Purification and characterization of smooth muscle myosin-associated phosphatase from chicken gizzards

Unfair competition governs the interaction of pCPI-17 with myosin phosphatase (PP1-MYPT1)

The small phosphoprotein pCPI-17 inhibits myosin light-chain phosphatase (MLCP). Current models postulate that during muscle relaxation, phosphatases other than MLCP dephosphorylate and inactivate pCPI-17 to restore MLCP activity. We show here that such hypotheses are insufficient to account for the observed rapidity of pCPI-17 inactivation in mammalian smooth muscles. Instead, MLCP itself is the critical enzyme for pCPI-17 dephosphorylation. We call the mutual sequestration mechanism through which pCPI-17 and MLCP interact inhibition by unfair competition: MLCP protects pCPI-17 from other phosphatases, while pCPI-17 blocks other substrates from MLCP’s active site. MLCP dephosphorylates pCPI-17 at a slow rate that is, nonetheless, both sufficient and necessary to explain the speed of pCPI-17 dephosphorylation and the consequent MLCP activation during muscle relaxation.

DOI:http://dx.doi.org/10.7554/eLife.24665.001

Reconstituted human myosin light chain phosphatase reveals distinct roles of two inhibitory phosphorylation sites of the regulatory subunit, MYPT1

The myosin light chain phosphatase (MLCP) is a cytoskeleton-associated protein phosphatase-1 (PP1) holoenzyme and a RhoA/ROCK effector, regulating cytoskeletal reorganization. ROCK-induced phosphorylation of the MLCP regulatory subunit (MYPT1) at two sites, Thr696 and Thr853, suppresses the activity, although little is known about the difference in the role. Here, we developed a new method for the preparation of the recombinant human MLCP complex and determined the molecular and cellular basis of inhibitory phosphorylation. The recombinant MLCP partially purified from mammalian cell lysates retained characteristics of the native enzyme, such that it was fully active without Mn²⁺ and sensitive to PP1 inhibitor compounds. Selective thio-phosphorylation of MYPT1 at Thr696 with ROCK inhibited the MLCP activity 30%, whereas the Thr853 thio-phosphorylation did not alter the phosphatase activity. Interference with the docking of phospho-Thr696 at the active site weakened the inhibition, suggesting selective autoinhibition induced by phospho-Thr696. Both Thr696 and Thr853 sites underwent autodephosphorylation. Compared with that of Thr853, phosphorylation of Thr696 was more stable, and it facilitated Thr853 phosphorylation. Endogenous MYPT1 at Thr696 was spontaneously phosphorylated in quiescent human leiomyosarcoma cells. Serum stimulation of the cells resulted in dissociation of MYPT1 from myosin and PP1C in parallel with an increase in the level of Thr853 phosphorylation. The C-terminal domain of human MYPT1(495–1030) was responsible for the binding to the N-terminal portion of myosin light meromyosin. The spontaneous phosphorylation at Thr696 may adjust the basal activity of cellular MLCP and affect the temporal phosphorylation at Thr853 that is synchronized with myosin targeting.

Purification and characterization of the plasmodial phosphatase that hydrolyses the phosphorylated light chain of Physarum myosin II from Physarum polycephalum

A phosphatase was purified through a combination of ion-exchange and hydrophobic chromatography followed by native PAGE from Physarum plasmodia. Recently, we demonstrated that this phosphatase isoform has a hydrolytic activity towards the PMLC (phosphorylated light chain of Physarum myosin II) at pH 7.6. The apparent molecular mass of the purified enzyme was estimated at approximately 50 kDa by means of analytical gel filtration. The enzyme was purified 340-fold to a final phosphatase activity of 400 pkat/mg of protein. Among the phosphorylated compounds tested for hydrolytic activity at pH 7.6, the enzyme showed no activity towards nucleotides. At pH 7.6, hydrolytic activity of the enzyme against PMLC was detected; at pH 5.0, however, no hydrolytic activity towards PMLC was observed. The Km of the enzyme for PMLC was 10 microM, and the V(max) was 1.17 nkat/mg of protein. Ca(2+) (10 microM) inhibited the activity of the enzyme, and Mg(2+) (8.5 microM) activated the dephosphorylation of PMLC. Mn(2+) (1.6 microM) highly stimulated the enzyme's activity. Based on these results, we concluded that the enzyme is likely to be a phosphatase with hydrolytic activity towards PMLC.

