Biomedical subjects
Publications and source records attributed to T P Burghardt.
In order to measure the cycling of a few ( approximately 6) myosin heads in contracting skeletal muscle, myofibrils were illuminated by Total Internal Reflection and observed through a confocal aperture. Myosin heads rotated at a rate approximately equal to the ATPase rate, suggesting that bulk ATPase of a whole muscle reflects the cycle frequency of individual heads.
It is well documented that muscle contraction results from cyclic rotations of actin-bound myosin cross-bridges. The role of actin is hypothesized to be limited to accelerating phosphate release from myosin and to serving as a rigid substrate for cross-bridge rotations. To test this hypothesis, we have measured actin rotations during contraction of a skeletal muscle. Actin filaments of rabbit psoas fiber were labeled with rhodamine-phalloidin. Muscle contraction was induced by a pulse of ATP photogenerated from caged precursor. ATP induced a single turnover of cross-bridges. The rotations were measured by anisotropy of fluorescence originating from a small volume defined by a narrow aperture of a confocal microscope. The anisotropy of phalloidin-actin changed rapidly at first and was followed by a slow relaxation to a steady-state value. The kinetics of orientation changes of actin and myosin were the same. Extracting myosin abolished anisotropy changes. To test whether the rotation of actin was imposed by cross-bridges or whether it reflected hydrolytic activity of actin itself, we labeled actin with fluorescent ADP. The time-course of anisotropy change of fluorescent nucleotide was similar to that of phalloidin-actin. These results suggest that orientation changes of actin are caused by dissociation and rebinding of myosin cross-bridges, and that during contraction, nucleotide does not dissociate from actin.
beta-cardiac myosin subfragment 1 (betaS1) tertiary structure and dynamics were characterized with proteolytic digestion, nucleotide analogue trapping kinetics, and intrinsic fluorescence changes accompanying nucleotide binding. Proteolysis of betaS1 produces the 25, 50, and 20 kDa fragments and a new cut within the 50-kDa fragment at Arg369. F-actin inhibits cleavage of the 50-kDa fragment and fails to inhibit cleavage at the 50/20 kDa junction, suggesting betaS1 presents an actoS1 conformation fundamentally different from skeletal S1. Time-dependent changes in Mg(2+)-ATPase accompanying proteolysis identifies cleavage points that lie within the energy transduction pathway. The nucleotide analogue trapping kinetics reveal the presence of a reversible weakly actin attached state. Comparison of nucleotide analogue induced betaS1 structures with the transient structures occurring during ATPase indicates analogue induced and transient structures are in a one-to-one correspondence. Tryptophan fluorescence enhancement accompanies the binding or trapping of nucleotide or nucleotide analogues. Isolation of Trp508 fluorescence shows it is an ATP-sensitive tryptophan and that its vicinity changes conformation sequentially with the transient intermediates accompanying ATPase. These studies elucidate energy transduction and suggest how mutations of betaS1 implicated in disease might undermine function, stability, or efficiency.
Myosin cross-bridge subfragment 1 (S1) is the ATP catalyzing motor protein in muscle. It consists of three domains that catalyze ATP and bind actin (catalytic), conduct energy transduction (converter), and transport the load (lever arm). Force development during contraction is thought to result from rotary lever arm movement with the cross-bridge attached to actin. To elucidate cross-bridge structure during force development, two crystal structures of S1 were extrapolated to working "in solution" or oriented "in tissue" forms, using structure-sensitive optical spectroscopic signals from two extrinsic probes. The probes were located at two interfaces containing the catalytic, converter, and lever arm domains of S1. Observed signals included circular dichroism (CD) and absorption originating from S1 in solution in the presence and absence of actin and fluorescence polarization from cross-bridges in muscle fibers. Theoretical signals were calculated from S1 crystal structure models perturbed with lever arm movement from swiveling at three conserved glycines, 699, 703, and 710 (chicken skeletal myosin numbering). Best agreement between the computed and observed signals gave structures showing that actin binding to S1 causes movement of the lever arm. A three-state model of S1 conformation during contraction consists of three actin-bound cross-bridge states observed from muscle fibers in isometric contraction, in the presence of MgADP, and in rigor. Structures best representing these states show that most of the lever arm rotation occurs between isometric contraction and the MgADP states, i.e., during phosphate release. Smaller but significant lever arm rotation occurs with ADP dissociation. Structural changes within the S1 interfaces studied are discussed in the accompanying paper [Burghardt et al. (2001) Biochemistry 40, 4834-4843].
