5908
Biochemistry 1981, 20, 5908-5913
Subunit Interactions of Skeletal Muscle Myosin and Myosin Subfragment 1. Formation and Properties of Thermal Hybrids? Morris Burke* and Mathoor Sivaramakrishnan
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ABSTRACT:
The formation of hybrid myosin and subfragment 1 species by incubation of these proteins with free alkali light chains at physiological ionic and temperature conditions is described. Exchange of bound alkali light chains on myosin by free alkali light chains under these conditions is readily demonstrated from the subunit composition of the isolated myosin. Therefore, the light chain exchange previously described for the one-headed subfragment l [Sivaramakrishnan, M., & Burke, M. (1981) J . B i d . Chem. 256,2607-26101 also occurs in the two-headed myosin molecule. It is found that the isozyme to hybrid transformation is dependent on both the temperature and the ionic strength of the incubation mixture but is relatively independent of pH in the range 6.5-8.0. A
comparison of the SFl(A1) SF1(A2)h system with the S F l (A2) S F l ( A l ) h system indicates that more hybrid is formed in the latter case. With the assumption that hybrid formation reflects the degree of reversible dissociation exhibited by the isozyme, under the particular experimental condition employed, the data signify that the subunit interactions in the two isozymes are not identical and that the heavy chain-A 1 interactions are significantly more stable than the heavy chain-A2 ones. An examination of the ATPase properties of the thermal hybrids in the presence and absence of actin indicates close similarities to their corresponding “native” isozymic counterparts.
I t is now well established that myosin isolated from vertebrate skeletal fast-twitch muscles is an oligomeric protein comprised of two heavy polypeptide chains and two pairs of light chains known as the 5,5’-dithiobis(2-nitrobenzoicacid) (DTNB)’ chains and alkali light chains (Gershman et al., 1969; Gazith et al., 1970; Weeds & Lowey, 1971). In the native molecule, the two heavy chains, for half their mass starting at their C termini, are wrapped around one another in the form of an a-helical coiled-coil cable (Lowey et al., 1969; Burke et al., 1973). The two heavy chains then separate, and their Nterminal halves are each separately folded into globular regions known as the myosin heads or subfragment 1 moieties. The light polypeptide chains are associated with the heavy chains in the globular head regions. The two DTNB light chains are thought to be identical (Weeds & Lowey, 1971; Collins, 1976) and to bind at or near the junction between the a-helical cable and the two heads (Weeds & Pope, 1977; Bagshaw, 1977). The alkali chains are associated with the heads and appear to exist as two chemically similar but distinct forms (Frank & Weeds, 1974) designated as A1 and A2. Stoichiometric analyses of the amounts of A1 and A2 associated with the myosin molecule give values of 1.3 and about 0.7 mol, respectively (Sarkar, 1972; Weeds et al., 1975). These nonintegral values for A1 and A2 are now known to be due to multiple isozymic forms of fast-twitch myosin (Holt & Lowey, 1977; Hoh et al., 1976) existing in different proportions depending on the species and muscles employed for the isolation of the myosin. Although these isozymes of myosin have been separated on the basis of their alkali light chain differences, there is clear evidence that the heavy chains are heterogeneous from both sequence analyses (Starr & Offer, 1973; Gallagher & Elzinga, 1980; Pope et al., 1977) and isoelectric focusing studies (John, 1980). The fact that there are two distinct alkali light chain subunits and apparently two distinct heavy chains in fast-twitch skeletal myosin (John, 1980) raises the question of whether there is a predilection for certain heavy chain-alkali
light chain combinations in vivo. Pope et al. (1977) have found that the heavy chains of subfragment 1 isozymes are heterogeneous, based on amino acid compositions of the N-terminal tripeptide, and this would argue against specific preferential interactions between the alkali light chains and particular heavy chains. The next point that can be raised is what is the physiological function for the myosin isoenzymes. Examination of the ATP functions has shown that there is no significant difference in catalysis between the two isozymes in the absence of actin (Weeds & Taylor, 1975). However, these workers have found that in the presence of actin the V,, and K, values for the actin-activated subfragment 1 MgATPase differ markedly for the two isozymes albeit at very low salt concentrations, significantly below that occurring in the muscle cell. Wagner et al. (1979) and Reisler (1980) have recently shown that this difference is ionic strength dependent and is practically abolished at ionic strengths comparable to physiological levels. Since it appears that the ATPase and actin binding functions require the specific interactions between the alkali light chains and the heavy chains to form the subfragment 1 complex, the nature of these interactions is crucial to our understanding of the function of subfragment 1 and myosin. Recent work from our laboratory (Sivaramakrishnan & Burke, 1981) has established that exchange between subunits of subfragment 1 isozymes readily occurs at ionic strengths and temperature conditions resembling those occurring in vivo. In the present paper, we examine the exchange of free alkali light chains for bound alkali light chains of subfragment 1 isozymes that occurs while the isozyme is active and hydrolyzing MgATP. We have studied the effects of ionic strength, pH, and temperature on this process. The data obtained indicate that the stabilities of the subunit interactions in SFl(A1) and SFl(A2) isozymes are not the same. From the degree of conversion to the hybrid species under identical
From the Department of Biology, Case Western Reserve University, Cleveland, Ohio 44106. Received January 27, 1981. This work was supported by grants from the National Institutes of Health (2-R01NS15319) and the National Science Foundation (PCM-8007876).
