Antibody and peptide probes of interactions between the SH1-SH2 region of myosin subfragment 1 and actin's N-terminus.

The negatively charged residues in the N-terminus of actin and the 697-707 region on myosin subfragment 1 (S-1), containing the reactive cysteines SH1 and SH2, are known to be important for actin-activated myosin ATPase activity. The relationship between these two sites was first examined by monitoring the rates of SH1 and SH2 modification with N-ethylmaleimide in the presence of actin and, secondly, by testing for direct binding of SH1 peptides to the N-terminal segment on actin. While actin alone protected SH1 from N-ethylmaleimide modification, this effect was abolished by an antibody against the seven N-terminal amino acids on actin, F(ab)(1-7), and was greatly reduced when the charge of acidic residues at actin's N-terminus was altered by carbodiimide coupling of ethylenediamine. Neither F(ab)(1-7) nor ethylenediamine treatment reversed the effect of F-actin on SH2 reactivity in SH1-modified S-1. These results show a communication between the SH1 region on S-1 and actin's N-terminus in the acto-S-1 complex. To test whether such a communication involves the binding of the SH1 site on S-1 to the N-terminal segment of actin, the SH1 peptide IRICRKG-NH2(4+) was used. Cosedimentation experiments revealed the binding of three to six peptides per actin monomer. Peptide binding to actin was affected slightly, if at all, by F(ab)(1-7). The antibody also did not change the polymerization of G-actin by the peptides. The peptides caused a small reduction in the binding of S-1 to actin and did not change the binding of F(ab)(1-7).(ABSTRACT TRUNCATED AT 250 WORDS)     

Spatial proximity of the glycine-rich loop and the SH2 thiol in myosin subfragment 1

Subfragment 1 (S1) prepared from rabbit skeletal muscle myosin was digested with trypsin to cleave the 95K heavy chain into three pieces, i.e., the 23K, 50K, and 20K fragments. The trypsin-treated S1 was then cross-linked with p-nitrophenyl iodoacetate. The cross-linker bridged one of the reactive thiols (SH2) in the 20K fragment and a lysine residue in the 23K fragment [Hiratsuka, T. (1987) Biochemistry 26, 3168-3173]. Location of the lysine residue was mapped along the 23K fragment by "end-label fingerprinting", which employed site-directed antibodies against the N-terminus of the 23K fragment and against the C-terminus of the 24K fragment (the 23K fragment plus nine extra residues at its C-terminus). The mapping revealed that Lys-184 or Lys-189 was the residue cross-linked with SH2. Since the cross-linker used here spans only several angstroms, the result indicates that Lys-184 or Lys-189 is very close to SH2 in the three-dimensional structure of myosin head. Examination of the primary structure of the 23K fragment has revealed that these lysine residues are in and very close to the so-called "glycine-rich loop", whose sequence is highly homologous to those of nucleotide-binding sites of various nucleotide-binding proteins.     

Actin and temperature effects on the cross-linking of the SH1-SH2 helix in myosin subfragment 1

Past biochemical work on myosin subfragment 1 (S1) has shown that the bent alpha-helix containing the reactive thiols SH1 (Cys(707)) and SH2 (Cys(697)) changes upon nucleotide and actin binding. In this study, we investigated the conformational dynamics of the SH1-SH2 helix in two actin-bound states of myosin and examined the effect of temperature on this helix, using five cross-linking reagents that are 5-15 A in length. Actin inhibited the cross-linking of SH1 to SH2 on both S1 and S1.MgADP for all of the reagents. Because the rate of SH2 modification was not altered by actin, the inhibition of cross-linking must result from a strong stabilization of the SH1-SH2 helix in the actin-bound states of S1. The dynamics of the helix is also influenced by temperature. At 25 degrees C, the rate constants for cross-linking in S1 alone are low, with values of approximately 0.010 min(-1) for all of the reagents. At 4 degrees C, the rate constants, except for the shortest reagent, range between 0.030 and 0.070 min(-1). The rate constants for SH2 modification in SH1-modified S1 show the opposite trend; they increase with the increases in temperature. The greater cross-linking at the lower temperature indicates destabilization of the SH1-SH2 helix at 4 degrees C. These results are discussed in terms of conformational dynamics of the SH1-SH2 helix.     

