Time-Resolved Structural Studies on Insect Flight Muscle after Photolysis of Caged-ATP

Analysis of transient structures by cryo-microscopy combined with rapid mixing of spray droplets

A simple method to determine transient conformations of biological molecules is described. The two reactants (e.g. protein complex and ligand) are mixed rapidly by the coalescence of spray droplets containing one component, with a thin, grid-supported aqueous film containing the other. The transient state is then trapped by rapid freezing, and investigated later by cryo-microscopy. Images of conformations associated with reaction times of 1-100 ms can be achieved by adjusting the delay between the droplet impact and freezing. The droplets (typically 1 micron in diameter) are propelled onto the grid by an atomizer spray. It is shown that the droplets impinging on the liquid film spread rapidly over its surface under the influence of surface tension, and only weakly disturb the underlying film, partially displacing its contents away from the point of impact. Experiments with sprayed salt solutions, using vesicles derived from erythrocytes as micro-osmometers, indicate that rapid mixing occurs both through the film and laterally, by diffusion. The spraying process does not produce any detectable concentration changes due to drying in either the droplets or the film, and the method is applicable to high-resolution imaging.

Relaxation from rigor of skinned trabeculae of the guinea pig induced by laser photolysis of caged ATP

The kinetics of ATP-induced rigor cross-bridge detachment were studied by initiating relaxation in chemically skinned trabeculae of the guinea pig heart using photolytic release of ATP in the absence of calcium ions (pCa > 8). The time course of the fall in tension exhibited either an initial plateau phase of variable duration with little change in tension or a rise in tension, followed by a decrease to relaxed levels. The in-phase component of tissue stiffness initially decreased. The rate then slowed near the end of the tension plateau, indicating transient cross-bridge rebinding, before falling to relaxed levels. Estimates of the apparent second-order rate constant for ATP-induced detachment of rigor cross-bridges based on the half-time for relaxation or on the half-time to the convergence of tension records to a common time course were similar at 3 x 10(3) M-1 s-1. Because the characteristics of the mechanical transients observed during relaxation from rigor were markedly similar to those reported from studies of rabbit psoas fibers in the presence of MgADP (Dantzig, J. A., M. G. Hibberd, D. R. Trentham, and Y. E. Goldman. 1991. Cross-bridge kinetics in the presence of MgADP investigated by photolysis of caged ATP in rabbit psoas muscle fibres. J. Physiol. 432:639-680), direct measurements of MgADP using [3H]ATP in cardiac tissue in rigor were made. Results indicated that during rigor, nearly 18% of the cross-bridges in skinned trabeculae had [3H]MgADP bound. Incubation of the tissue during rigor with apyrase, an enzyme with both ADPase and ATPase activity, reduced the level of [3H]MgADP to that measured following a 2-min chase in a solution containing 5 mM unlabeled MgATP. Apyrase incubation also significantly reduced the tension and stiffness transients, so that both time courses became monotonic and could be fit with a simple model for cross-bridge detachment. The apparent second-order rate constant for ATP-induced rigor cross-bridge detachment measured in the apyrase treated tissue at 4 x 10(4) M-1 s-1 was faster than that measured in untreated tissue. Nevertheless, this rate was still over an order of magnitude slower than the analogous rate measured in previous studies of isolated cardiac actomyosin-S1. These results are consistent with the hypothesis that the presence of MgADP bound cross-bridges suppresses the inhibition normally imposed by the thin filament regulatory system in the absence of calcium ions and allows cross-bridge rebinding and force production during relaxation from rigor.

