Fluorescence studies on heavy meromyosin-substrate interaction

Inhibition of myosin ATPase by beryllium fluoride

Inhibition of the myosin subfragment 1 (S-1) ATPase activity by beryllium fluoride was studied directly in the presence of MgATP and following preincubation of samples with MgADP. In both cases, the rates of inhibition were very slow, with kapp = 0.5 and 58 M-1 s-1, respectively, in analogy to the rates of inhibition of myosin ATPase by vanadate [Goodno, C. C. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 2620-2624]. The very different rates of inhibition in the presence of MgATP and on preincubation with MgADP suggested that beryllium fluoride binds to the M.ADP state of myosin. The slow inhibition rates and the nonlinear dependence of the observed rates on beryllium fluoride concentration were consistent with a two-step inhibition process involving a rapid binding equilibrium to yield a collisional complex, M.ADP.BeF3-, and its slow isomerization into M++.ADP.BeF3-. A third, much slower, step was required to account for the conversion of the stable M++.ADP.BeF3- to a virtually irreversibly inhibited complex. Kinetic description of the inhibition pathway was derived from the observed rates of inhibition of myosin ATPase, information on the binding of beryllium fluoride to M.ADP, and measurements of epsilon ADP chase from M++.epsilon ADP.BeF3-. The isomerization rate and equilibrium constants were 1.4 x 10(-2) s-1 and 50, respectively, and the overall binding constant of beryllium fluoride to M.ADP was 5 x 10(5) M-1. The inhibitory complex showed a 16% enhancement to tryptophan fluorescence of S-1 and a reduced quenching of epsilon ADP by acrylamide. It is concluded that M++.ADP.BeF3- is analogous to the M++.ADP.Vi and M**.ADP.Pi states of myosin.

o-Iodosobenzoic oxidation and cleavage of myosin subfragment 1

1. o-Iodosobenzoic acid (IOB) caused the formation of a disulfide bridge between SH1 and SH2 groups of myosin SF1 rendering inactive its ATPase activity. 2. IOB at high concentrations provoked fragmentation of SF1 at its tryptophan residues. 3. The main fragmentation point was located at 15 K from the amino terminus of the myosin heavy chain. 4. Actin was not fragmented by IOB. It protected SF1 tryptophans from IOB attack. 5. These results suggest a possible use of IOB as a reagent to study protein tryptophan under nondenaturing conditions.

Temperature-induced changes in the flexibility of the loop between SH1 (Cys-707) and SH3 (Cys-522) in myosin subfragment 1 detected by cross-linking

The ability of dibromobimane to cross-link SH1 (Cys-707) in the 21-kDa C-terminal segment to SH3 (Cys-522) in the 50-kDa middle segment of the myosin S1 heavy chain has been examined as a function of nucleotide binding and temperature. The results obtained indicate that, while the reagent rapidly reacts with SH1 at both 25 and 4 degrees C, its ability to cross-link to SH3 is highly dependent on temperature. At 25 degrees C, substantial cross-linking from monofunctionally labeled SH1 to SH3 occurs, in agreement with recent work of Mornet, Ue, and Morales (1985, Proc. Natl. Acad. Sci, USA 82, 1658-1662) and of Ue (1987, Biochemistry 26, 1889-1894) and with their conclusion that a loop, allowing SH1 and SH3 to reside at the cross-linking span of dibromobimane, preexists in the protein. At 4 degrees C, however, negligible amounts of cross-linking are observed whether or not a nucleotide is present, despite indications that SH1 is labeled rapidly by the reagent at this temperature. The inability to form this cross-link is not due to an alternate cross-link between monofunctionally labeled SH1 and another thiol in the 21-kDa segment. These results indicate that this loop exists at 25 degrees C and does not exist (or exists only transiently) at the lower temperature.

