase sedimented with ribosomal subunits containing L5 but did not sediment with those deficient in L5. These data indicate that L5 binds to the catalytic subunit of the type 1 protein phosphatase and may act as a target molecule for phosphatase in ribosomal function or other cell mechanisms.

Fibroblast contractility without an increase in basal myosin light chain phosphorylation in wild type cells and cells expressing the catalytic domain of myosin light chain kinase

We investigated the role of myosin light chain (MLC20) phosphorylation (MLC-P) in non-muscle contractility by comparing MLC-P and the contractile properties of wild type 3T3 fibroblasts and 3T3 fibroblasts expressing the catalytic domain of myosin light chain kinase (tMK). MLC-P is 0.96 MOL of PO4/mol of MOL20 in cell expressing tMK compared to 0.20 mol of PO4/mol of MLC20 in control cells. Expressing tMK also results in a 2-fold increase in cortical stiffness compared to control cells. Contractile properties were quantified by growing wild type and transfected fibroblasts in collagen and attaching the ensuing fibers to an apparatus for performing mechanical measurements. Serum stimulation resulted in a dose-dependent increase in force with maximal force generated in the presence of 30% (v/v) serum. Surprisingly, MLC-P did not increase in wild type cells following stimulation with 30% serum, and tMK expression did not affect the contractile properties of fibers made from these cells. Moreover, the dose responses to serum, maximal force, force-velocity relationships, and dynamic stiffness were similar in the wild type cells and fibroblasts expressing tMK. These data demonstrate that non-muscle cells can generate force without an increase in MLC-P, and that an increase in MLC-P does not affect the contractile properties of fibroblast fibers.

Intrasteric regulation of myosin light chain kinase

Ca2+/calmodulin activates myosin light chain kinase by reversal of an autoinhibited state. The effects of substitution mutations on calmodulin activation properties implicate 4 of the 8 basic residues between the catalytic core and the calmodulin-binding domain in maintaining autoinhibition. These residues are further amino-terminal to the basic residues comprising the previously proposed pseudosubstrate sequence and suggest involvement of the connecting region in intrasteric autoinhibition. The pseudosubstrate model for autoinhibition proposes that basic residues within the autoinhibitory region mimic basic residues in the substrate and bind to defined acidic residues within the catalytic core. Charge reversal mutations of these specific acidic residues, however, had little or no effect on the Km value for regulatory light chain. From a total of 20 acidic residues on the surface of the substrate binding lobe of the catalytic core, 7 are implicated in binding directly or indirectly to the autoinhibitory domain but not to the light chain. Only 2 acidic residues near the catalytic site may bind to the autoinhibitory domain and the arginine at P-3 in the light chain. Exposure of these 2 residues upon calmodulin binding may be necessary and sufficient for light chain phosphorylation.

Calmodulin and the regulation of smooth muscle contraction

Calmodulin, the ubiquitous and multifunctional Ca(2+)-binding protein, mediates many of the regulatory effects of Ca2+, including the contractile state of smooth muscle. The principal function of calmodulin in smooth muscle is to activate crossbridge cycling and the development of force in response to a [Ca2+]i transient via the activation of myosin light-chain kinase and phosphorylation of myosin. A distinct calmodulin-dependent kinase, Ca2+/calmodulin-dependent protein kinase II, has been implicated in modulation of smooth-muscle contraction. This kinase phosphorylates myosin light-chain kinase, resulting in an increase in the calmodulin concentration required for half-maximal activation of myosin light-chain kinase, and may account for desensitization of the contractile response to Ca2+. In addition, the thin filament-associated proteins, caldesmon and calponin, which inhibit the actin-activated MgATPase activity of smooth-muscle myosin (the cross-bridge cycling rate), appear to be regulated by calmodulin, either by the direct binding of Ca2+/calmodulin or indirectly by phosphorylation catalysed by Ca2+/calmodulin-dependent protein kinase II. Another level at which calmodulin can regulate smooth-muscle contraction involves proteins which control the movement of Ca2+ across the sarcolemmal and sarcoplasmic reticulum membranes and which are regulated by Ca2+/calmodulin, e.g. the sarcolemmal Ca2+ pump and the ryanodine receptor/Ca2+ release channel, and other proteins which indirectly regulate [Ca2+]i via cyclic nucleotide synthesis and breakdown, e.g. NO synthase and cyclic nucleotide phosphodiesterase. The interplay of such regulatory mechanisms provides the flexibility and adaptability required for the normal functioning of smooth-muscle tissues.

The calmodulin binding domain of nitric oxide synthase and adenylyl cyclase

Peptides corresponding to regions of the calmodulin-activated NO-synthase and of the calmodulin dependent adenylyl cyclase, which could represent the calmodulin binding domains of the two proteins, have been synthesized and tested for calmodulin binding. The chosen peptides were those in the sequence of the two proteins which most closely corresponded to the accepted general properties of the calmodulin binding domains, i.e., a hydrophobic sequence containing basic amino acids. In the case of the NO-synthase, the putative high-affinity calmodulin binding domain was identified by urea gel electrophoresis and fluorescence spectroscopy with dansylcalmodulin as peptide NO-30 (amino acids 725-754). The highest affinity calmodulin binding site of the calmodulin-dependent adenylyl cyclase was assigned to peptide AC-28 (amino acids 495-522) by titration with dansylcalmodulin and by the ability to inhibit the calmodulin-stimulated activity of purified calmodulin-stimulated adenylyl cyclase. The sequence 495-522 is located in the unit protruding into the cytosol from the sixth putative transmembrane domain of the molecule. It has the typical hydrophobic/basic composition of canonical calmodulin binding domains, and also contains, like most calmodulin binding domains, an aromatic amino acid in its N-terminal portion. It also contains two Cys residues in the central portion, which is an unusual feature of the calmodulin binding domain of this enzyme.