Cryo-electron microscopy of vitrified muscle samples

A great deal of information on the 3-dimensional structure of the protein assemblies involved in muscle contraction has been obtained using conventional transmission electron microscopy. In recent years, developments in cryo-electron microscopy have facilitated work with fully hydrated, non-chemically fixed specimens. It is shown how this technique can be used to visualize muscle sarcomere filaments in quasi-native conditions, to access hitherto inaccessible states of the crossbridge cycle, and to obtain new high resolution structural information on their 3-dimensional protein structure. A short introduction to the crossbridge cycle and its biochemically accessible states illustrates the problems amenable to studies using the electron microscope, as well as the possibilities offered by cryo-microscopy on vitrified samples. Work on vitrified cryo-sections and myosin filament suspensions demonstrates the accessibility of crossbridge states and gives implications on the gross structural features of myosin filaments. Recent studies on actin filaments and myosin (S1) decorated actin filaments provide the first high resolution data on vitrified samples. The use of photolabile nucleotide precursors allows the trapping of short lived states in the millisecond time range, thereby visualizing intermediate states of the crossbridge cycle.

Relaxation from rigor by photolysis of caged-ATP in different types of muscle fibres from Xenopus laevis

Using chemically skinned fast and slow fibres from the iliofibularis muscle of Xenopus laevis, we measured the force changes following laser pulse photolysis of caged-ATP at 4 degrees C in the presence and absence of added calcium. The time course of the early force change in the absence of calcium was used to derive an apparent second order rate constant for crossbridge detachment. These values were compared with previous model-dependent estimates stemming from force-velocity experiments. For fast muscle fibres, the value obtained here was equal to that obtained in the previous study, namely 4 x 10(5) M-1 S-1. For slow fibres, the value obtained from caged-ATP experiments was 1.5 x 10(4) M-1 S-1, whereas the value from force-velocity experiments was 20 times greater (2.9 x 10(5) M-1 S-1). The different values for slow fibres indicate that the model assumptions inherent in the analysis of the force-velocity experiments may not hold for all muscle types. For example, the process of dissociation of the actomyosin complex of slow myosins may be different from that of fast myosins. All observed or calculated kinetic transitions for the crossbridge cycle were slower in slow muscle fibres than in fast muscle fibres. These include the forward and backward rate constants for crossbridge attachment which were lower by a factor of three in slow fibres compared with fast fibres.

Time-resolved cryo-electron microscopic study of the dissociation of actomyosin induced by photolysis of photolabile nucleotides

The rapid release of a substrate or other ligand from photolabile precursors in a thin layer suspension of biological specimens followed by rapid freezing provides a method of trapping and visualizing short-lived states in a dynamic system. We demonstrate here the first successful application of this method to study the interaction of actin filaments with myosin subfragment 1 (S1) after release of nucleotides. The results obtained suggest that structural changes in actin filaments occur as a result of interaction with S1.

Biochemical and crystallographic characterization of a complex of c-Ha-ras p21 and caged GTP with flash photolysis

The GTP binding domain of the c-Ha-ras protooncogene product (p21'c) and the corresponding region from an oncogenic mutant form of the protein in which glycine at position 12 has been replaced by valine [p21'(G12V)] have been crystallized with P3-1-(2-nitro)phenylethylguanosine 5'-O-triphosphate (caged GTP) at their active sites. The crystals give x-ray diffraction patterns to a resolution of better than 0.3 nm. Photolysis can be achieved in the crystal, after which GTP hydrolysis takes place at the rate expected from solution studies. Complete x-ray data sets have been obtained for the starting caged-GTP state and the final GDP state after photolysis and hydrolysis, demonstrating the feasibility of time-resolved structural investigations of the process of GTP hydrolysis.

Flash photolysis of caged compounds: new tools for cellular physiology

Caged compounds are molecules or ions of physiological interest, e.g. ATP, IP3, cAMP, cGMP, GTP and Ca2+ rendered inactive by chemical modification. The modification introduces a photochemically labile bond, which on exposure to ultraviolet light cleaves rapidly, releasing the active compound. This article reviews some of the major advances and applications of the photorelease approach, and illustrates its potential in several areas of interest to cellular neuroscientists.