Supervillin slows cell spreading by facilitating myosin II activation at the cell periphery

During cell migration, myosin II modulates adhesion, cell protrusion and actin organization at the leading edge. We show that an F-actin- and membrane-associated scaffolding protein, called supervillin (SV, p205), binds directly to the subfragment 2 domains of nonmuscle myosin IIA and myosin IIB and to the N-terminus of the long form of myosin light chain kinase (L-MLCK). SV inhibits cell spreading via an MLCK- and myosin II-dependent mechanism. Overexpression of SV reduces the rate of cell spreading, and RNAi-mediated knockdown of endogenous SV increases it. Endogenous and EGFP-tagged SV colocalize with, and enhance the formation of, cortical bundles of F-actin and activated myosin II during early cell spreading. The effects of SV are reversed by inhibition of myosin heavy chain (MHC) ATPase (blebbistatin), MLCK (ML-7) or MEK (U0126), but not by inhibiting Rho-kinase with Y-27632. Flag-tagged L-MLCK co-localizes in cortical bundles with EGFP-SV, and kinase-dead L-MLCK disorganizes these bundles. The L-MLCK- and myosin-binding site in SV, SV1-171, rearranges and co-localizes with mono- and di-phosphorylated myosin light chain and with L-MLCK, but not with the short form of MLCK (S-MLCK) or with myosin phosphatase. Thus, the membrane protein SV apparently contributes to myosin II assembly during cell spreading by modulating myosin II regulation by L-MLCK.

Regulation of pulmonary arterial myosin phosphatase activity in neonatal circulatory transition and in hypoxic pulmonary hypertension: a role for CPI-17

Neonatal circulatory transition is dependent upon tightly regulated pulmonary circuit relaxation. Persistent pulmonary hypertension of the newborn (PPHN) is characterized by pulmonary arterial myocyte relaxation failure. We examined the effect of short course (72 hour) in vivo normobaric hypoxia in newborn swine on smooth muscle contractile enzyme activity and regulatory phosphoprotein abundance, in tissue homogenates of 2nd to 4th generation pulmonary arteries. Myosin light chain kinase (MLCK) and phosphatase (MLCP) protein contents were unchanged in hypoxic pulmonary arteries compared to controls. MLCP activity increased in normoxic animals from birth to day 3. This was ablated by hypoxia; phosphatase activity, measured as in vitro myosin light chain dephosphorylation, was decreased significantly (P < 0.005) in the hypoxic group. Inhibitory site phosphorylations of MLCP myosin binding subunit at threonines 696 and 850 were similar in both hypoxic and normoxic subjects, suggesting that downregulation of MLCP in hypoxia does not involve this pathway. However, content of regulatory protein CPI-17 (protein kinase C-related phosphatase inhibitor) increased from birth in hypoxic subjects (P < 0.05); active (phosphorylated) CPI-17 protein abundance declined after birth in normals, but increased in hypoxic arteries (P < 0.05). This corresponded with the decrease in phosphatase activity. We speculate that CPI-17 may play a role in myosin phosphatase upregulation during neonatal circulatory transition, and in hypoxic inhibition of pulmonary phosphatase activity in PPHN.

p116Rip Decreases Myosin II Phosphorylation by Activating Myosin Light Chain Phosphatase and by Inactivating RhoA

p116Rip was originally found to be a RhoA-binding protein, but its function has been unknown. Here, we clarify the function of p116Rip. Two critical findings were made. First, we found that p116Rip activated the GTPase activity of RhoA in vitro and that p116Rip overexpression in cells consistently diminished the epidermal growth factor-induced increase in GTP-bound RhoA. Second, p116Rip activated the myosin light chain phosphatase (MLCP) activity of the holoenzyme. p116Rip did not activate the catalytic subunit alone, indicating that the activation is due to the binding of p116Rip to the myosin phosphatase targeting subunit MYPT1. Interestingly, the activation of phosphatase was specific to myosin as substrate, and p116Rip directly bound to myosin, thus facilitating myosin/MLCP interaction. The gene silencing of p116Rip consistently and significantly increased myosin phosphorylation as well as stress fiber formation in cells. Based upon these findings, we propose that p116Rip is an important regulatory component that controls the RhoA signaling pathway, thus regulating MLCP activity and myosin phosphorylation in cells.

Dynamic changes in expression of myosin phosphatase in a model of portal hypertension

Myosin phosphatase is a target for signaling pathways that modulate calcium sensitivity of force production in smooth muscle. Myosin phosphatase targeting subunit 1 (MYPT1) isoforms are generated by cassette-type alternative splicing of exons in the central and 3' portion of the transcript. Exclusion of the 3' alternative exon, coding for the leucine zipper (LZ)-positive MYPT1 isoform, is associated with the ability to desensitize to calcium (relax) in response to NO/cGMP-dependent signaling. We examined expression of MYPT1 isoforms and smooth muscle phenotype in normal rat vessels and in a prehepatic model of portal hypertension characterized by arteriolar dilation. The large capacitance vessels, aorta, pulmonary artery, and inferior vena cava expressed predominantly the 3' exon-out/LZ-positive MYPT1 isoform. The first-order mesenteric resistance artery (MA1) and portal vein (PV) expressed severalfold higher levels of MYPT1 with predominance of the 3' exon-included/LZ-negative isoform. There was minor variation in the presence of the MYPT1 central alternative exons. Myosin heavy and light chain splice variants in part cosegregated with MYPT1 isoforms. In response to portal hypertension induced by PV ligature, abundance of MYPT1 in PV and MA1 was significantly reduced and switched to the LZ-positive isoform. These changes were evident within 1 day of PV ligature and were maintained for up to 10 days before reverting to control values at day 14. Alteration of MYPT1 expression was part of a complex change in protein expression that can be generalized as a modulation from a phasic (fast) to a tonic (slow) contractile phenotype. Implications of vascular smooth muscle phenotypic diversity and reversible phenotypic modulation in portal hypertension with regards to regulation of blood flow are discussed.