A spectroscopic and conformational study of pertussis toxin

The conformation of native pertussis toxin has been investigated by secondary structure prediction and by circular dichroism, fluorescence and second-derivative ultraviolet absorption spectroscopy. The far-ultraviolet circular dichroic spectrum is characteristic of a protein of high beta-sheet and low alpha-helix content. This is also shown by an analysis of the circular dichroic spectrum with the Contin programme which indicates that the toxin possesses 53% beta-sheet, 10% alpha-helix and 37% beta-turn/loop secondary structure. Second-derivative ultraviolet absorption spectroscopy suggests that 34 tyrosine residues are solvent-exposed and quenching of tryptophan fluorescence emission has shown that 4 tryptophan residues are accessible to iodide ions. One of these tryptophans appears to be in close proximity to a positively charged side-chain, since only 3 tryptophans are accessible to caesium ion fluorescence quenching. When excited at 280 nm, the emission spectrum contains a significant contribution from tyrosine fluorescence, which may be a consequence of the high proportion (55%) of surface-exposed tyrosines. No changes in the circular dichroic spectra of the toxin were found in the presence of the substrate NAD. However, NAD did quench both tyrosine and tryptophan fluorescence emission but did not change the shape of the emission spectrum, or the accessibility of the tryptophans to either the ionic fluorescence quenchers or the neutral quencher acrylamide.

51V NMR study of vanadate binding to myosin and its subfragment 1

The binding of various forms of vanadate to myosin and myosin subfragment 1 (S-1) was studied by 51V NMR at increasing vanadate concentrations between 0.06 and 1.0 mM. The distribution of the various forms of vanadate in the solution depended on the total concentration of vanadate. At low concentrations, the predominant vanadate form was monomeric, while at high concentration, it was tetrameric. The presence of myosin or S-1 in the solution produced a significant broadening of the signal of each form of vanadate, indicating that all of them bind to the protein. Addition of ATP, which does not affect the 51V NMR spectra in the absence of proteins, causes their significant alteration in the presence of myosin or S-1. The changes, which include the broadening of the signal of the monomeric and the narrowing of the signal of the oligomeric vanadate forms, indicate that more monomeric and less oligomeric vanadate binds to the proteins in the presence than in the absence of ATP. Irradiation by near-UV light in the presence of vanadate cleaves S-1 at three specific sites--at 23, 31, and 74 kDa from the N-terminus. The cleavages at 23 and 31 kDa are specifically inhibited by the addition of ATP. The vanadate-associated photocleavage of S-1 also depends on the total concentration of vanadate; it is observed only when the concentration of vanadate is at least 0.2 mM. This was also the lowest concentration at which oligomeric vanadate was detected in the 51V NMR spectra. From the parallel concentration dependence of the photocleavage and the appearance of the tetrameric vanadate, it is concluded that photocleavage occurs only when tetrameric vanadate binds to S-1.

MgADP-induced changes in the structure of myosin S1 near the ATPase-related thiol SH1 probed by cross-linking

The structural consequences of MgADP binding at the vicinity of the ATPase-related thiol SH1 (Cys-707) have been examined by subjecting myosin subfragment 1, premodified at SH2 (Cys-697) with N-ethylmaleimide (NEM), to reaction with the bifunctional reagent p-phenylenedimaleimide (pPDM) in the presence and absence of MgADP. By monitoring the changes in the Ca2(+)-ATPase activity as a function of reaction time, it appears that the reagent rapidly modifies SH1 irrespective of whether MgADP is present or not. In the absence of nucleotide, only extremely low levels of cross-linking to the 50-kDa middle segment of S1 can be detected, while in the presence of MgADP substantial cross-linking to this segment is observed. A similar cross-link is also formed if MgADP is added subsequent to the reaction of the SH2-NEM-pre-modified S1 with pPDM in the absence of nucleotide. Isolation of the labeled tryptic peptide from the cross-linked adduct formed with [14C]pPDM, and subsequent partial sequence analyses, indicates that the cross-link is made from SH1 to Cys-522. Moreover, it appears that this cross-link results in the trapping of MgADP in this S1 species. These data suggest that the binding of MgADP results in a change in the structure of S1 in the vicinity of the SH1 thiol relative to the 50-kDa "domain" which enables Cys-522 to adopt the appropriate configuration to enable it to be cross-linked to SH1 by pPDM.