Myosin subfragment 1 (S1) is the ATP catalyzing motor protein in muscle. It consists of three domains that catalyze ATP and bind actin (catalytic), conduct energy transduction (converter), and transport the load (lever arm). These domains interface in two places identified as interface I, containing the reactive thiol (SH1) and ATP-sensitive tryptophan (Trp510), and interface II, containing the reactive lysine residue (RLR). Two crystal structures of S1 were extrapolated to working "in solution" or oriented "in tissue" forms, using structure-sensitive optical spectroscopic signals from extrinsic probes located in the interfaces. Observed signals included circular dichroism (CD) and absorption originating from S1 in solution in the presence and absence of actin and fluorescence polarization from cross-bridges in muscle fibers. Theoretical signals were calculated from S1 crystal structure models perturbed with lever arm movement from swiveling at three conserved glycines, 699, 703, and 710 (chicken skeletal myosin numbering). Structures giving the best agreement between the computed and observed signals were selected as the representative forms. Both interfaces undergo dramatic conformational change during ATPase and force development. Changes at interface I suggest the molecular basis for the collisional quenching sensitivity of Trp510 to nucleotide binding. The probe conformation at SH1 suggests how it alters S1 ATPases. At interface II, the spatial relationship of the lever arm and the extrinsic probe at RLR suggests how the probe alters S1 ATPases and that it should inhibit lever arm movement during the power stroke. The latter possibility, if true, establishes a part of the corridor through which the lever arm swings during the power stroke. Global structural changes in actomyosin are discussed in the accompanying paper [Burghardt et al. (2001) Biochemistry 40, 4821-4833].
The effect of ionic strength on the conformation and stability of S1 and S1-nucleotide-phosphate analog complexes in solution was studied. It was found that increasing concentration of KCl enhances the reactivity of Cys(707) (SH1 thiol) and Lys(84) (reactive lysyl residue) and the nucleotide-induced tryptophan fluorescence increment. In contrast, high KCl concentration lowers the structural differences between the intermediate states of ATP hydrolysis in the vicinity of Cys(707), Trp(510) and the active site, possibly by increasing the flexibility of the molecule. High concentrations of neutral salts inhibit both the formation and the dissociation of the M**.ADP.Pi analog S1.ADP.Vi complex. High ionic strength profoundly affects the structure of the stable S1.ADP.BeF(x) complex, by destabilizing the M*.ATP intermediate, which is the predominant form of the complex at low ionic strength, and shifting the equilibrium to favor the M**.ADP.Pi state. The M*.ATP intermediate is destabilized by perturbation of ionic interactions possibly by disruption of salt bridges. Two salt-bridge pairs, Glu(501)-Lys(505) in the Switch II helix and Glu(776)-Lys(84) connecting the catalytic domain to the lever arm, seem most appropriate to consider for participating in the ionic strength-induced transition of the open M*.ATP to the closed M**.ADP.Pi state of S1.