On the mechanisms whereby temperature affects excitation-contraction coupling in smooth muscle

Moderate cooling of smooth muscle can modulate force production and may contribute to pathophysiological conditions, but the mechanisms underlying its effects are poorly understood. Interestingly, cooling increases force in rat ureter, but decreases it in guinea pigs. Therefore, this study used ureteric smooth muscle as a model system to elucidate the mechanisms of the effects of cooling on excitation-contraction coupling. Simultaneous recordings of force, intracellular [Ca(2+)], and electrical activity were made in intact ureter and ionic currents measured in isolated cells. The increase in force amplitude in rat ureter with cooling was found to be due to a significant increase in the duration of the Ca(2+) transient. This in turn was due to a marked prolongation of the action potential. In guinea pigs, both these parameters were much less affected by cooling. Examination of membrane currents revealed that differences in ion channel contribution to the action potential underlie these differences. In particular, cooling potentiated Ca(2+)-activated Cl(-) currents, which are present in rat but not guinea pig ureteric smooth muscle, and prolonged the plateau of the action potential and Ca(2+) entry. The force-Ca(2+) relationship revealed that the increased duration of the Ca(2+) transient was sufficient in the rat, but not in the guinea pig, to overcome kinetic lags produced in both species by cooling and potentiate force. Ca(2+) entry and release processes were largely temperature-insensitive, but the rate of relaxation was very temperature-sensitive. Effects of cooling on myosin light chain phosphatase, confirmed in experiments using calyculin A, appear to be the predominant mechanisms affecting relaxation. Thus, smooth muscle is diverse in its response to temperature, even when experimental variables, such as the mode of stimulation, are removed. Although the biochemical and mechanical events accompanying contraction are likely to be affected in similar ways by temperature, differences in electrical events lead to subsequent differences in these processes between smooth muscles.

Myosin light chain phosphorylation is correlated with cold-induced changes in platelet shape

Chilling induces shape changes in platelets from disks to spheres with abundant filopodia. Such changes were time-dependent and correlated well with the phosphorylation of 20-kDa myosin light chain (LC20). Both the shape changes and the phosphorylation were reversible. After the platelets had been chilled, myosin became incorporated into the Triton X-insoluble fraction. When the chilled platelets were immunocytochemically stained, anti-myosin antibody was localized with filamentous structures inside the filopodia. These results suggest that LC20 phosphorylation and subsequent interactions with actin filaments play a crucial role in the cold-induced changes in platelet shape and in the formation of filopodia.

Is myosin phosphatase regulated in vivo by inhibitor-1? Evidence from inhibitor-1 knockout mice

1. The Ca(2+) sensitivity of smooth muscle contractility is modulated via regulation of phosphatase activity. Protein phosphatase inhibitor-1 (I-1) is the classic type-1 phosphatase inhibitor, but its presence and role in cAMP-dependent protein kinase (PKA) modulation of smooth muscle is unclear. To address the relevance of I-1 in vivo, we investigated smooth muscle function in a mouse model lacking the I-1 protein (I-1((-/-)) mice). 2. Significant amounts of I-1 protein were detected in the wild-type (WT) mouse aorta and could be phosphorylated by PKA, as indicated by (32)P-labelled aortic extracts from WT mice. 3. Despite the significant presence of I-1 in WT aorta, phenylephrine and KCl concentration- isometric force relations in the presence or absence of the PKA pathway activator isoproterenol (isoprenaline) were unchanged compared to I-1((-/-)) aorta. cGMP-dependent protein kinase (PKG) relaxation pathways were also not different. Consistent with these findings, dephosphorylation rates of the 20 kDa myosin light chains (MLC(20)), measured in aortic extracts, were nearly identical between WT and I-1((-/-)) mice. 4. In the portal vein, I-1 protein ablation was associated with a significant (P < 0.05) rightward shift in the EC(50) of isoproterenol relaxation (EC(50) = 10.4 +/- 1.4 nM) compared to the WT value (EC(50) = 3.5 +/- 0.2 nM). Contraction in response to acetylcholine as well as Ca(2+) sensitivity were similar between WT and I-1((-/-)) aorta. 5. Despite the prevalence of I-1 and its activation by PKA in the aorta, I-1 does not appear to play a significant role in contractile or relaxant responses to any pharmacomechanical or electromechanical agonists used. I-1 may play a role as a fine-tuning mechanism involved in regulating portal vein responsiveness to beta-adrenergic agonists.