Tryptophan-130 is the most reactive tryptophan residue in rabbit skeletal myosin subfragment-1

Rabbit skeletal muscle myosin subfragment-1 (S-1) was reacted with dimethyl(2-hydroxy-5-nitrobenzyl)sulfonium bromide (DHNBS) resulting in modification of 0.8 tryptophan residues per S-1. In order to assign the most reactive tryptophan of the 5 S-1 tryptophans, antibodies were raised in rabbits against bovine serum albumin modified with DHNBS. The antibodies reacted with the 27 kDa tryptic fragment of DHNBS-treated S-1, indicating that the reactive tryptophan resides on this domain. The 27 kDa fragment was isolated from DHNBS-treated S-1 and was further cleaved at a single cysteine residue by 2-nitro-5-thiocyanobenzoic acid. This cleavage resulted in two peptides, each of them containing one tryptophan. The antibodies reacted with the smaller peptide consisting of residues 122-204. The only tryptophan residing on this peptide is Trp130, and this is therefore the most reactive tryptophan of S-1.

ATP-induced structural change in myosin subfragment-1 revealed by the location of protease cleavage sites on the primary structure

To understand the nature of the ATP-induced structural change in myosin subfragment-1, rabbit and chicken skeletal subfragments-1s were cleaved by various proteolytic enzymes in the absence, and in the presence, of ATP and the exact locations of the cleavage sites that were affected by ATP were determined from the amino end analysis of fragments by the use of a protein sequencer. It was found that subtilisin cleaved a site between Gln27 and Asn28 of rabbit subfragment-1 and between Gln28 and Asn29 of chicken subfragment-1 only in the presence of ATP. Thermolysin cleaved a site between Pro31 and Phe32 of chicken subfragment-1 in the presence of ATP, but the same site of rabbit subfragment-1 was not cleaved. The location of these sites is quite similar to the ATP-induced chymotryptic cleavage site of chicken gizzard heavy meromyosin, between Trp29 and Ser30 as reported by others. It is suggested, therefore, that the structure and the ATP-induced structural change in the regions are similar in these subfragment-1s. ATP also changes the cleavage rate of the 26K-50K junction by many proteases. Exact cleavage sites were determined and the relationship between their location and the suppression or the enhancement by ATP of the cleavage was studied. It was found that the cleavage sites were restricted to a quite narrow region and only the cleavage by thermolysin that attacked the middle of the region was enhanced by ATP. The distribution of the cleavage sites and the effect of ATP suggest that ATP induces drastic structural change at the middle of the 26K-50K junction region. The region attacked easily by many proteases coincided very well with a hydrophilic region indicated by the hydropathy index. The region probably protrudes outside and is, therefore, easily attacked by many proteases.

Visualization of domains in native and nucleotide-trapped myosin heads by negative staining

Electron microscopy of negatively stained vertebrate skeletal muscle myosin molecules has revealed substructure suggestive of globular domains in the head portions of the molecule. This head substructure has been examined after both low and high electron doe. The results suggest it is probably not an artefact of radiation damage. The most common appearance is of one or two stain-filled clefts which run roughly perpendicular to the long axis of the head, giving rise to the appearance of two or three domains in a line. A large domain is located at the end of the head, while two smaller domains are arranged between this and the head-tail junction. The size of the large distal domain (about 10 nm long and about 7 nm wide at its widest point) is similar in heads showing either two or three domains. Stable analogues of M.ATP and M.ADP.Pi, the predominant complexes present during hydrolysis of ATP by myosin, were prepared by crosslinking the two reactive SH groups (SH1 and SH2) in the myosin head heavy chain with N,N'-p-phenylenedimaleimide (pPDM) in the presence of ADP, and by forming a complex with vanadate ion and ADP. At this resolution (approximately 2 nm) the heads of these modified molecules did not appear markedly different from those of the untreated protein, although there was a small increase in the number of straight as opposed to curved heads after cross-linking with pPDM.