Differential scanning calorimetry of the unfolding of myosin subfragment 1, subfragment 2, and heavy meromyosin

The thermal unfolding of rabbit skeletal heavy meromyosin (HMM), myosin subfragment 1, and subfragment 2 has been studied by differential scanning calorimetry (DSC). Two distinct endotherms are observed in the DSC scan of heavy meromyosin. The first endotherm, with a Tm of 41 degrees C at pH 7.9 in 0.1 M KCl, is assigned to the unfolding of the subfragment 2 domain of HMM based on scans of isolated subfragment 2. The unfolding of the subfragment 2 domain is reversible both in the isolated form and in HMM. The unfolding of subfragment 2 in HMM can be fit as a single two-state transition with a delta Hvh and delta Hcal of 161 kcal/mol, indicating that subfragment 2 exists as a single domain in HMM. The unfolding of subfragment 2 is characterized by an extraordinarily large delta Cp of approximately 30,000 cal/(deg.mol). In the presence of nucleotides, the high-temperature HMM endotherm with a Tm of 48 degrees C shifts to higher temperature, indicating that this peak corresponds to the unfolding of the subfragment 1 domain. This assignment has been confirmed by comparison with isolated subfragment 1. The stabilizing effect of AMPPNP was significantly greater than that of ADP. The vanadate-trapped ADP species was slightly more stable than M.AMPPNP with a Tm at 58 degrees C. The unfolding of subfragment 1, both in the isolated form and in HMM, was irreversible. Only a single endotherm was noted in the DSC scans of the subfragment 1 domain of HMM and in freshly prepared subfragment 1 complexes.(ABSTRACT TRUNCATED AT 250 WORDS)     

The reactive SH1 and SH2 cysteines in myosin subfragment 1 are cross-linked at similar rates with reagents of different length

Nuclei prepared from confluent and mitotically arrested populations of human diploid fibroblast-like cells of different $\text{in}\text{vitro}$ ages were subjected to digestion by micrococcal nuclease and DNase I. There was no age or culture state variation in the susceptibility of DNA to micrococcal nuclease digestion. There was, however, an age related inhibition of DNA digestion by DNase I in nuclei from older confluent but not older arrested cells. It is suggested that this is the result of an age related masking by nucleosome core histones which limits the accessibility of DNA to enzymatic activities in older confluent cells.     

Interaction of myosin subfragment 1 with fluorescent ribose-modified nucleotides. A comparison of vanadate trapping and SH1-SH2 cross-linking

The environment near the ribose binding site of skeletal myosin subfragment 1 (S1) was investigated by use of two adenosine 5'-diphosphate analogues with fluorescent groups attached at the 2'- and 3'-hydroxyls of the ribose ring. We have compared steady-state and time-resolved fluorescent properties of the reversibly bound S1-nucleotide complexes and the complexes generated by N,N'-p-phenylenedimaleimide (pPDM) thiol cross-linking or vanadate (Vi) trapping. A new fluorescent probe, 2'(3')-O-[N-[2-[[[5-(dimethylamino)naphthyl]sulfonyl] amino]ethyl]carbamoyl]adenosine 5'-diphosphate (DEDA-ADP), which contains a base-stable carbamoyl linkage between the ribose ring and the fluorescent dansyl group, was synthesized and characterized. For comparison, we performed parallel experiments with 2'(3')-O-(N-methylanthraniloyl)adenosine 5'-diphosphate (MANT-ADP) [Hiratsuka, T. (1983) Biochim. Biophys. Acta 742, 496-508]. Solute quenching studies indicated that both analogues bound reversibly to a single cleft or pocket near the ribose binding site. However, steady-state polarization measurements indicated that the probes were not rigidly bound to the protein. The quantum yields of both fluorophores were higher for the complexes formed after trapping with pPDM or Vi than for the reversibly bound complexes. Both DEDA-ADP and MANT-ADP, respectively, had nearly homogeneous lifetimes free in solution (3.65 and 4.65 ns), reversibly bound to S1 (12.8 and 8.6 ns), and trapped on S1 by pPDM (12.7 and 8.7 ns) or Vi (12.8 and 8.6 ns). In contrast to the quantum yields, the lifetimes were not increased upon trapping, compared to those of the reversibly bound states. These results suggested that static quenching in the reversibly bound complex was relieved upon trapping. Taken together, the results suggest that there was a conformational change near the ribose binding site upon trapping by either pPDM or Vi. On the basis of the quantum yield, lifetime, polarization, and solute accessibility studies, we could not detect differences between the S1-pPDM-nucleotide analog complex and the S1-Vi-nucleotide analogue complex for either analogue. Thus, previously observed differences with the adenine modified nucleotide analogue 1,N6-ethenoadenosine diphosphate (epsilon ADP) could not be detected with these ribose-modified probes, indicating that structural differences may be localized to the adenine binding site and not transmitted to the region near the ribose ring.     

