Measurement of ATPase activities of myosin at the level of tracks and single molecules

Single molecule turnover of fluorescent ATP by myosin and actomyosin unveil elusive enzymatic mechanisms

Benefits of single molecule studies of biomolecules include the need for minimal amounts of material and the potential to reveal phenomena hidden in ensembles. However, results from recent single molecule studies of fluorescent ATP turnover by myosin are difficult to reconcile with ensemble studies. We found that key reasons are complexities due to dye photophysics and fluorescent contaminants. After eliminating these, through surface cleaning and use of triple state quenchers and redox agents, the distributions of ATP binding dwell times on myosin are best described by 2 to 3 exponential processes, with and without actin, and with and without the inhibitor para-aminoblebbistatin. Two processes are attributable to ATP turnover by myosin and actomyosin respectively, whereas the remaining process (rate constant 0.2–0.5 s⁻¹) is consistent with non-specific ATP binding to myosin, possibly accelerating ATP transport to the active site. Finally, our study of actin-activated myosin ATP turnover without sliding between actin and myosin reveals heterogeneity in the ATP turnover kinetics consistent with models of isometric contraction.

With fluorescence based-TIRF microspectroscopy, Ušaj et al. unveil mechanistic details about the ATP turnover rates by myosin and actomyosin with enzymatic reaction pathways that were not possible to obtain from ensemble studies. This study could be important to the field of molecular motors.

Direct real-time detection of the structural and biochemical events in the myosin power stroke

A principal goal of molecular biophysics is to show how protein structural transitions explain physiology. We have developed a strategic tool, transient time-resolved FRET [(TR)(2)FRET], for this purpose and use it here to measure directly, with millisecond resolution, the structural and biochemical kinetics of muscle myosin and to determine directly how myosin's power stroke is coupled to the thermodynamic drive for force generation, actin-activated phosphate release, and the weak-to-strong actin-binding transition. We find that actin initiates the power stroke before phosphate dissociation and not after, as many models propose. This result supports a model for muscle contraction in which power output and efficiency are tuned by the distribution of myosin structural states. This technology should have wide application to other systems in which questions about the temporal coupling of allosteric structural and biochemical transitions remain unanswered.

Single turnovers of fluorescent ATP bound to bipolar myosin filament during actin filaments sliding

The nucleotide turnover rates of bipolar myosin thick filament along which actin filament slides were measured by the displacement of prebound fluorescent ATP analog 2'(3')-O-[N-[2-[(Cy3)]amindo]ethyl] carbamoyl]-adenosine 5' triphosphate (Cy3-EDA-ATP) upon flash photolysis of caged ATP. The fluorescence of the thick filament where actin filament slides decayed with two exponential processes. The slower rate constant was the same as that without actin filament. Along bipolar myosin thick filament, actin filaments slide at a fast speed towards the central bare zone (forward), but more slowly away from the bare zone (backward). The displacement rate constant of fluorescent ATP from the myosin filament where actin filament moved forward was 5.0 s(-1), whereas the rate constant where the actin filament slid backward was 1.7 s(-1). These findings suggest that the slow ADP release rate is responsible for the slow backward sliding movement.

A prism combination for near isotropic fluorescence excitation by total internal reflection

Total internal reflection fluorescence (TIRF) microscopy is finding increasing application for selectively detecting molecules at or near a glass-water surface. As with all fluorescence methods, the efficiency of excitation of a fluorophore is potentially sensitive to the polarization state of the source. In TIRF, s-polarized excitation produces an evanescent field that is perpendicular to the incident plane (y direction), whereas p-polarized light generates a more complex pattern but one dominated by a field that is vertical to the surface (z direction). Thus, fluorophores whose absorption dipoles are fixed in the x direction are not favourably aligned for excitation. Here we describe a beam-splitting prism arrangement that allows excitation by two orthogonal beams, thus giving isotropic excitation in the x-y plane with s-polarized light. With linearly polarized light at the magic angle, near isotropic excitation in three dimensions should be achieved. This prism design should find application in polarized fluorescence microscopy to investigate the rotational motions of macromolecules or to minimize flickering of fluorescence emission arising from molecular rotations in single molecule studies.

Single molecule research on surfaces: from analytics to construction and back

The study of single molecules opens a new dimension in understanding nature down to its finest ramifications. While much progress was achieved in the last decade concerning the detection techniques, suitable techniques for manipulating and handling the biomolecules still bear a challenge. Primarily, the task is keeping an individual, active molecule of a certain lifespan in the spot. Here, we will focus on techniques for the functional immobilization of (single) molecules on surfaces to enable their observation at one position over a time period. Presenting the main methods of reversible immobilization we will accentuate the chelator lipid concept as combining all features prerequisite for functional, reversible and well-defined immobilization. This will also show that single molecule research in principle is the synthesis of an insight into the function of nature and nano-biotechnology (manipulation): thus of analytics, construction, and back.

Myosin monomer density and exchange in synthetic thick filaments investigated using fluorescence microscopy with single molecule sensitivity

The number of myosin molecules in synthetic thick filaments, prepared by dialysis at 0.12 M NaCl and pH 7.0, was estimated to be between 400 and 800 molecules per micrometre under conditions appropriate for in vitro motility assays. This estimate was based on a number count of Cy3-labelled myosin molecules incorporated into filaments at a nominal ratio of 1:1000. At this dilution, single fluorescent spots were resolved corresponding to individual labelled myosins. The spots usually bleached with a one- or two-step profile but, in around 30% of the cases, fluctuations were observed indicating that additional photophysical or photochemical events had occurred. Myosin molecules were shown to exchange between filaments in suspension on a time-scale of several hours at 4 degrees C, but the reaction was only 75% complete after 48 h, suggesting a non-exchangeable core. However, myosin exchange does not appear to be the predominant source of the fluctuations in fluorescence intensity in the single molecule assays.