0006-296018 110420-5908$01.25/0
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Abbreviations used: DTNB, 5,5’-dithiobis(2-nitrobenzoic acid); GdneHCI, guanidine hydrochloride; SFl (Al) and SF1(A2), subfragment 1 isozymes containing the A1 and A2 alkali light chains, respectively; S F l (A1)h and SFl(A2)h,thermal hybrids of subfragment 1 containing the A1 and A2 chains, respectively; HC, heavy chain subunit of subfragment 1; NaDodSO,, sodium dodecyl sulfate.
0 1981 American Chemical Society
VOL. 20, N O . 20, 1 9 8 1
THERMAL MYOSIN AND SUBFRAGMENT I HYBRIDS
conditions, it appears that the A2 interaction with the heavy chain is intrinsically weaker than that occurring with the AI light chain, with respect to both ionic strength and temperature. These studies raise the possibility that the off and on states of the alkali subunits on the myosin heads may have some physiological role. ATPase measurements of these isolated hybrids show that in the presence or absence of actin they are similar to their native isozymic counterparts. Since it was of interest to see whether a physiologically more relevant molecule, containing two heads and a pair of DTNB light chains, would behave in a similar manner to the singleheaded species, the possibility that light chain exchange can occur in myosin has also been studied. The results obtained by incubating a standard myosin preparation with either free AI or A2 result in reciprocal changes in the alkali light chain composition of the isolated myosin, indicating that exchange of bound by free alkali light chains can occur on the heavy chains of the myosin molecule. Materials and Methods Deionized water prepurified in a Millipore QTM system was used throughout. Myosin was prepared by the procedure of Godfrey & Harrington (1972). Actin was prepared as described by Spudich & Watt (1971). The subfragment l isozymes were prepared by digestion of myosin with a-chymotrypsin and separated on DEAE-cellulose by the methods of Weeds & Taylor (1975). Alkali light chains were prepared by denaturing myosin in 6 M GdnHCI following the procedures described by Holt & Lowey (1975). These alkali light chains were separated on DEAE-cellulose, and only those fractions which were pure by NaDodSO, gel electrophoretic analysis employing Coomassie Brilliant Blue as the protein stain were p l e d and concentrated for further studies. Protein concentrations were measured either by absorption, employing values of 5.5.7.5, and 2.0 for myosin, subfragment I , and alkali light chains, respectively, or by the Lowry procedure (Lowry et al., 1951) and the Bradford method (Bradford, 1976). Hybridization experiments were done by incubating a particular isozyme of subfragment 1 (1 mg/mL) with the required molar excess of the free alkali light chain of the alternate isozyme in a solution of the desired molarity of KCI buffered by 0.05 M imidazole, pH 7.0, and 5 m M dithiothreitol. When the studies were done at higher temperatures, the pHs of the buffer solutions were adjusted such that they were equal to 7.0 at the respective temperatures. In addition, the solvent contained M MgATP. After incubation for the required period of time a t the desired temperature, the solutions were cooled on ice. The incubated solution was then divided into two parts, and one part was dialyzed vs. 0.05 M imidazole, pH 7.0 (at 4 "C), for ion-exchange chromatography on DEAE-cellulose to separate the subfragment 1 species (Weeds & Taylor, 1975; Wagner & Weeds, 1977). The second portion was dialyzed against 0.01 M sodium pyrophosphate, 0.1 M glycine, and 0.01 M 8-mercaptoethanol, pH 6.5, for subsequent electrophoretic separation on nondenaturing polyacrylamide gels. These gels were comprised of 5% acrylamide and 0.135% bis(acry1amide) polymerized in the same solvent hut without 2-mercaptoethanol present. Electrophoresis was done on an LKB multiphor apparatus with the cooling water temperature set at 6 OC. The gels were stained with Coomassie Brilliant Blue and destained by the procedures of Weber & Osborn (1969). NaDcdSO, gel electrophoresis was done on 10% acrylamide and 0.27% bis(acrylamide) by employing the proadures of Weber & Osborn (1969) except that 0.05 M imidazole, pH 7.0, and 0.1% NaDodSO, was used as the electrophoresis buffer. The amounts
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