Conformational transitions in the myosin head induced by temperature, nucleotide and actin. Studies on subfragment-1 of myosins from rabbit and frog fast skeletal muscle with a limited proteolysis method

Tryptic digestion patterns reveal a close similarity of the substructure of frog subfragment-1 (S1) to that established for rabbit S1. The 97-kDa heavy chain of chymotryptic S1 of frog myosin is preferentially cleaved into three fragments with apparent molecular masses of 29 kDa, 49 kDa and 20 kDa. These fragments correspond to the 27-kDa, 50-kDa and 20-kDa fragments of rabbit S1, respectively; this is indicated by the sequence of their appearance during digestion, by the suppression by actin of the generation of the 49-kDa and 20-kDa peptides, and by a nucleotide-promoted cleavage of the 29-kDa peptide to a 24-kDa fragment and the 49-kDa peptide to a 44-kDa fragment, analogous to the nucleotide-promoted cleavage of the 27-kDa and 50-kDa fragments of rabbit S1 to the 22-kDa and 45-kDa peptides. The same changes in the digestion patterns as those produced by the presence of nucleotide (ATP or its beta,gamma-imido analog AdoP P[NH]P) at 25 degrees C were observed when the digestion was carried out at 0 degrees C in the absence of nucleotide. The low-temperature-induced changes were particularly well seen in the preparations from frog myosin. The presence of ATP or AdoP P[NH]P at 0 degrees C enhanced, whereas the complex formation with actin prevented, the low-temperature-induced changes. The results are consistent with there being two fundamental conformational states of the myosin head in an equilibrium that is dependent on the temperature, the nucleotide bound at the active site, and the presence or absence of actin.

Both the 25-kDa and 50-kDa domains in myosin subfragment 1 are close to the reactive thiols

The thiol-specific photoactivatable reagent benzophenone-4-iodoacetamide can be incorporated into myosin subfragment 1 (S1), accompanied by an increase of Ca2+-ATPase and the loss of K+-ATPase activities, a characteristic property of S1 when reactive sulfhydryl 1 (SH-1) is modified. After trypsin cleavage, 25-kDa, 50-kDa, and 20-kDa fragments were found upon NaDodSO4/polyacrylamide gel electrophoresis of the unphotolyzed sample, whereas only the 50-kDa fragment and a 45-kDa fragment appeared in the photolyzed sample, indicating that the NH2-terminal 25-kDa fragment was crosslinked to the COOH-terminal 20-kDa fragment via SH-1. When photolysis was carried out in the presence of Mg2+ and ATP or Mg2+ and adenosine 5-[beta, gamma-imido]triphosphate (AdoPP[NH]P), a 70-kDa band, attributable to a crosslinked (50 kDa + 20 kDa) species, was also observed. This suggests that the conformational change induced by nucleotide binding reduces the distance between the 50-kDa region and the label on SH-1. Similar results were obtained when labeling and photolysis were carried out on trypsin-nicked S1, in which the 25-kDa, 50-kDa, and 20-kDa fragments are held together noncovalently. Further, when labeling with benzophenone-4-iodoacetamide was carried out in the presence of Mg-ATP, which increases the reactivity of another thiol, presumably SH-2, both 45-kDa and 70-kDa species were formed upon photolysis in the absence of ATP, suggesting that SH-2 is close to the 50-kDa region. More of the 70-kDa species was formed, at the expense of the 45-kDa species, when photolysis was carried out in the presence of Mg-ATP. Partial heat denaturation preferentially reduced the crosslinking between the reactive thiols and the 50-kDa region.