Calyculin-A induces focal adhesion assembly and tyrosine phosphorylation of p125(Fak), p130(Cas), and paxillin in Swiss 3T3 cells

Treatment of intact Swiss 3T3 cells with calyculin-A, an inhibitor of myosin light chain (MLC) phosphatase, induces tyrosine phosphorylation of p125(Fak) in a sharply concentration- and time-dependent manner. Maximal stimulation was 4.2 +/- 2.1-fold (n = 14). The stimulatory effect of calyculin-A was observed at low nanomolar concentrations (<10 nM); at higher concentrations (>10 nM) tyrosine phosphorylation of p125(Fak) was strikingly decreased. Calyculin-A induced tyrosine phosphorylation of p125(Fak) through a protein kinase C- and Ca(2+)-independent pathway. Exposure to either cytochalasin-D or latrunculin-A, which disrupt actin organization by different mechanisms, abolished tyrosine phosphorylation of p125(Fak) in response to calyculin-A. Treatment with high concentrations of platelet-derived growth factor (20 ng/ml) which also disrupt actin stress fibers, completely inhibited tyrosine phosphorylation of p125(Fak) in response to calyculin-A. This agent also induced tyrosine phosphorylation of the focal adhesion-associated proteins p130(Cas) and paxillin. These tyrosine phosphorylation events were associated with a striking increase in the assembly of focal adhesions. The Rho kinase (ROK) inhibitor HA1077 that blocked focal adhesion formation by bombesin, had no effect on the focal adhesion assembly induced by calyculin-A. Thus, calyculin-A induces transient focal adhesion assembly and tyrosine phosphorylation of p125(Fak), p130(Cas), and paxillin, acting downstream of ROK.

Divergence in murine myometrium spontaneous and oxytocin-stimulated contractile responses to serine/threonine protein phosphatase-1 inhibition

Reversible phosphorylation is essential in regulating uterine contractions. Identification, characterization, and functional understanding of myometrium protein phosphatase(s) are lacking. Okadaic acid (OA), which inhibits protein phosphatase-1 (PP1) and PP2A, has been shown to alter uterine contractions. Experiments were conducted to determine the 1) identity of the myometrial OA-sensitive PP, 2) influence of OA on spontaneous and oxytocin (OT)-stimulated myometrial contractions, and 3) expression of uterine PPs during sexual development. Western blot analysis indicated the presence of PP1(alpha) and PP2A in immature and mature mice. As determined by immunohistochemistry, gonadotropin-stimulated adult mouse uteri contain PP1(alpha) in longitudinal and circular myometrial layers and endometrial epithelium. Conversely, PP2A was localized to the endometrial stroma. Cumulative addition of OA (n = 9; 10, 100, 250, 500, 1000 nM) did not significantly alter spontaneous contractions of mouse uterine horns in comparison to vehicle-treated controls (n = 9). By the end of the test period OA- and vehicle-treated uteri displayed a comparable decline in uterine contractions to 79.2% and 63.7%, respectively, of basal contractile activity. Pretreatment of uterine tissue with OA (1 microM; n = 7) significantly reduced contractile response to increasing concentrations of OT (8, 16, 32, 64 nM) in comparison to vehicle pretreatment (dimethyl sulfoxide; n = 7). At the end of the OT-administration period, contractile activity was 160.4% and 67.3% of basal contractile activity for vehicle (no OA) and OA-pretreated groups, respectively. During the early prepubertal period PP1(alpha) was expressed in longitudinal myometrium and absent in circular myometrium; whereas, during the transition to sexual maturity PP1(alpha) was observed in both the longitudinal and circular myometrium. In summary, these studies have indicated 1) that PP1 is the primary myometrial OA-sensitive PP; 2) that inhibition of PP1 had no effect on spontaneous contractions, whereas it markedly inhibited OT-stimulated uterine contractions; and 3) that PP1 is differentially expressed in the circular and longitudinal myometrium in relation to sexual development.

The presence of an extractable substance in the CSF of humans with cerebral vasospasm after subarachnoid haemorrhage that correlates with phosphatase inhibition

