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Calcium Ion-Regulated Thin Filaments from Vascular Smooth Muscle

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Calcium Ion-Regulated Thin Filaments from Vascular Smooth Muscle Biochem. J. (1980) 185, 355-365 355 Printed in Great Britain Calcium Ion-Regulated Thin Filaments from Vascular Smooth Muscle Steven B. MARSTON, Rachel M. TREVETT and Michael WALTERS Imperial Chemical Industries Limited, Pharmaceuticals Division, Mereside, Alderley Park, Macclesfield, Cheshire SK10 4TG, U.K. (Received 20 June 1979) Myosin and actin competition tests indicated the presence of both thin-filament and myosin-linked Ca2+-regulatory systems in pig aorta and turkey gizzard smooth-muscle actomyosin. A thin-filament preparation was obtained from pig aortas. The thin filaments had no significant ATPase activity [1.1 + 2.6 nmol/mg per min (mean + S.D.)], but they activated skeletal-muscle myosin ATPase up to 25-fold [500nmol/mg of myosin per min (mean + S.D.)] in the presence of 1O-4M free Ca2+. At 10-8M-Ca2+ the thin filaments activated myosin ATPase activity only one-third as much. Thin-filament activation of myosin ATPase activity increased markedly in the range 10-6-10-5M- Ca2+ and was half maximal at 2.7 x 10-6M (pCa2+ 5.6). The skeletal myosin-aorta-thin- filament mixture gave a biphasic ATPase-rate-versus-ATP-concentration curve at 10-8M-Ca2+ similar to the curve obtained with skeletal-muscle thin filaments. Thin fila- ments bound up to 9.5,umol of Ca2+/g in the presence of MgATP2-. In the range 0.06- 27,uM-Ca2+ binding was hyperbolic with an estimated binding constant of (0.56 + 0.07) x 106 M-1 (mean + S.D.) and maximum binding of 8.0 + 0.8,umol/g (mean + S.D.). Significantly less Ca2+ bound in the absence of ATP. The thin filaments contained actin, tropomyosin and several other unidentified proteins. 6M-Urea/poly- acrylamide-gel electrophoresis at pH8.3 showed proteins that behaved like troponin I and troponin C. This was confirmed by forming interspecific complexes between radio- active skeletal-muscle troponin I and troponin C and the aorta thin-filament proteins. The thin filaments contained at least 1.4,umol of a troponin C-like protein/g and at least 1.1,umol of a troponin I-like protein/g. Smooth muscle, like any other muscle, contracts Bremel, 1975) and many older results were reinter- by means of the interaction of the contractile preted in terms of regulatory mechanisms involving proteins myosin and actin. Tension and work are myosin. According to current models Ca2+ regulates produced during the interaction at the expense of smooth-muscle myosin in two ways: by direct inter- ATP hydrolysis at the myosin active site. The inter- action with a regulatory light chain on the myosin action is controlled by free Ca2+ concentrations in molecule and by regulating a myosin light-chain the range 10-6-10-5 M. (Ebashi & Endo, 1968; kinase, which activates myosin by phosphorylating Sparrow et al., 1970; Weber & Murray, 1973; its regulatory light chain. The relative importance of Sobieszek & Bremel, 1975). the two mechanisms is uncertain (Aksoy et al., Ca2+ may act either by modulating the myosin or 1976; Sobieszek & Small, 1977; Small & Sobieszek, the actin-containing thin filaments (Lehman et al., 1977a,b; Chacko et al., 1977; Adelstein et al., 1977; 1972; Lehman & Szent-Gyorgyi, 1975). Previous Sherry et al., 1978; Mrwa et al., 1979). Only a few work on smooth-muscle regulation produced in- workers dissent from the view that myosin regu- direct evidence for a regulatory mechanism linked to lation is paramount (Ebashi et al., 1977). the thin filaments and possibly analogous to the The evidence given against the existence of a thin- troponin-tropomyosin system of vertebrate striated filament-linked regulatory system is 3-fold. First, the muscle (Ebashi et al., 1966; Carsten, 1971; Sparrow myosin competition test for thin-filament-linked & Van Bockxmeer, 1972; Ito & Hotta, 1976). More regulation gave a negative result (Bremel, 1974; recently it was discovered that smooth muscle was Bremel et al., 1977). Secondly, smooth-muscle thin- myosin regulated (Bremel, 1974; Sobieszek & filament preparations were not Ca2+ sensitive (Sobieszek & Small, 1976). Thirdly, no troponin-like Abbreviation used: SDS, sodium dodecyl sulphate. proteins could be identified by SDS/polyacryl- Vol. 185 0306-3275/80/020355-11 $1.50/1 356 S. B. MARSTON, R. M. TREVETT AND M. WALTERS amide-gel electrophoresis of gizzard or vascular from rabbit skeletal muscle (Perry, 1955) and from actomyosin (Sobieszek & Bremel, 1975; Driska & pig aorta (Sobieszek & Bremel, 1975). Pig aorta Hartshorne, 1975; Sparrow & Van Bockxmeer, tropomyosin was made as described by Bailey 1972). (1948). Rabbit skeletal-muscle troponin I and In our hands, however, the myosin-competition troponin C were gifts from Dr. D. A. Mercola, test gave a positive result, indicating the presence