Specific cross-linking of the SH1 thiol of skeletal myosin subfragment 1 to F-actin and G-actin.

Recently, we reported that (maleimidobenzoyl)-G-actin (MBS-G-actin), which was resistant to the salt and myosin subfragment 1 (S-1) induced polymerizations, reacts reversibly and covalently in solution with the S-1 heavy chain at or near the strong F-actin binding region [Bettache, N., Bertrand, R., & Kassab, R. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 6028-6032]. Here, we have readily converted the MBS-G-actin into MBS-F-actin in the presence of phalloidin and salts. The binding of S-1 to the two actin derivatives carrying on their surface free reactive maleimidobenzoyl groups was investigated comparatively in cross-linking experiments performed under various conditions to probe further the molecular structure of the actin-heavy chain complex before and after the polymerization process. Like MBS-G-actin, the isolated MBS-F-actin, which did not undergo any intersubunit cross-linking, bound stoichiometrically to S-1, generating two kinds of actin-heavy chain covalent complexes migrating on electrophoretic gels at 180 and 140 kDa. The relative extent of their production was essentially dependent on pH for both G-and F-actins. At pH 8.0, the 180-kDa species was predominant, and at pH 7.0, the amount of the 140-kDa adduct increased at the expense of the 180-kDa entity. The cross-linking of MBS-F-actin to S-1 led to the superactivation of the MgATPase substantiating the ability of this derivative to stimulate the S-1 ATPase as the native protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

UV-induced vanadate-dependent modification and cleavage of skeletal myosin subfragment 1 heavy chain. 2. Oxidation of serine in the 23-kDa NH2-terminal tryptic peptide

Myosin subfragment 1 (S1) can be specifically photomodified at the active site without polypeptide chain cleavage by irradiating the stable MgADP-orthovanadate-S1 complex with UV light above 300 nm [Grammer, J. C., Cremo, C. R., & Yount, R. G. (1988) Biochemistry (preceding paper in this issue)]. Here, the UV spectral properties of photomodified S1 were used to determine the nature and location of the photomodified residue(s) within S1. By comparison of the unusual pH dependence of the UV absorption spectrum of the photomodified S1 to that of the S1-MgADP-Vi complex as a control, the photomodified residue(s) was (were) localized to the 23-kDa NH2-terminal tryptic peptide of the heavy chain. NaBH4 reduced the photomodified S1, but not the control, to regenerate the original spectral properties and ATPase activities of the unmodified S1. Amino acid analysis of photomodified S1 reduced with NaB3H4 gave only [3H]serine, suggesting the hydroxyl group of serine had been oxidized to a "serine aldehyde". The pH dependence of the absorption spectrum of the photomodified enzyme can be explained by an equilibrium between a chromophoric enolate anion of the serine aldehyde (favored in base) and less chromophoric keto and enol forms (favored in acid). The oxidized serine(s) was (were) shown to be directly involved with the vanadate-dependent photocleavage of the S1 heavy chain previously described by Grammer et al. (1988). This serine(s) is (are) likely to be important to the binding and hydrolysis of the gamma-PO4 of ATP at the active site of S1.     

