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Functional and Structural Similarities Between the Inhibitory Region Of Proc. NatI. Acad. Sci. USA Vol. 87, pp. 7285-7289, September 1990 Biochemistry Functional and structural similarities between the inhibitory region of troponin I coded by exon VII and the calmodulin-binding regulatory region of the catalytic subunit of phosphorylase kinase (actin/troponin C/actomyosin ATPase/skeletal muscle) HEMANT K. PAUDEL AND GERALD M. CARLSON* Department of Biochemistry, College of Medicine, University of Tennessee, Memphis, 800 Madison Avenue, Memphis, TN 38163 Communicated by Edmond H. Fischer, July 1, 1990 ABSTRACT A sequence homology has been noted between the 'y subunit is capable of mimicking inhibition of actomy- the carboxyl quarter ofthe catalytic y subunit ofphosphorylase osin ATPase by TnI. kinase and the region oftroponin I coded by exon VII. Because this portion of troponin I contains the inhibitory region that interacts with actin and troponin C, we have emined whether MATERIALS AND METHODS the y subunit of phosphorylase kinase can functionally mimic Enzymes, Proteins, and Tetradecapeptide. Phosphorylase troponin I by also interacting with actin and troponin C. We kinase was isolated from white skeletal muscle of New have found that troponin C not only activates the isolated y Zealand White rabbits through the DEAE-cellulose chroma- subunit of phosphorylase kinase but also binds with approxi- tography step as described (6, 18). The catalytic 'y subunit mately the same affinity as calmodulin. Although actin had no was isolated in 8 M urea/0.1 M H3PO4/1 mM EDTA/1 mM effect on the activity of the y subunit alone, it did inhibit the dithiothreitol, pH 3.3, as described (6). Dilution bufferfor the activity of y-calmodulin and y-troponin C complexes. Con- isolated y subunit was 50 mM Tris HCI/50 mM 83- versely, the y subunit was able to inhibit actomyosin ATPase glycerophosphate/0.1 mM EDTA/0.1 mM dithiothreitol, pH in a process that could be overcome by calmodulin. These 6.8. Phosphorylase b was isolated as described (19) and results suggest that actin and calmodulin (or troponin C) residual AMP was removed by treatment with cocoanut compete for binding to the y subunit. Moreover, the structural charcoal. Bovine brain CaM, rabbit muscle actin and myosin, and functional similarities between the y subunit and troponin and bovine serum albumin were from Sigma. TnC from rabbit I suggest that the y subunit of phosphorylase kinase may have skeletal muscle was generously provided by James D. Potter evolved from the fusion of a protein kinase protogene with a of the University of Miami School of Medicine and was progenitor of exon VII of troponin I. homogenous by SDS/PAGE. The synthetic tetradecapeptide substrate (SDQEKRKQISVRGL), corresponding to the Phosphorylase kinase is a Ca2+-dependent regulatory en- phosphorylation site of phosphorylase b (20), was kindly zyme in the cascade activation of glycogenolysis (1, 2). Its provided by Thomas J. Fitzgerald of St. Jude Children's requirement for Ca2+ ions is thought to couple muscle Research Hospital, Memphis, TN. contraction with energy production (3). The enzyme is a Protein Concentrations. The concentrations of phosphory- hexadecameric oligomer with the subunit stoichiometry lase kinase and phosphorylase b were determined spectro- (a282y282)2 (4, 5). The a and P subunits are inhibitory (6, 7), photometrically by using published absorbance indices (18, the y subunit is catalytic (8, 9), and the 8 subunit is calmodulin 21). Solutions of CaM were prepared based on its dry weight. (CaM) (4). This integral CaM subunit inhibits an intrinsic The concentration of TnC was determined by the Bio-Rad Ca2'-independent activity of the catalytic subunit (10), stim- protein assay using CaM as the standard. When the effects of ulates its overall phosphotransferase activity while making it TnC and CaM were being compared, their concentrations Ca2+-sensitive (11), and may limit the number of promoters were carefully matched by using the Bio-Rad protein assay. that associate (12). The concentration of the y subunit was determined as de- Reimann et al. (9) first suggested that the 386-residue scribed (6). All other protein concentrations were determined catalytic y subunit may interact with CaM through some by the Bio-Rad assay using bovine serum albumin as the domain contained within its carboxyl-terminal 110 amino standard. acids because in this region the homology between the y Activity Assays. The phosphorylation of phosphorylase b subunit and other protein kinases completely disappears. and tetradecapeptide was followed using a filter paper assay Since then this region has been evaluated by sequence (22) and a phosphocellulose strip assay (23), respectively. comparisons with other known CaM-binding proteins, by a The standard assays were performed at 30°C and pH 6.8 and computer algorithm designed to screen for basic amphipathic contained (at final concentrations) 50 mM Tris HCI, 50 mM helices, and by synthetic peptides. These approaches have f3-glycerophosphate, 0.1 mM dithiothreitol, 0.1 mM EDTA, targeted about three-fourths of the 110 residues as being 0.3 mM CaCI2, 10 mM Mg(CH3CO2)2, 1.5 mM [y-32P]ATP potentially involved in the binding of CaM (13-17). Within (ICN), and phosphorylase b (3 mg/ml) or 50 ,M tetradeca- this region ofthe y subunit, a limited sequence homology with peptide. The assays were initiated by addition of MgATP and troponin I (TnI) also has been noted (16, 17). Because this samples were removed at 15-min intervals for determination homologous region in TnI has been implicated in the binding of 32p incorporation into substrate. of both troponin C (TnC) and actin, we have examined The actomyosin ATPase activity was determined essen- whether these latter two proteins influence the properties of tially by the method of Pollard (24). When isolated 'y subunit the isolated y subunit of phosphorylase kinase and whether was included in these assays, it was renatured by diluting 1: 100 with 15 mM Hepes/15 mM KCI/50 ,M dithiothreitol/ The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" Abbreviations: CaM, calmodulin; TnI, troponin I; TnC, troponin C. in accordance with 18 U.S.C. §1734 solely to indicate this fact. *To whom reprint requests should be addressed. 7285 Downloaded by guest on September 27, 2021 7286 Biochemistry: Paudel and Carlson Proc. Natl. Acad. Sci. USA 87 (1990) 50MM EDTA/0.3 mM CaCl2, pH 6.8, and concentrated using an Amicon microconcentrator. Solutions of myosin, actin, CaM, and bovine serum albumin were prepared in the same buffer. Assay components were mixed in the proportions 0 described in the figure legends and incubated for 10 min prior E to assay. The assays were performed at 30'C and pH 6.8 and were initiated by addition of 10 ,ul of MgATP to 100 pl of a 40 assay mixture. Final concentrations in the standard assays were 14 mM Hepes, 14 mM KCI, 45 ,M dithiothreitol, 45 ,M EDTA, 0.27 mM CaCI2, 58 mM urea, 2 mM MgCI2, 1 mM 0) O } [y-32P]ATP, actin (18 Ag/ml), and myosin (30 Ag/ml). Any additional components are listed in the figure legends. At appropriate intervals 20 1.l was removed from the assay mixtures and pipetted into vials containing 20 ,ul of an acid 0.0 1.0 2.0 mixture [1.4 M sulfuric and 4.3% (wt/vol) silicotungstic] and Concentration (,uM) 100 IlI oforganic solvent [isobutanol/benzene, 1: 1 (vol/vol)]. FIG. 2. Stimulation by CaM and TnC of the phosphorylase After vigorously mixing for 2 sec, 20 Al of 10% (wt/vol) conversion activity of the isolated y subunit. Prior to assay the ammonium molybdate was added and the sample was mixed isolated y subunit was diluted 1:10 with dilution buffer and 10 Al of on a vortex mixer for 10 sec. Then 20 ,ul was removed from CaM or TnC was added to 40 ,ul of the diluted y subunit. A 10-Jl the upper organic phase and spotted on filter paper, and sample was removed from this y-CaM or y-TnC mixture and added radioactivity was measured in a liquid scintillation counter to 50 ,l of an assay mixture containing all components except MgATP. After 15 min the reactions were initiated with 10 Al of for the amount of [32p]p, liberated. Control tubes containing MgATP. In addition to the standard components the phosphorylation all components ofthe assay mixture except myosin were used assays contained y subunit (1.1 ,ug/ml), 0.1 M urea, and the indicated for generating background blanks. concentrations of CaM or TnC. The ordinate is product formed per 20 ,ul in 15 min. o, CaM; e, TnC; *, y subunit alone. (Inset) Double reciprocal plot of the data [1/p] refers to the reciprocal concentra- RESULTS tions of CaM or TnC. The apparent values for maximal velocity are A prominent sequence homology between the carboxyl ter- 10.6 and 1.1 mol of phosphate transferred per min per mol of y minus ofthe y subunit ofphosphorylase kinase and TnI, from subunit and for half-maximal activation are 0.31 and 0.36 ,uM for rabbit fast skeletal muscle, is evident between residues 100 CaM and TnC, respectively. The maximal velocity (V) for the reconstituted y-CaM complex corresponds to about 1% of the and 115 ofTnI and residues 298 and 325 ofthe y subunit (Fig. maximal activity calculated for the y subunits within the holoenzyme 1). Maximal alignment ofthe sequences requires a 12-residue assayed at pH 8.2. loop in the y subunit. This homologous region of TnI (10 of 16 residues identical) is contained entirely within its so-called acceptor substrates for phosphoryl transfer (Fig.
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