The fluorescence intensity difference between rabbit skeletal myosin subfragment 1 (S1) and nucleotide-bound or trapped S1 isolates ATP-sensitive tryptophans (ASTs) emission from the total tryptophan signal. Neutral (acrylamide) quenching of the ASTs is sensitive to the binding or trapping of nucleotide to the active site of S1. Anion (I(-)) quenching of the ASTs, sensitive to charge separation in the tryptophan micro environment, is negligible. These findings suggest the ASTs sense conformational change during ATPase from negatively charged surroundings. Specific chemical modifications of S1 identified the location of the ASTs. Trp131 was quenched by chemical modification, and its emission was isolated by taking the intensity difference between unmodified and modified S1. Trp131 fluorescence intensity and quenching constant do not distinguish among the bound or trapped nucleotides, suggesting that the vicinity of Trp131 does not change conformation during the ATPase cycle and eliminating Trp131 as an AST. Trp510 fluorescence was quenched by 5'-iodoacetamidofluorescein (5'IAF) modification of the reactive thiol (SH1) of S1. The tryptophan emission enhancement increment due to active site trapping decreases linearly with SH1 modification and extrapolates to 0 for 100% modification. These data identify Trp510 as the primary AST in skeletal S1 in agreement with observations from Dictyostelium (Batra and Manstein (1999) Biol. Chem. 380, 1017-1023) and smooth muscle S1 (Yengo et al. (2000) Biophys. J. 78, 242A). With Trp510 identified as the sole AST, fluorescence difference spectroscopy provides a novel means to monitor the concentration of myosin transient intermediates in ATP hydrolysis.
Trinitrophenylation of the reactive lysine (Lys84) in skeletal myosin subfragment 1 (S1) introduces a chiral probe (TNP) into an interface of the catalytic and lever arm domains of S1 [Muhlrad (1977) Biochim. Biophys. Acta 493, 154-166]. Characteristics of the TNP absorption and circular dichroism (CD) spectra in TNP-modified S1 (TNP-Lys84-S1), and the Lys84 trinitrophenylation rate in native S1, indicate a one-to-one correspondence between ATPase transients and trapped phosphate analogues. Phosphate analogue-induced structures of TNP-Lys84-S1 were modeled using the crystallographic coordinates of S1 [Rayment et al. (1993) Science 261, 50-58] with swivels at Gly699 and Gly710 to approximate conformational changes during ATPase. The CD and absorption spectral characteristics of the model structures were compared to those observed for analogue-induced structures. The model calculations, first tested on a trinitrophenylated hexapeptide with known structure, were applied to TNP-Lys84-S1. They showed that ATP binding initiates swiveling at Gly699 and that swiveling at both Gly710 and Gly699 accompanied ATP splitting just prior to product release. The computed lever arm trajectory during ATPase suggests (i) a plausible mechanism for the nucleotide-induced inhibition of Lys84 trinitrophenylation, and (ii) trinitrophenylation-induced changes in S1 Mg2+- and K+-EDTA ATPase are from collision of the lever arm with TNP at Lys84. TNP is a site-specific structural perturbant of S1 and a chiral reporter group for the effect of Lys84 modification on dynamic S1 structure. As such, TNP-Lys84-S1 is equivalent to a genetically engineered mutant with intrinsic sensitivity to structure local to the modified residue.
Magnesium (Mg2+) is the physiological divalent cation stabilizing nucleotide or nucleotide analog in the active site of myosin subfragment 1 (S1). In the presence of fluoride, Mg2+ and MgADP form a complex that traps the active site of S1 and inhibits myosin ATPase. The ATPase inactivation rate of the magnesium trapped S1 is comparable but smaller than the other known gamma-phosphate analogs at 1.2 M-1 s-1 with 1 mM MgCl2. The observed molar ratio of Mg/S1 in this complex of 1.58 suggests that magnesium occupies the gamma-phosphate position in the ATP binding site of S1 (S1-MgADP-MgFx). The stability of S1-MgADP-MgFx at 4 degrees C was studied by EDTA chase experiments but decomposition was not observed. However, removal of excess fluoride causes full recovery of the K+-EDTA ATPase activity indicating that free fluoride is necessary for maintaining a stable trap and suggesting that the magnesium fluoride complex is bonded to the bridging oxygen of beta-phosphate more loosely than the other known phosphate analogs. The structure of S1 in S1-MgADP-MgFx was studied with near ultraviolet circular dichroism, total tryptophan fluorescence, and tryptophan residue 510 quenching measurements. These data suggest that S1-MgADP-MgFx resembles the M**.ADP.Pi steady-state intermediate of myosin ATPase. Gallium fluoride was found to compete with MgFx for the gamma-phosphate site in S1-MgADP-MgFx. The ionic radius and coordination geometry of magnesium, gallium and other known gamma-phosphate analogs were compared and identified as important in determining which myosin ATPase intermediate the analog mimics.