Studies of ligand-induced conformational perturbations in myosin subfragment 1. An examination of the environment about the SH2 and SH1 thiols using a photoprobe

The effect of ligand binding on the environment near the SH2 and SH1 thiols in myosin subfragment 1 has been investigated by photocross-linking after specific labeling of these thiols individually with 4-(N-maleimido)benzophenone (MBP). On photolysis, cross-linking occurred from SH2-MBP to the middle 50-kDa segment, and subsequent immunopeptide mapping revealed that the cross-link was made to a peptide stretch 31-32 kDa from the N terminus in the absence of MgADP, whereas in its presence the cross-link occurred at about 60-61 kDa from the N terminus. Photolysis of SH1-MBP in the absence of MgADP resulted in a major cross-link to the 27-kDa N-terminal segment and minor cross-links to the 50-kDa middle segment. In the presence of MgADP, no new cross-link occurred but the amount of cross-linking to the 50-kDa segment increased at the expense of the other. Immunopeptide mapping indicated that the regions in the 27- and 50-kDa peptides that were cross-linked to SH1-MBP are at about 14-16 and 55-56 kDa from the N terminus respectively. These results indicate that when nucleotide binds to S1, SH2 is displaced relative to the 50-kDa segment, whereas the local environment around SH1 does not change significantly because photolysis in the presence of MgADP resulted in a change at the site of cross-linking for SH2-MBP but caused only a redistribution of the relative amounts of the cross-links formed from SH1-MBP.     

Excitation energy transfer studies on the proximity between SH1 and the adenosinetriphosphatase site in myosin subfragment 1

Excitation energy transfer studies were carried out to determine the distance between the adenosinetriphosphatase (ATPase) site and a unique "fast-reacting" sulfhydryl (referred to as SH1) in myosin subfragment 1. The fluorescent moiety of the probe N-(iodoacetyl)-N'-(5-sulfo-1-naphthyl)ethylene-diamine was used as the donor attached at SH1. The chromophoric nucleotide analogue 2'(3')-0-(2,4,6-trinitrophenyl)adenosine 5'-diphosphate was used as the acceptor noncovalently bound at the ATPase site. The energy transfer efficiency was found to be 56% by measuring the decrease in donor fluorescence lifetime. The critical transfer distance, R0(2/3), was determined to be 40.3 A. Since both donor and acceptor are likely to be rigidly attached, a statistical interpretation of the data was applied (Hillel, Z., & Wu, C.-W. (1976) Biochemistry 15, 2105] to determine distances. The method yielded the following conclusions: most probable distance = 38.7 A; maximum possible distance = 52 A; 10% probability for the distance to be less than 20 A; 3% probability to be less than 15 A. It may be concluded that despite the great influence that the two sites exert on each other, it is not likely that SH1 interacts directly with the ATPase site in myosin subfragment 1. This conclusion is in agreement with the findings of Wiedner et al. [Wiedner, H., Wetzel, R., & Eckstein, F. (1978) J. Biol. Chem. 253, 2763] and Botts et al. [Botts, J., Ue., K., Hozumi, T., & Samet, J. (1979) Biochemistry 18, 5157].     

Actin-binding peptide obtained by the cyanogen bromide cleavage of the 20-kDa fragment of myosin subfragment-1