of A.R.C. Unit, Department of Zoology, University of thin-filament-linked regulation in both pig aorta and Oxford, Oxford, U.K. They were radioactively turkey gizzard actomyosin. We therefore decided to labelled by incubating with a 1.5-fold molar excess re-investigate the question of Ca2+-regulated thin of iodo['4C]acetamide in 0.6M-KCl/10mM-Tris, filaments in smooth muscle. We believe the work pH8.0, at 40C for 12h. Free radioactive label was presented here refutes the evidence previously given then removed by dialysis (Marston & Weber, 1975). against thin-filament regulation. This paper describes how we prepared thin filaments from pig Reconstituted actomyosin aorta smooth muscle which activated myosin ATPase measurements were made by using ATPase activity, which were regulated by Ca2+ in actomyosin reconstituted from rabbit skeletal- the range 106--10-5M and which contained muscle myosin and pig aorta thin filaments. Myosin troponin-I- and troponin-C-like proteins. and thin filaments were mixed at the appropriate concentrations, usually 0.5 mg of myosin/ml + 1 mg of thin filaments/ml in 0.6 M-KCI/10 mM-imidazole Materials and Methods (pH 7.0)/5 mM-MgCl2/10 mM-sodium azide/0.2mM- Preparation ofpig aorta thinfilaments dithiothreitol. The mixture was left for 6 h so that the two proteins could hybridize and then dialysed over- Pig aorta (250g) was minced and then homo- night against ATPase buffer [60mM-KCl/10mM- genized in 1.5 vol. of extraction buffer [80mM-KCl/ imidazole (pH 7.0 at 250C)/5 mM-MgCl2/l0mM- 4mM-MgCl2/4 mM-EGTA/20mM-4-morpholinepro- sodium azide/0.5 mM-dithiothreitoll. Pure myosin panesulphonic acid (pH 7.0)/0.5 mM-dithiothreitol/ and pure thin-filament controls were treated in the 10mM-ATP] (Sparrow et al., 1970) at 40C. After same way. For the myosin competition experiments 10min the residue was sedimented (5min at skeletal-muscle myosin and smooth-muscle acto- 14000g) and then homogenized in a further 1.5 vol. myosin were hybridized by this procedure. of extraction buffer. After 10min the residue was again sedimented. The washed residue was homo- Assay ofA TPase activity genized in another 1.5vol. of extraction buffer, left The ATPase activity of myosin, thin filaments for 2h to solubilize an actin-rich actomyosin and and reconstituted actomyosin was measured at the residue was removed by a further centrifugation. 25 0C in ATPase buffer. The reaction was started by ATP (from a 100mM pH 7.0 stock solution) was adding MgATP2- to 2 mm and terminated after 0, 1, added to the supernatant to bring the concentration 2 or 3min with an equal volume of 5% trichloro- to 15 mM and the pH was adjusted back to 7.0. The acetic acid. Pi released was measured by the supernatant was clarified by centrifugation for Taussky & Schorr (1953) method; it was linear with 30min at 14000g and filtering through a fine nylon time for at least 5 min and so the rate was calcu- mesh. Solid KCI was added to a concentration of lated by a least-squares fit of the Pi-versus-time data. 0.5M to optimize the dissociation of actin from Some measurements of ATPase activity were myosin and the solubility of myosin. The thin made over a range of MgATP2- concentrations filaments were then isolated by high-speed centrifu- from 10-6 to 10-3 M. In these experiments 2.5 mm- gation for 3h at 230000g. The pellets of thin fila- phosphocreatine and 1 mg of creatine kinase/ml ments were soaked in a small volume of ATPase were used to regenerate ATP from ADP. The buffer (for composition see under 'Reconstituted reaction was terminated with 5 mM-p-hydroxy- actomyosin') overnight and then resuspended by mercuribenzoate. Protein was by using a loose-fitting Teflon/glass homogenizer. precipitated Finally, aggregated material was removed by low- ZnSO4 and subsequently removed by Ba(OH)2 speed centrifugation (1000g). Average yield was (Somogyi, 1945). The creatine released was assayed 0.41+0.08mg of protein/g of tissue (mean+S.D.; by the Eggleton et al. (1943) method. 11 preparations). Maintenance ofCa2+ concentration Ca2+ concentrations were maintained in the range The otherproteins 10-8 to 10-4M by use of CaEGTA buffers (Portzehl Pig aorta actomyosin was prepared by the et al., 1964). In general the CaCl2 concentration was method of Sparrow et al. (1970); turkey gizzard constant at 102pM, this value being standardized by actomyosin was prepared by the method of atomic-absorption spectroscopy. Ca2+ concen- Sobieszek & Bremel (1975). Myosin was prepared tration was then varied by adding 0-2 mM-EGTA. 1980 SMOOTH-MUSCLE THIN FILAMENTS 357 The Ca2+concentration was calculated by using the 1972). Some urea gels were run with radioactively ion-binding computer programme of Perrin & Sayce labelled troponin I and troponin C. To assay the (1967) with the stability constants listed by White & position and amount of radioactivity the stained gels Thorson (1971).
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