The mechanism of muscle contraction

Knowledge of the mechanism of contraction has been obtained from studies of the interaction of actin and myosin in solution, from an elucidation of the structure of muscle fibers, and from measurements of the mechanics and energetics of fiber contraction. Many of the states and the transition rates between them have been established for the hydrolysis of ATP by actin and myosin subfragments in solution. A major goal is to now understand how the kinetics of this interaction are altered when it occurs in the organized array of the myofibril. Early work on the structure of muscle suggested that changes in the orientation of myosin cross-bridges were responsible for the generation of force. More recently, fluorescent and paramagnetic probes attached to the cross-bridges have suggested that at least some domains of the cross-bridges do not change orientation during force generation. A number of properties of active cross-bridges have been defined by measurements of steady state contractions of fibers and by the transients which follow step changes in fiber length or tension. Taken together these studies have provided firm evidence that force is generated by a cyclic interaction in which a myosin cross-bridge attaches to actin, exerts force through a "powerstroke" of 12 nm, and is then released by the binding of ATP. The mechanism of this interaction at the molecular level remains unknown.

Fluorescence studies on the nucleotide- and Ca2+-binding domains of molluscan myosin

The effects of nucleotides and Ca2+ on the intrinsic tryptophan fluorescence of molluscan myosin and its proteolytic fragments were studied. By using these proteins from the scallop, Pecten maximus, the existence of two distinct tryptophan-containing domains was established, which respond independently to ATP and Ca2+-specific binding. The latter is located in the 'neck' region of the myosin, which constitutes the regulatory domain. Subfragment 1, lacking the regulatory domain, responded only to ATP binding. On the other hand a tryptic fragment comprising the regulatory domain responded only to Ca2+ binding. Subfragment 1, containing the regulatory domain, responded to both ATP and Ca2+, but its ATPase activity was Ca2+-insensitive. By contrast, the ATPase activity of HMM was Ca2+-sensitive. Increasing the ionic strength had a detrimental effect on Ca2+-sensitivity, and fluorescence studies on solubilized myosin were therefore of limited value. Myosin and its fragments from other molluscan species which were investigated produced similar changes to those of Pectan maximus.

Molecular movements promoted by metal nucleotides in the heavy-chain regions of myosin heads from skeletal muscle

Molecular movements generated in the heavy-chain regions (27-50-20(X 10(3)) Mr) of myosin S1 on interaction with nucleotides ATP, AMPPNP, ADP and PPi were investigated by limited proteolysis of several enzyme-metal nucleotide complexes in the absence and presence of reversibly bound and crosslinked F-actin. The rate and extent of the nucleotide-promoted conversion of the NH2-terminal 27 X 10(3) Mr and 50 X 10(3) Mr segments into products of 22 X 10(3) Mr and 45 X 10(3) Mr, respectively, were estimated to determine the amplitude of the molecular movements. The 22 X 10(3) Mr peptide was identified by amino acid sequence studies as being derived from cleavage of the peptide bond between Arg and Ile (at position 23 to 24). The 45 X 10(3) Mr peptide, previously shown to represent the NH2-terminal part of the 50 X 10(3) Mr region, would be connected to the adjacent C-terminal 20 X 10(3) Mr region by a pre-existing loop segment of about 5 X 10(3) Mr; the proteolytic sensitivity of the latter region is increased particularly by nucleotide binding. The tryptic reaction proved to be a sensitive indicator of the conformational state of the liganded heavy chain as the rate of peptide bond cleavage in the two regions is dependent on the nature of the bound ligand; it decreases in the order: ATP greater than AMPPNP greater than ADP greater than PPi. It depends also on the nature of the metal present, Mg2+ and Ca2+ being much more effective than K+. Binding of F-actin to the S1-MgAMPPNP complex affords significant protection against breakdown of 27 X 10(3) Mr and 50 X 10(3) Mr peptides, but with concomitant hydrolysis of the 50 X 10(3) Mr-20 X 10(3) Mr junction. Additionally, interaction of MgATP with HMM modulates the tryptic fission of the S1-S2 region. The overall data provide a molecular support for the two-state model of the myosin head and emphasize the involvement of the 50 X 10(3) Mr unit in the mechanism of coupling between the actin and nucleotide binding sites.