Partial agonists are effective tools for advancing development of highly selective drugs and providing insights into molecular regulation of cellular functions. Here, we explore the impact of a partial agonist on key aspects of cholecystokinin (CCK) receptor regulation, its lateral mobility and cellular trafficking, in native pancreatic acinar cells and Chinese hamster ovary cells expressing CCK receptor (CHO-CCKR). We developed and characterized a novel fluorescent partial agonist, rhodamine-Gly-[(Nle28, 31)CCK-26-32]-phenethyl ester, that binds specifically and with high affinity to CCK receptors. Such analogs are fully efficacious pancreatic acinar cell secretagogues without supramaximal inhibition that mobilize intracellular calcium with little or no increase in phospholipase C (PLC) activity. Despite minimal phosphorylation of CCK receptors in response to this partial agonist, receptor trafficking was the same as that observed with full agonist (CCK). This included normal internalization via clathrin-dependent endocytosis in CHO-CCKR cells and insulation on the surface of pancreatic acinar cells. Also, as with CCK-occupied receptor, fluorescence recovery after photobleaching of partial agonist-occupied receptor on the acinar cell surface demonstrated a marked temperature-dependent slowing of its rate of diffusion. This was similarly associated with resistance to acid-induced dissociation of ligand. Thus some key molecular regulatory mechanisms for CCK receptor internalization and insulation may be initiated by cellular signaling cascades that are not dependent on PLC activation or receptor phosphorylation.
2-[(2-Nitrophenyl)amino]ethyl triphosphate (NPhAETP) is the smallest ATP analogue that serves as a substrate for the actin-activated ATPase of myosin subfragment 1 (S1) and supports the development of active tension in skinned fibers. 2-(Phenylamino)ethyl triphosphate (PhAETP), in which the nitro group on the phenyl ring of NPhAETP is substituted by a H atom, is also a substrate of the actin-activated ATPase but does not support active tension [Wang, D., Pate, E., Cooke, R., and Yount, R. (1993) J. Muscle Res. Cell Motil. 14, 484-497]. We compared the S1-catalyzed hydrolysis of these analogues, their ability to support the formation of stable complexes with S1 and phosphate analogues, and their effect on S1 conformation. The analogues were hydrolyzed by S1 under various conditions both in the presence and in the absence of actin. In some cases, the effects of the two analogues are similar to each other and to those of ATP; they protect S1 from heat denaturation at 40 degreesC and inhibit the formation of the N-terminal 29 kDa fragment during the tryptic digestion of S1 and the modification of Lys-83 with trinitrobenzene sulfonate. However, in other cases, the effect of the two analogues is different; the effect of NPhAETP resembles that of ATP. NPhAETP and ATP decrease while PhAETP increases the rate of reaction of the SH1 thiol (Cys-707) with coumarin maleimide. The diphosphate forms of the two analogues induce a much smaller change in the near-UV CD spectrum of S1 than ADP. NPhAEDP forms stable complexes with S1 in the presence of beryllium fluoride (BeFx), aluminum fluoride (AlF4-), or vanadate (Vi) phosphate analogues, while the S1.PhAEDP complex is stable in the presence BeFx but much less stable with AlF4- and Vi. These results indicate that the S1.PhAEDP.Pi state is poorly populated during the PhAETP hydrolysis. The models of the atomic structure of S1 complexed by the two analogues show that PhAETP, unlike NPhAETP or ATP, does not form a H bond with Tyr-134 in S1, which is the probable structural reason of the lack of tension development, with PhAETP as the substrate.
