The association of actin and myosin in the presence of gamma-amido-ATP proceeds mainly via a complex with myosin in the closed conformation

Mn(2+)-nucleotide coordination at the myosin active site as detected by pulsed electron paramagnetic resonance

Pulsed electron paramagnetic resonance at the microwave K(a) band (~30 GHz) was used to study the coordination of adenosine nucleotides to Mn(2+) at the active site of myosin ATPase and in solution. We have found that the electron spin echo (ESE) field sweep, electron-nuclear double resonance (ENDOR) and ESE envelope modulation (ESEEM) techniques are not sufficiently specific for reliable differentiation between the solvated and myosin-bound Mn·nucleotide complexes. Therefore, to directly detect binding of the Mn·nucleotide to myosin, we used nonhydrolizable nucleotide analogs, site-directed spin labeling, and pulsed electron-electron double resonance to detect spin probe-manganese dipolar interaction. We found that under substoichiometric conditions, both Mn·AMPPNP and Mn·ADP·AlF(4) form a complex with myosin, and Mn·ADP does not form such a complex. This correlates well with the biological dissociation of Mg·ADP from myosin after the hydrolysis of ATP. The analysis of (31)P ENDOR spectra reveals that in Mn·AMPPNP, Mn·ATP, and Mn·ADP at myosin or in solution, the nucleotide is coordinated to Mn(2+) by two phosphate groups, whereas in Mn·ADP·AlF(4), only one phosphate group is coordinated. The observation of two phosphates and one nitrogen in the coordination sphere of Mn·ADP in solution by ESEEM spectroscopy suggests that a significant population of Mn ions is coordinated by two ADP molecules, one of which is coordinated by phosphates, and the other one, by a nitrogen atom. The developed approach will be generally useful for monitoring the metal-protein binding when such binding does not provide reliable spectroscopic signatures.

Early stages of the recovery stroke in myosin II studied by molecular dynamics simulations

The recovery stroke is a key step in the functional cycle of muscle motor protein myosin, during which pre-recovery conformation of the protein is changed into the active post-recovery conformation, ready to exersice force. We study the microscopic details of this transition using molecular dynamics simulations of atomistic models in implicit and explicit solvent. In more than 2 μs of aggregate simulation time, we uncover evidence that the recovery stroke is a two-step process consisting of two stages separated by a time delay. In our simulations, we directly observe the first stage at which switch II loop closes in the presence of adenosine triphosphate at the nucleotide binding site. The resulting configuration of the nucleotide binding site is identical to that detected experimentally. Distribution of inter-residue distances measured in the force generating region of myosin is in good agreement with the experimental data. The second stage of the recovery stroke structural transition, rotation of the converter domain, was not observed in our simulations. Apparently it occurs on a longer time scale. We suggest that the two parts of the recovery stroke need to be studied using separate computational models.

Rapidly photoactivatable ATP probes for specific labeling of tropomyosin within the actomyosin protein complex

Tropomyosin-specific photoaffinity adenosine triphosphate (ATP) probes have been first developed, in which a diazirine moiety is incorporated into the γ-phosphate group as a rapidly carbene-generating photophore. These probes clearly labeled tropomyosin in the presence of other actomyosin components, that is, myosin, actin, and troponins. The specific labeling of tropomyosin was easily identified by selective trapping of the photo-incorporated ATP probe on Fe(3+)-immobilized metal ion affinity chromatography (IMAC) beads. The characteristic nature of tropomyosin-specific photocross-linking was further confirmed with a biotin-carrying derivative of the ATP probe. These data suggest that the tropomyosin on the actin filament assembly is located in close proximity to the ATP binding cavity of myosin.

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 Mg(2+)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·AlF(4)(-)>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.

The mechanism of the reverse recovery step, phosphate release, and actin activation of Dictyostelium myosin II

The rate-limiting step of the myosin basal ATPase (i.e. in absence of actin) is assumed to be a post-hydrolysis swinging of the lever arm (reverse recovery step), that limits the subsequent rapid product release steps. However, direct experimental evidence for this assignment is lacking. To investigate the binding and the release of ADP and phosphate independently from the lever arm motion, two single tryptophan-containing motor domains of Dictyostelium myosin II were used. The single tryptophans of the W129+ and W501+ constructs are located at the entrance of the nucleotide binding pocket and near the lever arm, respectively. Kinetic experiments show that the rate-limiting step in the basal ATPase cycle is indeed the reverse recovery step, which is a slow equilibrium step (k(forward) = 0.05 s(-1), k(reverse) = 0.15 s(-1)) that precedes the phosphate release step. Actin directly activates the reverse recovery step, which becomes practically irreversible in the actin-bound form, triggering the power stroke. Even at low actin concentrations the power stroke occurs in the actin-attached states despite the low actin affinity of myosin in the pre-power stroke conformation.