The cellular events leading to cerebral vasospasm after subarachnoid haemorrhage are poorly understood, although an increase in smooth muscle myosin light chain phosphorylation has been observed. This study set out to determine if phosphatase inhibition may be involved in the pathological maintenance of tension observed during vasospasm. We found that 1 nM okadaic acid, a type 2A protein phosphatase inhibitor, elicited an increase in rate of O(2) consumption in the porcine carotid artery similar to that by cerebrospinal fluid (CSF) from vasospastic patients (CSF(V), n=5) (control 0.23+/-0.03, CSF(V) 0.84+/-0.16 and okadaic acid 0.85+/-0.02 micromol min(-1) g dwt(-1)). It was also observed that phosphatase inhibition with 1 nM okadaic acid significantly slowed relaxation after a stretch in a similar fashion to CSF(V) haemorrhage. CSF from vasospastic subarachnoid haemorrhage patients, but not from those without vasospasm, contains an extractable substance which modulates myosin light chain phosphorylation in vitro. A phosphatase preparation obtained from the porcine carotid artery dephosphorylated 63+/-2% of the phosphorylated (MLC(20)) substrate in vitro, and non-vasospastic CSF treated enzyme dephosphorylated 60+/-2.6%. Okadaic acid inhibited phosphatase dephosphorylated only 7.5+/-1% of the substrate where CSF(V) treated enzyme dephosphorylated 22+/-2.8% of the substrate. We conclude that inhibition of smooth muscle phosphatase may be involved in the mechanisms associated with cerebral vasospasm after subarachnoid haemorrhage.

Role of the N-terminal region of the regulatory light chain in the dephosphorylation of myosin by myosin light chain phosphatase

Myosin regulatory light chain (RLC) is phosphorylated at various sites at its N-terminal region, and heterotrimeric myosin light chain phosphatase (MLCP) has been assigned as a physiological phosphatase that dephosphorylates myosin in vivo. Specificity of MLCP toward the various phosphorylation sites of RLC was studied, as well as the role of the N-terminal region of RLC in the dephosphorylation of myosin by MLCP. MLCP dephosphorylated phosphoserine 19, phosphothreonine 18, and phosphothreonine 9 efficiently with almost identical rates, whereas it failed to dephosphorylate phosphorylated serine 1/serine 2. Deletion of the N-terminal seven amino acid residues of RLC markedly decreased the dephosphorylation rate of phosphoserine 19 of RLC incorporated in the myosin molecule, whereas this deletion did not significantly affect the dephosphorylation rate of isolated RLC. On the other hand, deletion of only four N-terminal amino acid residues showed no effect on dephosphorylation of phosphoserine 19 of incorporated RLC. The inhibition of dephosphorylation by deletion of the seven N-terminal residues was also found with the catalytic subunit of MLCP. Phosphorylation at serine 1/serine 2 and threonine 9 did not influence the dephosphorylation rate of serine 19 and threonine 18 by MLCP. These results suggest that the N-terminal region of RLC plays an important role in substrate recognition of MLCP.

Dephosphorylation of distinct sites on the 20 kDa myosin light chain by smooth muscle myosin phosphatase

The dephosphorylation of the myosin light chain kinase and protein kinase C sites on the 20 kDa myosin light chain by myosin phosphatase was investigated. The myosin phosphatase holoenzyme and catalytic subunit, dephosphorylated Ser-19, Thr-18 and Thr-9, but not Ser-1/Ser-2. The role of noncatalytic subunits in myosin phosphatase was to activate the phosphatase activity. For Ser-19 and Thr-18, this was due to a decrease in Km and an increase in k(cat) and for Thr-9 to a decrease in Km. Thus, the distinction between the various sites is a property of the catalytic subunit.

The exogenously added small subunit of smooth muscle myosin phosphatase increases the Ca2+ sensitivity of the contractile apparatus in the permeabilized porcine renal artery

The effects of the small noncatalytic subunit of myosin light chain phosphatase (MLCPsr) on the Ca2+-induced contraction of smooth muscle were investigated in the Triton X-100-permeabilized porcine renal artery. The full-length recombinant chicken MLCPsr obtained by the bacterial expression system induced an additional contraction at a constant [Ca2+]i and shifted the [Ca2+]i-force relation curve to the left. A deletion mutant containing the N-terminal 78 amino acids of MLCPsr retained the full action, compared with the full-length MLCPsr, while the deletion of this region completely abolished its effect. The process of relaxation was also delayed by the fragment containing the N-terminal 78 amino acids. These results indicated that MLCPsr increases the Ca2+ sensitivity of the contractile apparatus while the N-terminal 78 amino acids are responsible for this effect in vascular smooth muscle.

Inhibitory effect of calyculin A, a Ser/Thr protein phosphatase type I inhibitor, on renin secretion