The 20-kDa fragment of myosin subfragment-1 heavy chain was cleaved with cyanogen bromide. Gel electrophoresis of the fragmented peptides indicated the presence of 20-, 18-, 16-, 14-, 12-, and 10-kDa peptides in addition to two peptides smaller than 10 kDa. The renaturation procedure of Muhlrad and Morales (Muhlrad, A., and Morales, M. (1984) Proc. Natl. Acad. Sci. U. S. A. 81, 1003-1007) was applied to the mixture of these peptides. The peptides larger than 10 kDa, which contain both the reactive SH1 and SH2 groups, were precipitated with F-actin by ultracentrifugation. The 10-kDa peptide was purified and was identified as p10 of Elzinga and Collins (Elzinga, M., and Collins, J. H. (1977) Proc. Natl. Acad. Sci. U. S. A. 74, 4281-4284). The renaturation procedure was applied to the purified 10-kDa peptide. The 10-kDa peptide was also precipitated with F-actin by ultracentrifugation. Affinity of the 10-kDa peptide for F-actin was determined with an increase of turbidity, and the apparent dissociation constant was 0.94 microM. Results are consistent with our proposition that a binding site for F-actin exists around the SH1 and SH2 groups of subfragment-1 (Katoh, T., Imae, S., and Morita, F. (1984) J. Biochem. 95, 447-454; Katoh, T., and Morita, F. (1984) J. Biochem. 96, 1223-1230).     

A second consensus sequence of ATP-requiring proteins resides in the 21-kDa C-terminal segment of myosin subfragment 1

Previous comparisons of sequence homologies of ATP-requiring enzymes have defined three consensus sequences which appear to be involved in the binding of the nucleotide. One of these was identified in the N-terminal 27-kDa segment of the myosin heavy chain but the other two sequences have not hitherto been located in myosin. The present paper proposes that one of these other two consensus sequences is in the 21-kDa C-terminal portion of S1 and that it may contribute to the ATP binding domain.     

Thiol-specific cross-linkers of variable length reveal a similar separation of SH1 and SH2 in myosin subfragment 1 in the presence and absence of MgADP.

A series of thiol-specific cross-linking reagents were prepared for studying the kinetics of cross-linking between SH1 (Cys(707)) and SH2 (Cys(697)) in rabbit skeletal muscle myosin subfragment 1. The reagents were of the type RSS(CH(2))(n)()SSR, with R = 3-carboxy-4-nitrophenyl and n = 3, 6, 7, 8, 9, 10, and 12, spanning distances from 9 to 20 A. The reactions were monitored spectrophotometrically by measuring the release of 2-nitro-5-thiobenzoate. Reaction rates for modification of SH1 (k(1)) and for cross-linking (k(2)) were measured by the decrease of the K(+)(EDTA)-ATPase activity and the decrease of the Ca(2+)-ATPase activity, respectively, and corrected for the different reactivities of C(n). Cross-linking rates in the presence and absence of MgADP showed similar dependence on the length of the reagents: While the cross-linking rates for n = 3 or n = 6 were close to those for n = 0 (Ellman's reagent), those for n = 7 and 8 were significantly increased. Thus the distance between SH1 and SH2 appears to be equal in both states and can be estimated as >/=15 A, based on the length of the reagent with n = 8 in stretched conformation. Under rigor conditions, reactivity of SH1 differed significantly from that in the presence of MgADP, presumably because of shielding through a lipophilic domain. Similarly, the cross-linking rates k(2) for C(3), C(6), and C(7) in the absence of MgADP were ca. 15 times lower than in the presence of MgADP, suggesting a change in the structure of the SH2 region that depends on nucleotide binding. The results are discussed in terms of recent X-ray structures of S1 and S1-MgADP [Rayment et al. (1993) Science 261, 50-58; Gulick et al. (1997) Biochemistry 36, 11619-11628].     

2,4-Dinitrophenol reduces the reactivity of Lys553 in the lower 50-kDa region of myosin subfragment 1

2,4-Dinitrophenol (DNP) increases the affinity of myosin for actin and accelerates its Mg²⁺ATPase activity, suggesting that it acts on a region of the myosin head that transmits conformational changes to actin- and ATP-binding sites. The binding site/s for DNP are unknown; however similar hydrophobic compounds bind to the 50-kDa subfragment of the myosin head, near the actin-binding interface. In this region, a helix-loop-helix motif contains Lys553, which is specifically labeled with the fluorescent probe 6-[fluorescein-5(and 6)-carboxamido] hexanoic acid succinimidyl ester (FHS). This reaction is sensitive to conformational changes in the helix-loop-helix and the labeling efficiency was reduced when S1 was bound to actin, DNP or nucleotide analogs. The nucleotide analogs had a range of effects (PPi > ADP·AlF₄⁻ > ADP) irrespective of the open-closed state of switch 2. The greatest reduction in labeling was in the presence of actin or DNP. When we measured the effect of each ligand on the fluorescence of FHS previously attached to S1, only DNP quenched the emission. Together, the results suggest that the helix-loop-helix region is flexible, it is part of the communication pathway between the ATP- and actin-binding sites of myosin and it is proximal to the region of myosin where DNP binds.     