Identification of an active site peptide of skeletal myosin after photoaffinity labeling with N-(4-azido-2-nitrophenyl)-2-aminoethyl diphosphate

The active site of skeletal myosin has been photoaffinity labeled (approximately equal to 50%) by the ADP analog N-(4-azido-2-nitrophenyl)-2-aminoethyl triphosphate (NANDP) following the cobalt phenanthroline active site trapping procedure of Wells and Yount [Wells, J. A. & Yount, R. G. (1979) Proc. Natl. Acad. Sci. USA 76, 4966-4970]. Extensive proteolytic digestion of [3H]NANDP-labeled myosin subfragment one yielded two major peptides, P1 and P2, which were purified by reversed-phase high-performance liquid chromatography. These peptides represented 50% of all labeled amino acids and contained 1 mol of the unusual amino acid epsilon-N-trimethyllysine. Analysis of P2 by Edman techniques gave a sequence Val-Asn-Pro-Tyr-Lys(Me3)-X-Leu-Pro-Val-Tyr, which corresponds to an identical sequence for residues 125-134 determined by Tong and Elzinga [Tong, S. W. & Elzinga, M. (1983) J. Biol. Chem. 258, 13100-13110] for a segment of rabbit skeletal myosin heavy chain in which X is Trp-130. P1 was identical to P2 except it contained an additional three amino acids, Asn-Pro-Gln, at the COOH-terminal end. Amino acid composition, sequence data, spectral measurements, and location of radioactive label in both P1 and P2 all indicate Trp-130 is the major site of labeling by NANDP. The adjacent epsilon-N-trimethyllysine may provide part of the binding site for the triphosphate portion of ATP.

Temperature-induced ultraviolet absorption changes of heavy meromyosin. An application of a computerized spectrophotometer system

The UV absorption of HMM (heavy meromyosin) was measured at various temperatures with a computerized spectrophotometer system. HMM showed temperature-induced absorption changes in the presence and absence of nucleotides. The temperature-induced absorption change at 293 nm, which is due to conformational changes around the tryptophan residues of HMM, was enhanced in the presence of nucleotides. The temperature-induced difference spectra of HMM + AMPPNP relative to HMM obtained by using a conventional spectrophotometer [(1977) J. Biochem. (Tokyo) 81, 313-320] could be reproduced by subtracting the temperature-induced spectral changes of HMM from those of HMM + AMPPNP.

The competition between adenosine triphosphate and inorganic pyrophosphate for myosin and its suppression by substoichiometric actin concentrations

Inorganic pyrophosphate (PPi) inhibits not only Mg2+-ATPase activity of myosin subfragment 1 (S-1) but abolishes also the ATP-induced increment of tryptophan fluorescence of subfragment 1. At the concentrations used (25-50 micron ATP, 12 mm PPi) these effects of PPi were abolished by substoichiometric actin concentrations (approx. 0.1 microM actin vs. approx. 1 microM S-1), where ATPase activity was barely stimulated by actin.

Fluorescence energy transfer between the myosin subfragment-1 isoenzymes and F-actin in the absence and presence of nucleotides