We have recently shown that several putative selective inhibitors of Ca2+-calmodulin-dependent myosin light chain kinase (MLCK), such as ML-9 [1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine], reversibly stimulate renin secretion [C. S. Park, S.-H. Chang, H. S. Lee, S.-H. Kim, J. W. Chang, and C. D. Hong. Am. J. Physiol. 271 (Cell Physiol. 40): C242-C247, 1996]. We hypothesized that Ca2+ inhibits renin secretion, via phosphorylation of 20-kDa myosin light chain (MLC20), by activating MLCK. In the present studies, we have investigated the types of protein phosphatase (PP) involved in the control of renin secretion through inhibition of MLC dephosphorylation using inhibitors of various types of serine/threonine-specific protein phosphatases. Cyclosporin A, a putative inhibitor of PP type 2 (calcineurin), was without effect. Calyculin A and okadaic acid, putative selective inhibitors of both PP type 1 (PP1) and type 2A (PP2A), significantly inhibited renin secretion under control conditions. Calyculin A had inhibitory effects at least 10-fold more potent than okadaic acid, suggesting that PP1, rather than PP2A, is involved in the control of renin secretion. Furthermore, calyculin A blocked the reversal of renin secretion preinhibited by raised intracellular Ca2+ concentrations in a concentration-dependent manner. Calyculin A (10(-6) M) significantly inhibited renin secretion stimulated by lowering intracellular Ca2+ concentrations and blocked the stimulatory effect of ML-9 on renin secretion. Taking all of these results into consideration, we hypothesize that dephosphorylation of MLC20 by Ca2+-independent PP1 stimulates renin secretion, whereas phosphorylation of MLC20 by Ca2+-calmodulin-dependent MLCK inhibits it. This hypothesized regulatory model of renin secretion predicts that the rate of renin secretion at a given time is determined by the ratio of phosphorylated to dephosphorylated MLC20, which is, in turn, determined by the dynamic balance between activity of MLCK and MLC phosphatase.

The delta isoform of protein phosphatase type 1 is localized in nucleolus and dephosphorylates nucleolar phosphoproteins

The immunolocalization and substrates of protein phosphatases present in nucleolus were investigated using Swiss 3T3 cells and Novikoff hepatoma ascites cells. The protein phosphatase activity was detected in the extract of the isolated nucleoli and its activity was inhibited by okadaic acid with IC50 value of 160 nM. Immunoblotting assay indicated that PP1c delta but not PP1c alpha, PP1c gamma 1, and PP2Ac was localized in the isolated nucleoli. Confocal microscopy showed that PP1c delta was localized in nucleoli, nuclei, and cytosol, though the intensity of fluorescence at the nucleoli was stronger than that of the cytosol or nuclei. PP1c delta was co-localized with the major nucleolar phosphoprotein B23 at nucleoli. The phosphatase was capable of dephosphorylating several proteins in the nucleolus, including B23. The Km of PP1 for the recombinant B23.1, phosphorylated by endogenous kinase(s), was 3.5 microM. These results indicate that PP1c delta is the major serine/threonine phosphatase present in nucleolus and it dephosphorylates nucleolar phosphoproteins, including B23.

Possible involvement of the novel CPI-17 protein in protein kinase C signal transduction of rabbit arterial smooth muscle

1. CPI-17 has recently been identified as a novel protein in vascular smooth muscle. In vitro , its phosphorylation and thiophosphorylation by protein kinase C (PKC) specifically inhibits the type 1 class of protein phosphatases, including myosin light chain (MLC) phosphatase. 2. Both of the phosphorylated CPI-17 states dose-dependently potentiated submaximal contractions at constant [Ca2+] in beta-escin-permeabilized and Triton X-100-demembranated arterial smooth muscle, but produced no effect in intact and less intensely permeabilized (alpha-toxin) tissue. Thiophosphorylated CPI-17 (tp-CPI) induced large contractions even under Ca2+-free conditions and decreased Ca2+ EC50 by more than an order of magnitude. Unphosphorylated CPI-17 produced minimal but significant effects. 3. tp-CPI substantially increased the steady-state MLC phosphorylation to Ca2+ ratios in beta-escin preparations. 4. tp-CPI affected the kinetics of contraction and relaxation and of MLC phosphorylation and dephosphorylation in such a manner that indicates its major physiological effect is to inhibit MLC phosphatase. 5. Results from use of specific inhibitors in concurrence with tp-CPI repudiate the involvement of general G proteins, rho A or PKC itself in the Ca2+ sensitization by tp-CPI. 6. Our results indicate that phosphorylation of CPI-17 by PKC stimulates binding of CPI-17 to and subsequent inhibition of MLC phosphatase. This implies that CPI-17 accounts largely for the heretofore unknown signalling pathway between PKC and inhibited MLC phosphatase.

Regulation of large calcium-activated potassium channels by protein phosphatase 2A