Purification and characterization of actin-activatable, Ca2+-sensitive myosin II from Acanthamoeba

Approximately 8-10 mg of highly actin-activatable, CA2+-sensitive Acanthamoeba myosin II can be isolated in greater than 98% purity from 100 g of amoeba by the new procedure described in detail in this paper. The enzyme isolated by this procedure can be activated by actin because its heavy chains are not fully phosphorylated (Collins, J. H., and Korn, E. D. (1980) J. Biol Chem. 255, 8011-8014). We now show that Acanthamoeba myosin II Mg2+-ATPase activity is more highly activated by Acanthamoeba actin than by muscle actin. Also, actomyosin II ATPase is inactive at concentrations of free Mg2+ lower than about 3 mM and fully active at Mg2+ concentrations greater than 4 mM. Actomyosin II Mg2+-ATPase activity is stimulated by micromolar Ca2+ when assayed over the narrow range of about 3-4 mM Mg2+ but is not affected by Ca2+ at either lower or higher concentrations of Mg2+. The specific activity of te actomyosin II Mg2+-ATPase also increases with increasing concentrations of myosin II when the free Mg2+ concentration is in the range of 3-4 mM but is independent of the myosin II concentration at lower or higher concentrations of Mg2+ . This marked effect of the Mg2+ concentration on the Ca2+-sensitivity and myosin concentration-dependence of th specific activity of actomyosin II ATPase activity does not seem to be related to the formation of myosin filaments, and to be related to the formation of myosin filaments, and myosin II is insoluble only at high concentrations of free Mg2+ (6-7 mM) were neither of these effects is observed. Also, the Mg2+ requirements for actomyosin II ATPase activity and myosin II insolubility can be differentially modified by EDTA and sucrose.     

The MgADP-induced decrease of the SH1-SH2 fluorescence resonance energy transfer distance of myosin subfragment 1 occurs in two kinetic steps

The fluorescence resonance energy transfer distance between 5-[2-[iodoacetyl)amino)ethyl]aminoaphthalene-1-sulfonic acid covalently attached to the SH1 thiol of myosin subfragment 1 as the energy donor and N-(4-dimethylamino-3,5-dinitrophenyl)maleimide attached to SH2 as the energy acceptor has been found to decrease by about 7 A in the presence of MgADP (Dalby, R. E., Weiel, J., and Yount, R. G. (1983) Biochemistry 22, 4696-4706; Cheung, H. C., Gonsoulin, F., and Garland, F. (1985) Biochim. Biophys. Acta 832, 52-62). Fluorescence stopped-flow experiments on the same system have yielded biphasic traces which are resolvable into a fast and slow component, k1 and k2, respectively. Results of experiments in which k1 and k2 were measured as a function of excess ADP concentration showed: 1) a nonlinear dependence of the apparent rate constants on [ADP]; 2) k1 is a factor of 10 faster than k2. These results are consistent with the 3-step mechanism previously proposed for nucleotide binding to myosin S1 (Garland, F., and Cheung, H. C. (1979) Biochemistry 18, 5281-5289). Kinetic experiments in which the anisotropy of the donor was monitored show this quantity to be unchanged over the course of the reaction. The biphasic decrease of donor intensity is assigned to an increase in energy transfer efficiency which, from the above results, is due to a decrease in donor-acceptor distance, occurring in two steps. The fast step is associated with a 4-5-A decrease of the donor-acceptor separation, while the slow step is associated with a further decrease of approximately 2 A.     

Binding between maleimidobenzoyl-G-actin and myosin subfragment 1.