X-ray structure determination of telokin, the C-terminal domain of myosin light chain kinase, at 2.8 A resolution

The three-dimensional structure of telokin, an acidic protein identical to the C-terminal portion of smooth muscle myosin light chain kinase from turkey gizzard, has been determined at 2.8 A resolution and refined to a crystallographic R-factor of 19.5% for all measured X-ray data from 30 A to 2.8 A. Crystals used in the investigation belonged to the space group P3(2)21, with one molecule per asymmetric unit and unit cell dimensions of a = b = 64.4 A and c = 50.6 A. Telokin contains 154 amino acid residues, 103 of which were visible in the electron density map. The overall molecular fold of telokin consists of seven strands of antiparallel beta-pleated sheet that wrap around to form a barrel. There is also an extended tail of eight amino acid residues at the N terminus that does not participate in beta-sheet formation. The beta-barrel can be simply envisioned as two layers of beta-sheet, nearly parallel to one another, with one layer containing four and the other three beta-strands. This type of beta-barrel, as seen in telokin, was first observed for the CH2 domain of an immunoglobulin fragment Fc. Telokin is an intracellular protein and, as such, does not contain the disulphide linkage between beta-strands B and F normally observed in the immunoglobulin constant domains. It does, however, contain two cysteine amino acid residues (Cys63 and Cys115) that are situated at structurally identical positions to those forming the disulphide linkage in the immunoglobulin constant domain.

Structural basis of the intrasteric regulation of myosin light chain kinases

The smooth muscle myosin light chain kinase (smMLCK) catalytic core was modeled by using the crystallographic coordinates of the cyclic AMP-dependent protein kinase catalytic subunit (cAPK) and a bound pseudosubstrate inhibitor peptide, PKI(5-24). Despite only 30% identity in amino acid sequence, the MLCK sequence can be readily accommodated in this structure. With the exception of the short B-helix, all major elements of secondary structure in the core are very likely conserved. The active site of the modeled MLCK complements the known requirements for peptide substrate recognition. MLCK contains a pseudosubstrate sequence that overlaps the calmodulin binding domain and has been proposed to act as an intrasteric inhibitor and occupy the substrate binding site in the absence of Ca(2+)-calmodulin. The pseudosubstrate sequence can be modeled easily into the entire backbone of PKI(5-24). The results demonstrate that the intrasteric model for regulation of MLCK by intramolecular competitive inhibition is structurally plausible.

Target enzyme recognition by calmodulin: 2.4 A structure of a calmodulin-peptide complex

The crystal structure of calcium-bound calmodulin (Ca(2+)-CaM) bound to a peptide analog of the CaM-binding region of chicken smooth muscle myosin light chain kinase has been determined and refined to a resolution of 2.4 angstroms (A). The structure is compact and has the shape of an ellipsoid (axial ratio approximately 2:1). The bound CaM forms a tunnel diagonal to its long axis that engulfs the helical peptide, with the hydrophobic regions of CaM melded into a single area that closely covers the hydrophobic side of the peptide. There is a remarkably high pseudo-twofold symmetry between the closely associated domains. The central helix of the native CaM is unwound and expanded into a bend between residues 73 and 77. About 185 contacts (less than 4 A) are formed between CaM and the peptide, with van der Waals contacts comprising approximately 80% of this total.

The MgATP-binding site on chicken gizzard myosin light chain kinase remains open and functionally competent during the calmodulin-dependent activation-inactivation cycle of the enzyme

An ATP-like affinity labeling reagent, 5'-[p-(fluorosulfonyl)benzoyl]adenosine (FSBA), was used to probe the MgATP-binding site of smooth muscle myosin light chain kinase from chicken gizzard (smMLCK) and its calmodulin (CaM) complex. Native smMLCK has an absolute requirement for the binding of the calcium complex of CaM for expression of its catalytic activity. FSBA reacted with smMLCK-CaM and with the CaM-free, inactive enzyme as well. Both reactions were dependent on time and FSBA concentration. Reaction was accompanied by the incorporation of covalently bound [14C]FSBA into smMLCK protein at a molar ratio of approximately 1:1 in each case. p-(Fluorosulfonyl)benzoic acid, an analogue of FSBA lacking the adenosine targeting group, did not react at a significant rate with either form of smMLCK. Reaction of CaM-free and CaM-bound smMLCK with FSBA displayed saturation kinetics. The first-order rate constants for the conversion of the reversible, noncovalent enzyme-FSBA complex to form the irreversibly inhibited, covalently modified enzyme were similar for both smMLCK and smMLCK-CaM, 0.15 and 0.07 min-1, respectively. The concentrations of FSBA yielding the half-maximal rate of inactivation, KI, were essentially identical--0.65 and 0.64 mM, respectively--for smMLCK and smMLCK-CaM. MgATP, but not MgGTP or a substrate peptide, potently inhibited reaction with FSBA. Inhibition by MgATP was competitive. The measured inhibitory constant for MgATP was essentially the same--33 versus 34 microM--for both smMLCK and smMLCK-CaM. It therefore is concluded that the MgATP-binding site on smMLCK remains accessible and recognizable as such when the enzyme becomes inactivated upon dissociation of CaM.