The nature of the interaction between the nucleotide ATP and hsp90 was investigated by observing fluorescence quenching of the four tryptophan residues in hsp90 as a function of quencher type and temperature. ATP and acrylamide quench the fluorescence from tryptophan free in solution principally by static and collisional mechanisms, respectively. Acrylamide quenching of tryptophan fluorescence in hsp90 is also principally collisional and identifies two classes of residues, one readily accessible to quenching the other less accessible. ATP quenching of tryptophan fluorescence in hsp90 is more complex exhibiting no overall preferred mechanism. However, ATP competitively inhibits acrylamide quenching of the readily accessible class of tryptophan residues by static quenching with the quenching constant providing an upper limit for the ATP dissociation constant. The ATP-free tryptophan dissociation constant is more than a factor of three larger than that for ATP-hsp90 suggesting that the ATP-hsp90 interaction is specific. The static quenching of tryptophan fluorescence in hsp90 by ATP implies that the nucleotide binds in close proximity to one or more of the tryptophan residues.
The conformation of myosin subfragment 1 (S1) in the vicinity of the ATP sensitive tryptophan (Trp510) and the highly reactive thiol (SH1), both residing in the "probe-binding" cleft at the junction of the catalytic and lever arm domains, was studied to ascertain its role in the mechanism of energy transduction and force generation. In glycerinated muscle fibers in rigor, a fluorescent probe linked to SH1 detects a strained probe-binding cleft conformation following a length transient by altering emission intensity without detectably rotating. In myosin S1 in solution, the optical activity of Trp510 senses conformation change in the probe-binding cleft caused by substrate analog trapping of S1 in various structures attainable transiently during normal energy transduction. Also in S1 in solution, the induced optical activity of a fluorescein probe linked to SH1 shows sensitivity to changing probe-binding cleft conformation caused by nucleotide binding to the S1 active site. The changes in the optical activity of Trp510 and SH1 bound fluorescein in response to nucleotide or nucleotide analog binding are interpreted structurally using the S1 crystallographic coordinates and aided by a model of energy transduction that pivots at Gly699 to change probe-binding cleft conformation and to displace the S1 lever arm as during force generation. The crystallographic structure of the probe-binding cleft in S1 resembles most the nucleotide bound conformation in the native protein. A different structure, generated by pivoting at Gly699, better resembles the native rigor conformation of the probe-binding cleft. Pivoting at Gly699 rotates probes at SH1 suggesting that length transients on fibers in rigor do not cause pivoting at Gly699 or reverse the power stroke.
The circular dichroism (CD) spectrum was measured from vanadate (Vi) cyclic esters of chiral vicinal diols, hydroxycarboxylates, and cyclodextrines as a function of Vi concentration ([Vi]) and at the lowest energy transitions of the vanadium. At low [Vi] and in the presence of excess vicinal diols, hydroxycarboxylates, or cyclodextrines the CD signal intensity scales linearly with [Vi] indicating the predominance of a monomeric cyclic ester. At higher [Vi], the signal intensity in the presence of the vicinal diols and hydroxycarboxylates become nonlinear in [Vi], indicating formation of a dimeric cyclic ester. Vanadium-51 NMR (51V-NMR) indicates the coordination geometry of several of these model Vi centers in solution and identifies the CD signals characteristic to Vi trigonal bipyramidal (tbp) and octahedral (Oh) coordination geometries from monomeric and dimeric species. The CD spectra from monomeric and dimeric forms of the tbp-coordinated model compounds have two apparent transitions with amplitudes of opposite sign at wavelengths > or = 240 nm. Spectra from the monomeric and dimeric Oh coordinated species are distinct from the tbp-type spectra over the same wavelength domain because of the presence of two additional transitions with opposite sign amplitudes. These model spectra were compared to the vanadate CD spectra from Vi bound to rabbit myosin subfragment 1 (S1) in solution, in the presence of divalent metal cations (MeVi-S1) or trapped with MeADP (MeADPVi-S1). Polymeric MeVi binds to the active site of S1 and the vanadate centers in MnVi-S1 or CoVi-S1 produce a CD signal resembling that from the tbp model. The trapped ATPase transition state analog MeADPVi produces a different CD signal resembling that from the Oh model.