It is well known that the structural interactions between S-1 moieties of myosin molecules ("cross bridges") and actin molecules in polymerized ("F") form are thought to underlie muscle contraction. It is surmised that such interactions are unitary (actin:S-1 = 1:1), but actual demonstration thereof is handicapped by intrinsic properties of the proteins. Recently, it has been reported that chemically modified [with m-maleimidobenzoyl-N-hydroxysuccinimide ester (MBS)] actin maintains its monomeric ("G") form and makes a stable unitary complex with S-1 but does not activate the S-1 ATPase [Bettache, N., Bertrand, R., & Kassab, R. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 6028-6032]. However, we recently showed that when MBS-G-actin and S-1 are covalently cross-linked by 1-ethyl-3-[3-(dimethylamino)propyl]-carbodiimide (EDC), ATPase activity is restored [Hozumi, T. (1991) Biochem. Int. 23, 835-843]. Here we investigated the interface between MBS-G-actin and S-1 using the techniques of tryptic digestion and EDC-cross-linking. MBS-G-actin specifically protected the N-terminal region of S-1 heavy chain against tryptic cleavage at the 25 kDa/50 kDa junction, which is different from the effect that a protomer within F-actin has on the protection of the 25 kDa/50 kDa junction. In addition, the cross-linking pattern between MBS-G-actin and S-1 was different from that between F-actin and S-1. When MBS-G actin was cross-linked to trypsin-treated S-1, no cross-linked product was observed.(ABSTRACT TRUNCATED AT 250 WORDS)     

The interaction of skeletal myosin subfragment 1 with the polyanion, heparin

The association between chymotryptic skeletal muscle myosin subfragment 1 (S1) and the polyanion, heparin, was investigated as an experimental approach in probing the functional importance of the cationic sites on S1 and their involvement in ionic interactions within the myosin head during energy transduction. The direct binding of heparin, used at micromolar concentrations, and its influence on the structural and functional properties of S1 were followed by gel chromatography, electron microscopy, chemical cross-linking techniques and limited digestion studies. 1. The limited tryptic digestion of S1 showed that the presence of heparin, as well as of the homopolymer, poly-(L-glutamic acid) causes a specific structural change in the 50-kDa heavy chain region of S1 and accelerates the breakdown of this segment into a 45-kDa species by a proteolytic cleavage restricted to its COOH-terminal portion. Under similar experimental conditions, the binding of MgATP and MgADP to S1 led also to the 50-kDa----45-kDa conversion, suggesting that the S1-nucleotide interactions exhibit some resemblances to the polyanion-S1 binding of polyanionic ligands to S1. This particular area is adjacent to the actin site containing the 45-kDa and 20-kDa segments of the S1 heavy chain. On the other hand, the polyanions as well as nucleotides induced changes in the interface between the heavy chain and the alkali light chains. 2. Moreover, the binding of heparin to S1 resulted in the self-association of the enzyme and the production of stable small S1 oligomers, most likely dimers, which were demonstrated by the alteration of the size of the S1 particles examined by electron microscopy and their freezing by chemical cross-linking agents. These findings are relevant to the recently reported property of skeletal chymotryptic S1 to form dimers under convenient ionic conditions, in particular in the presence of Mg-nucleotides. The interaction of cationic sites on S1 and possibly on the 50-kDa region of the heavy chain with polyanions promotes the dimerization of the S1 molecules. The binding of S1 to F-actin abolished S1 aggregation.     

Proteolysis and binding of myosin subfragment 1 to actin

Rates of proteolytic cleavage of myosin subfragment 1 were measured in the absence and presence of different amounts of actin. The rates of tryptic digestion at the 50K/20K junction and papain digestion at the 25K/50K junction of the myosin head were progressively inhibited with increasing substoichiometric molar ratios of actin to myosin subfragment 1. The percentage inhibitions of digestion reactions corresponded precisely to the molar compositions of actin-subfragment 1 solutions and demonstrated that equimolar complexes of these proteins were responsible for the observed changes in the proteolysis of myosin heads.