Vasodilating agents induce relaxation of mesangial cells, in part through cGMP-mediated activation of large calcium-activated potassium channels (BKCa). Normally quiescent in cell-attached patches, the response of BKCa to nitric oxide, atrial natriuretic peptide, and dibutyryl cGMP (Bt2cGMP) is characterized by a biphasic increase and then decrease ("rundown") in open probability. Using the patch-clamp method in conjunction with phosphatase inhibitors, we investigated whether the run-down phase was the result of dephosphorylation by an endogenous protein phosphatase. In cell-attached patches, cantharidic acid (500 nM), okadaic acid (100 nM), and calyculin A (100 nM), nondiscriminant inhibitors of protein phosphatases 1 (PP1) and 2A (PP2A) at these concentrations, caused a significantly greater and sustained response of BKCa to Bt2cGMP. Within 2 min, the response of BKCa to the combination of cantharidic acid and Bt2cGMP was greater than the response to these agents added separately. Incubation of mesangial cells with okadaic acid for 20 min at a concentration (5 nM) specific for PP2A increased the basal open probability of BKCa and completely inhibited rundown after activation by Bt2cGMP. Incubation with calyculin A (10 nM), a more potent inhibitor of PP1, did not affect BKCa activity. In inside-out patches, Bt2cGMP plus MgATP caused a sustained activation of BKCa that was inhibited by exogenous PP2A but not PP1. It is concluded that either BKCa or a tightly associated regulator of BKCa is a common substrate for endogenous cGMP-activated protein kinase, which activates BKCa, and PP2A, which inactivates BKCa, in human mesangial cells.

Purification and characterization of a kinase-associated, myofibrillar smooth muscle myosin light chain phosphatase possessing a calmodulin-targeting subunit

A myofibrillar form of smooth muscle myosin light chain phosphatase (MLCPase) was purified from turkey gizzard myofibrils, and it was found to be closely associated with the myosin light chain kinase (MLCKase). For this reason we have named this phosphatase the kinase- and myosin-associated protein phosphatase (KAMPPase). Subunits of the KAMPPase could be identified during the first ion exchange chromatography step. After further purification on calmodulin (CaM) and on thiophosphorylated regulatory myosin light chain affinity columns we obtained either a homogenous preparation of a 37-kDa catalytic (PC) subunit or a mixture of the PC subunit and variable amounts of a 67-kDa targeting (PT) subunit. The PT subunit bound the PC subunit to CaM affinity columns in a Ca2+-independent manner; thus, elution of the subunits required only high salt concentration. Specificity of interaction between these subunits was shown by the following observations: 1) activity of isolated PC subunit, but not of the PTC holoenzyme, was stimulated 10-20-fold after preincubation with 5-50 microM of CoCl2; 2) the pH activity profile of the PC subunit was modified by the PT subunit (the specific activity of the PTC holoenzyme was higher at neutral pH and lower at alkaline pH); and 3) affinity of the holoenzyme for unphosphorylated myosin was 3-fold higher, and for phosphorylated myosin it was 2-fold lower, in comparison with that of the purified PC subunit. KAMPPase was inhibited by okadaic acid (Ki = 250 nM), microcystin-LR (50 nM) and calyculin A (1.5 microM) but not by arachidonic acid or the heat-stable inhibitor (I-2), which suggested that this is a type PP1 or PP2A protein phosphatase.

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.

Interactions of the subunits of smooth muscle myosin phosphatase

Myosin phosphatase from smooth muscle consists of a catalytic subunit (PP1c) and two non-catalytic subunits, M130 and M20. Interactions among PP1c, M20, and various mutants of M130 were investigated. Using the yeast two-hybrid system, PP1c was shown to bind to the NH2-terminal sequence of M130, 1-511. Other interactions were detected, i.e. PP1c to PP1c, M20 to the COOH-terminal fragment of M130, and dimerization of the COOH-terminal fragment of M130. Mutants of M130 were constructed to localize the PP1c and light chain binding regions. Results from the two-hybrid system indicated two binding sites for PP1c on M130: one site in the NH2-terminal 38 residues and a weaker site(s) in the ankyrin repeats region. Inhibition of PP1c activity with phosphorylase a by the M130 mutants also was consistent with the assignment of these two sites. Overlay assays showed binding of phosphorylated light chain to the ankyrin repeats, probably in the COOH-terminal repeats. Activation of PP1c with phosphorylated light chain required binding sites for PP1c and substrate, plus an additional sequence COOH-terminal to the ankyrin repeats. Thus, activation of phosphatase and binding of PP1c and substrate are properties of the NH2-terminal one-third of M130.

Modulation of the calcium sensitivity of the smooth muscle contractile apparatus: molecular mechanisms, pharmacological and pathophysiological implications

Smooth muscle contraction is the basis of the physiological reactivity of several systems (vascular, respiratory, gastrointestinal, urogenital ...). Hyperresponsiveness of smooth muscle may also contribute to a variety of problems such as arterial hypertension, asthma and spontaneous abortion. An increase in cytoplasmic calcium concentration ([Ca2+]i) is the key event in excitation-contraction coupling in smooth muscle and the relationship linking the [Ca2+]i value to the force of contraction represents the calcium sensitivity of the contractile apparatus (CaSCA). Recently, it has become evident that CaSCA can be modified upon the action of agonists or drugs as well as in some pathophysiological situations. Such modifications induce, at a fixed [Ca2+]i value, either an increase (referred to as sensitization) or a decrease (desensitization) of the contraction force. The molecular mechanisms underlying this modulation are not yet fully elucidated. Nevertheless, recent studies have identified sites of regulation of the actomyosin interaction in smooth muscle. Sensitization primarily results from the inhibition of myosin light chain phosphatase (MLCP) by intracellular messengers such as arachidonic acid or protein kinase C. In addition, phosphorylation of thin filament-associated proteins, caldesmon and calponin, increases CaSCA. Activation of small (monomeric) G-proteins such as rho or ras is also involved. Desensitization occurs as a consequence of phosphorylation of myosin light chain kinase (MLCK) by the calcium-calmodulin activated protein kinase II, or stimulation of MLCP by cyclic GMP-activated protein kinase. In the present review, examples of physiological modulation of CaCSA as well as pharmacological and pathophysiological implications are illustrated for some smooth muscles.