It is widely conjectured that muscle shortens because portions of myosin molecules (the "cross-bridges") impel the actin filament to which they transiently attach and that the impulses result from rotation of the cross-bridges. Crystallography indicates that a cross-bridge is articulated-consisting of a globular catalytic/actin-binding domain and a long lever arm that may rotate. Conveniently, a rhodamine probe with detectable attitude can be attached between the globular domain and the lever arm, enabling the observer to tell whether the anchoring region rotates. Well-established signature effects observed in shortening are tension changes resulting from the sudden release or quick stretch of active muscle fibers. In this investigation we found that closely correlated with such tension changes are changes in the attitude of the rhodamine probes. This correlation strongly supports the conjecture about how shortening is achieved.
The interaction of myosin with actin, coupled with hydrolysis of ATP, is the molecular basis of muscle contraction. The head segment of myosin, called S1, contains the distinct binding sites for ATP and actin and is responsible for the ATPase activity. The myosin-catalyzed ATP hydrolysis consists of several intermediate steps and each step is accompanied by conformational changes in the S1 segment. The rate-limiting step of the ATP hydrolysis is the dissociation of the S1 x ADP x Pi complex which is accelerated by actin. The substitution of Pi with phosphate analogs (PA), such as vanadate, beryllium fluoride (BeFx) or aluminum fluoride (AlF4-), yields stable complexes which mimic the intermediates of the ATP hydrolysis. In this work, tertiary structure changes in S1 in the vicinity of aromatic residues was studied by comparing near-UV circular dichroism (CD) spectra from S1-nucleotide-phosphate analog complexes in the presence of Mg2+ and other cations. A significant difference between the MgATP and MgADP spectra indicated notable tertiary structural changes accompanying the M**ADP x Pi --> M*ADP transition. The spectra of the S1 x MgADP x BeFx and S1 x MgADP x AlF4- complexes resemble to those obtained upon addition of MgATPgammaS and MgATP to S1, and correspond to the M* x ATP and M** x ADP x Pi intermediates, respectively. We have found recently that the presence of divalent metal cations (Me2+) is essential for the formation of stable S1 x MeADP x PA complexes. Moreover, the nature of the metal cations strongly influences the stability of these complexes [Peyser, Y. M., et al. (1996) Biochemistry 35, 4409-4416]. In the present work we studied the effect of Mg2+, Mn2+, Ca2+, Ni2+, Co2+, and Fe2+ on the near-UV CD spectrum of the ATP, ADP, ADP x BeFx, and ADP x AlF4- containing S complexes. The CD spectra obtained with ADP, ATP ADP x BeFx and ADP x AlF4- were essentially identical in the presence of Co2+ and rather similar in the case of Ca2+, while they were partially different in other cases. An interesting correlation was found between actin activation and ATP versus ADP difference spectra in the presence of various metal ions. The distribution of the fractional concentration of the intermediates of ATP hydrolysis was estimated in the presence of each cation from the CD spectra with phosphate analogs. In the presence of Mg2+ the predominant intermediate is the M** x ADP x Pi state, which is in accordance with the kinetic studies. On the other hand with non-native cations the predominant intermediate is the M* x ADP state and the release of ADP is the rate limiting step in the myosin-catalyzed ATP hydrolysis. According to the results, the near-UV CD spectrum originating from aromatic residues in S1 not only can distinguish identifiable states in the ATP hydrolysis cycle but can also pinpoint to changes in the tertiary structure caused by complex formation with nucleotide or nucleotide analog and various divalent metal cations. These findings, that are correlative with actin activation, and thus with the power stroke, suggest new strategies for perturbing S1 structure in the continuous efforts directed toward the elucidation of the mechanism of muscle contraction.