The unique fast-reacting cysteine residue (SH1) of myosin subfragment 1 (S1), prepared by chymotryptic digestion, and cysteine 373 of actin have been labelled selectively with the fluorescent probes, N-(bromoacetyl)-N'-(1-sulpho-5-naphthyl)ethylenediamine (1,5-BrAEDANS) and 5-(iodoacetamido)fluorescein (5-IAF), whose spectral properties render them a particularly effective donor-acceptor pair in fluorescence energy transfer studies. The transfer efficiency of 40-45% represented a spatial separation of the chromophores of about 5 nm, which is in reasonable agreement with the value of 6 nm reported earlier for similarly labelled S1, prepared by papain digestion, and actin [Takashi, R. (1979) Biochemistry, 18, 5164-5169]. This transfer efficiency did not change when the doubly-labelled binary complex was formed: (1) with acto-S1(A1) or acto-S1(A2) at 10-200 mM KCl, pH 7-8 and different buffer conditions; (2) with either S1 isoenzyme and regulated actin (i.e. actin with tropomyosin and troponin) both in the presence and absence of Ca2+ or when the donor and acceptor attachment sites were reversed. Analysis of donor and acceptor polarized fluorescence showed that the chromophores are not randomly orientated (i.e. chi 2 not equal to 2/3), but they do have some motion relative to either protein. From a knowledge of the limiting values for chi 2, the intersite distance for donor and acceptor chromophores was calculated to be in the range 3.9-6.7 nm. Addition of MgATP to the doubly-labelled acto-S1 complex eliminated energy transfer but this was recovered when ATP hydrolysis was completed. By utilizing the known binding constants between S1, actin and either MgADP or MgAdoPP[NH]P (magnesium adenosine 5'-[beta, gamma-imido]triphosphate) [Konrad, M. and Goody, K. (1982) Eur. J. Biochem. 128, 547-555; Greene, L. E. and Eisenberg, E. (1980) J. Biol. Chem. 255, 543-548], the concentrations of all species present at equilibrium were determined. Experimental conditions were chosen to maximise the amount of ternary acto-S1-nucleotide complex (approximately equal to 50%) and minimise the amount of binary complex (less than or equal to 2%). The spatial separation of the chromophore interaction sites in the ternary complex was found to be the same with both nucleotides and indistinguishable from that found with the binary complex. A similar strategy was employed to compare the conformations of the binary and ternary complexes by 1H-NMR spectroscopy. In these experiments about 90% of the S1 was in the form of the ternary complex. There was no noticeable change in the acto-S1 spectra upon addition of either MgAdoPP[NH]P or MgADP. These observations support the conclusion that there is no large change in structure in the 'rigor' binary acto-S1 complex when it binds either ADP or AdoPP[NH]P.

The process in which nucleotide is buried into the active site of heavy meromyosin

The process in which nucleotide is buried into the active site of heavy meromyosin was studied with stopped-flow apparatus by monitoring the time-course of the large fluorescence increase of 1,N6-ethenoadenosine triphosphate (epsilon-ATP) when it binds from acrylamide-containing solutions. We have recently reported that free epsilon-ATP fluorescence is effectively quenched by acrylamide while bound epsilon-ATP is resistant to quenching by acrylamide. In the present study it was found that in the first step the phosphate moiety binds at a high rate, while the adenine moiety is still on the rim of the active site; the adenine moiety is then pulled into a crevice, and finally epsilon-ATP hydrolysis occurs.

Skeletal muscle myosins from the yak (Bos grunniens), cattle (Bos taurus) and their hybrids

The skeletal muscle myosins of the yak (Bos grunniens), of cattle (Bos taurus) and of their first and second filial generation hybrids have been studied by ATPase measurements, fluorescence spectroscopy, near-ultraviolet circular dichroism and peptide mapping on polyacrylamide gels. The ATPase activities, the intrinsic tryptophan fluorescence enhancement upon addition of ATP and the circular dichroism spectra of the four myosins were closely comparable. Peptide maps of the myosin heavy chains indicate extensive sequence homologies but do reveal differences between the myosins of the yak and cattle.

Tryptic digestion as a probe of myosin S-1 conformation

One of the products of the limited tryptic hydrolysis of chymotryptic myosin subfragment 1 is the 27,000-dalton NH2-terminal fragment. This fragment is generated by two parallel routes from either the 75,000- or 95,000-dalton peptide of the heavy chain: (i) through a 20,500-dalton precursor or (ii) directly without participation of a precursor. Lowering of pH and temperature and increasing of ionic strength inhibited route i digestion in comparison to route ii. MgATP and its derivatives in millimolar concentration substantially suppressed route i digestion. Suppression of route i digestion depended on the concentration of MgATP. It occurred after a lag phase when the ratio of MgATP to subfragment 1 concentrations was greater than 0.5. In contrast, the MgATP-induced increase in tryptophan fluorescence of myosin subfragment 1 appeared without a lag phase. The generation of the 27,000-dalton fragment by either route was not affected by F-actin however, the suppression of route i digestion induced by MgADP was abolished when myosin subfragment 1 was in ternary complex with actin and MgADP. We conclude that the 27,000/50,000-dalton hinge region is a flexible domain of the myosin head and that conformation of this region is sensitive to the presence of nucleotides and actin and to variations in ambient factors.