Polyamines inhibit myosin phosphatase and increase LC20 phosphorylation and force in smooth muscle

The increase in Ca(2+)-activated force caused by polyamines in beta-escin-permeabilized guinda pig ileum is shown to be associated with increased myosin 20-kDa light chain (LC20) phosphorylation and shortening velocity. Myosin LC20 dephosphorylation with arrested kinase activity was slower in the presence of 1 mM spermine. Smooth muscle phosphatases (SMP-I, -II, -III, and -IV) isolated from turkey gizzard are all active against phosphorylated LC20, but only SMP-III and -IV dephosphorylate heavy meromyosin (HMM). Spermine inhibited SMP-III activity toward LC20 but stimulated HMM dephosphorylation, whereas SMP-IV was inhibited with both substrates. In contrast, SMP-I and -II were stimulated by spermine. The relative effects of different polyamines correlated with an increasing number of positive charges. Spermine did not affect binding of SMP-IV to myosin and did not dissociate any of the subunits of the enzyme. Incubation of permeabilized strips with SMP-IV resulted in attenuated responses to Ca2+, an effect that was opposed by spermine and abolished by microcystin-LR. We conclude that spermine selectively inhibits myosin phosphatase activity and suggest that polyamines function as endogenous myosin phosphatase inhibitors.

A latent form of protein phosphatase 1 alpha associated with bovine heart myofibrils

The catalytic subunit of the major protein phosphatase associated with bovine cardiac myofibrils was purified to homogeneity. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of the enzyme revealed only one band with an apparent molecular weight of 37,000. On gel filtration chromatography, the phosphatase activity and the protein co-eluted as a single peak with an apparent molecular weight of 37,000. The purified enzyme was identified as the catalytic subunit of protein phosphatase 1, as determined by sensitivity to inhibitor 1, inhibitor 2, okadaic acid and by specific immunostaining. Evidence obtained with specific antipeptide antibodies demonstrated that this myofibril protein phosphatase was predominantly the alpha isoform of protein phosphatase 1. The purified catalytic subunit was completely inactive. It was activated by pretreatment with Co2+/trypsin in the presence of high ionic strength. Treatment with trypsin alone did not activate the latent enzyme. The enzyme was also activated by Co2+ or Mn2+ alone but not by Ca2+, Mg2+, Ni2+, Cu2+ or Zn2+. Activation of the enzyme was not reversed by removal of Co2+, but Mn(2+)-activated phosphatase activity was partially reversed when Mn2+ was removed. The catalytic subunit could form a 1:1 complex with inhibitor 2 in vitro. The resulting holoenzyme was also activated by pretreatment with Co2+. Since phosphatase 1 alpha is the major phosphatase associated with cardiac myofibril, it is suggested that it is responsible for the dephosphorylation of myosin and other myofibril phosphoproteins.

Purification and characterization of the myofibrillar form of myosin light-chain phosphatase from turkey gizzard smooth muscle

The myofibrillar form of smooth-muscle myosin light-chain phosphatase (MLCP) was isolated from turkey gizzards. The enzyme was extracted from washed myofibrils and purified by affinity chromatography on a column of thiophosphorylated myosin 20 kDa light-chain (LC20). The purified enzyme was a monomeric protein of 35 kDa and bound to unphosphorylated myosin with a binding constant of 5.45 x 10(4) M-1. It dephosphorylated both isolated phosphorylated light-chain (PLC20) and intact myosin as well as myosin light-chain kinase. The enzyme activity was stimulated by Mn2+, Mg2+, Ca2+, and inhibited by Co2+ ions. Okadaic acid inhibited the phosphatase activity with an IC50 (concentration required for 50% inhibition) value of 250 nM, that is around 25-times higher than required for type 1 protein phosphatase; however, the heat stable inhibitor-2 had no effect. The unique properties of the myofibrillar phosphatase as compared to smooth-muscle phosphatases so far described, suggest that the myofibrillar MLCP is a novel protein of this class. It has been also suggested that in vivo the myofibrillar MLCP exists in a complex with myosin light-chain kinase (MLCK) and a 63 kDa protein.