Acrylamide quenching of tryptophan 510 (Trp510) fluorescence in rabbit skeletal myosin subfragment 1 (S1) indicates the conformation of the probe binding cleft, containing the highly reactive thiol (SH1) and Trp510, in the presence of nucleotides or nucleotide analogs trapped in the active site of S1 [Park et al. (1996) Biochim. Biophys. Acta 1296, 1-4]. The Trp510 quenching efficiency shows that the probe binding cleft closes slightly in the presence of beryllium fluoride trapped MgADP (MgADPBeFx-S1) and most tightly in the presence of vanadate trapped MgADP (MgADPVi-S1) with aluminum fluoride and scandium fluoride trapped MgADP (MgADPA1F4-S1 and MgADPScFx-S1) falling in between in the order MgADPBeFx > MgADPA1F4 > MgADPScFx > MgADPVi. These nucleotide analogs are identified with sequential structural changes in MgATP during hydrolysis in the same order with beryllium fluoride occurring earliest in the ATPase cycle. Correlation of the separation distance of the gamma-phosphate analog metal from the oxygen connecting it to the beta-phosphate in ADP, to the extent of cleft closure, suggests that this distance in the nucleotide transition state determines the conformation of the probe binding cleft. Trp510 quenching efficiency was also measured as a function of the base moiety of the vanadate trapped Mg-nucleotide diphosphate (MgNDPVi-S1). The extent of cleft closure is largest in the presence of the natural substrate NDP and follows the order MgADPVi > MgCDPVi > MgUDPVi > MgIDPVi > MgGDPVi with very little difference between MgADPVi and MgCDPVi. These data follow the order of the effectiveness of the corresponding nucleotide triphosphates to support force production in muscle fibers [Pate et al. (1993) J. Biol. Chem. 268, 10046-10053]. In both the fiber and S1, it appears that the 6-position amino group of the bases of ADP and CDP is required to properly anchor the nucleotide in the active site, possibly at tyrosine 135 as suggested by X-ray crystallographic studies [Fisher et al. (1995) Biochemistry 34, 8960-8972]. Finally, the Trp510 quenching efficiency was measured as a function of the size of the divalent cation trapped in the active site of S1 with ADPVi. These data failed to show a correlation suggesting that the divalent cation is not involved with the propagation of influence from the active site to the probe binding cleft. The forgoing experiments suggest that the changing conformation of ATP during hydrolysis, parameterized by the increasing distance between the beta- and the gamma-phosphate, stresses the active site of S1 through protein-nucleotide contacts at the gamma-phosphate and nucleotide base. The stress-induced strain in the cross-bridge may be the mechanism by which energy in ATP is transferred to the myosin structure.
The xanthene probes 5'-iodoacetamido-fluorescein and -tetramethylrhodamine specifically modify skeletal muscle myosin subfragment 1 (S1) at the reactive thiol residue (SH1) and fully quench the fluorescence emission from tryptophan residue 510 (Trp510) in S1 (T.P. Burghardt and K. Ajtai, Biophys. Chem., 60 (1996) 119; K. Ajtai and T.P. Burghardt, Biochemistry, 34 (1995) 15943). The difference between the fluorescence intensity obtained from S1 and probe-modified S1 comes solely from Trp510 in chymotryptic S1, a protein fragment that contains five tryptophan residues. The rotary strength and quantum efficiency of Trp510 were measured using difference signals from fluorescence detected circular dichroism (FDCD) and fluorescence emission spectroscopy. These structure-sensitive signals indicate that the binding of nucleotide or nucleotide analogs to the active site of S1 causes structural changes in S1 at Trp510 and that a one-to-one correspondence exists between Trp510 conformation and transient states of myosin during contraction. The Trp510 rotary strength and quantum efficiency were interpreted structurally in terms of the indole side-chain conformation using model structures and established computational methods.