Magnesium adenosine 5'-diphosphate influences proteolytic susceptibility of myosin in myofibrils

The proteolytic susceptibility of the subfragment 2/light meromyosin junction [heavy meromyosin (HMM) junction] of myosin was employed as a probe of the cross-bridge conformation. The proteolysis was carried out in the myofibrils where myosin assembled in arrays typical of the in vivo organization. When subfragment I formation was inhibited by saturating the Nbs2 [5,5'-dithiobis(2-nitrobenzoic acid)] light chains with Mg2+ ions, chymotrypsin attacked exclusively the HMM junction. The rate of this attack was assessed by measuring the rate of HMM formation by quantitative polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and by following absorbance changes associated with the solubilization of myofibrillar suspensions. Under rigor conditions, the myofibrils were relatively resistant to the chymotryptic attack. The presence of MgAMP-PNP or MgPPi did not affect the rate of proteolytic attack. On the other hand, binding of MgADP had a powerful stimulating influence on the HMM site digestibility. The dissociation constant for the effect of MgADP was 10 microM less than Kd less than 50 microM. MgADP did not exercise its unique effect through destabilization of myosin filaments or through dissociation of the actomyosin complex. These results are explained in terms of a change in the myosin cross-bridge conformation brought about by the binding of MgADP to the active site.

Fourier transform infrared absorption studies on the sulfhydryl groups in heavy meromyosin

Infrared absorptions of heavy meromyosin solutions were studied in the frequency range of 2600 cm-1 to 1800 cm-1 with a Fourier transform infrared spectrophotometer. An absorption band characteristic of the stretching vibration of sulfhydryl groups was found at about 2565 cm-1. By comparison with the infrared absorption spectrum of a cysteine solution, the absorption band of sulfhydryl groups in heavy meromyosin showed that the absorption intensity is much stronger, the absorption peak shifts to a lower wavenumber and the width of the absorption band is much broadened. These results indicate that the sulfhydryl groups in heavy meromyosin are strongly hydrogen-bound. The additions of ATP and ADP increased the absorption intensity of the absorption band, suggesting the that hydrogen-bonded structure involving the sulfhydryl groups becomes more strengthened on the binding of ATP and ADP. This indicates that myosin heads change conformation around the sulfhydryl groups during ATP hydrolysis.

Energetics and kinetics of interconversion of two myosin subfragment-1.adenosine 5'-diphosphate complexes as viewed by phosphorus-31 nuclear magnetic resonance

The 31P NMR spectrum of MgADP bound to myosin subfragment-1 (S-1) at 0 degrees C contains two resolved beta-phosphate resonances corresponding to two interconvertible conformations of the S-1 . ADP complex [Shriver, J. W., & Sykes, B. D. (1981) Biochemistry 20, 2004]. The two conformations, MT*ADP and MR*ADP, are in slow exchange on the NMR time scale, and the rates of interconversion are less than 20 s-1. This is consistent with transient kinetic experiments reported in the literature and allows a determination of the rate constants of interconversion: k+ approximately equal to k- approximately equal to 7 s-1 at 0 degrees C. The relative population of the two conformations is highly temperature dependent, and only one form is significantly populated at 25 degrees C. Simulations of the 31P NMR spectra are used to evaluate an equilibrium constant at various temperatures from 0 to 25 degrees C. The standard enthalpy and entropy differences for the R leads to T transition are determined from the variation of the relative free energies of the two states as a function of temperature: delta H degree = 15 (+/- 2) kcal/mol and delta S degree = 55 (+/- 5) cal/(deg mol) (K = 1 at 271 K). This suggests that a significant conformational change occurs in the R leads to T transition with MgADP bound in the active site. However, the entropy and enthalpy differences are nearly compensatory at physiological temperatures. At 25 degrees C the endothermic R leads to T transition is entropy driven, and delta G degree = 1.4 kcal/mol.