Photolabile chelators for the rapid photorelease of divalent cations

The properties of a recently synthesized photolabile chelator for divalent cations are described, the affinity of which for Ca2+ changes by some 5 orders of magnitude on illumination. The compound 1-(2-nitro-4,5-dimethoxyphenyl)-N,N,N',N'-tetrakis[(oxycarbonyl)me thyl]-1,2-ethanediamine (DM-nitrophen) binds Ca2+ (Kd approximately 5.0 x 10(-9) M) and Mg2+ (Kd approximately 2.5 x 10(-6) M) with relatively high affinities. On exposure of the DM-nitrophen-Ca2+ complex to UV light in the 350-nm range, the chelator is cleaved yielding iminodiacetic products with a much lower affinity for Ca (Kd approximately 3 x 10(-3) M) and the free [Ca2+] increases. The quantum yield for Ca2+ release is 0.18. In experiments with chemically skinned skeletal muscle fibers, a fully relaxed fiber equilibrated with DM-nitrophen-Ca2+ complex produced maximal contraction after a single flash from a frequency-doubled ruby laser (347 nm). Half-maximal tension was achieved in approximately 40 ms, some 5 times faster than that obtained after a rapid solution change from a Ca2+-free to a Ca2+-containing solution. In experiments with resealed human erythrocyte ghosts, irradiation of ghosts containing the DM-nitrophen-Ca2+ complex activates a Ca2+-dependent K+ efflux pathway, which is not observed in the absence of illumination. DM-nitrophen is sufficiently stable and photolabile to be used as a caged Ca (or caged Mg) for the rapid photoinitiation of divalent cation-dependent processes over a wide concentration range with a significant increase in temporal resolution over conventional mixing methods.

Cryo-electron microscopy of insect flight muscle thick filaments. An approach to dynamic electron microscope studies

Suspensions of isolated insect flight muscle thick filaments were embedded in layers of vitreous ice and visualized in the electron microscope under liquid nitrogen conditions. The unfixed, unstained, unsupported and fully hydrated filaments were observed under various biochemical conditions. We demonstrate here the first successful application of this method to thick filaments, and show that this is a possible approach to following dynamic processes by rapid freezing and electron microscopy.

A low cost high intensity flash device for photolysis experiments

Novel techniques of flash photolysis experiments require high intensity light sources in the near UV. We describe here a simple and inexpensive flash device which may complete with bulky and expensive laser systems if the experiments do not necessitate very short light pulses. Using a particular optical arrangement and stored electrical energy, a variation of the parameters voltage and capacitance led to a difference in light output by a factor of more than two. The system is used to relax both skeletal and smooth muscle fibres in the rigor state by releasing up to 2 mM ATP from 12.5 mM caged-ATP.

Relaxation of chemically skinned guinea pig taenia coli smooth muscle from rigor by photolytic release of adenosine-5'-triphosphate

The mechanical events following release of ATP from P3-1-(2-nitro)phenylethyladenosine-5'-triphosphate (caged-ATP) in skinned guinea pig taenia coli smooth muscle in rigor were investigated. A rigor force of about 25-35% of the maximal active force was obtained by removing ATP at the plateau of a maximal active contraction. In the rigor solution free-Mg2+ was 2 mM, ionic strength 90 mM and pH 7.0. When caged-ATP (12.5 mM) was diffused into the preparation there was no change in the rigor force. Photolytic production of about 2 mM ATP was achieved with a xenon flash lamp. Following illumination, force decreased with an approximate initial rate constant of 0.7 s-1. The rate of relaxation was increased in the presence of inorganic phosphate (at 3 mM: 1.3 s-1; 10 mM: 2.2 s-1). At higher Mg2+ concentrations the rate of relaxation was slower (5 mM: 0.2 s-1) and at lower concentrations the rate was faster (0.5 mM: 1.2 s-1). An increased rate of relaxation was observed when ionic strength was increased to 150 mM (2.2 s-1). Phosphate increased the rate of relaxation at the different levels of Mg2+ (0.5-10 mM) and ionic strength (90, 150 mM). In preparations shortened (by 1-3%) to give reduced rigor force, a small transient increase in tension was recorded after ATP release. In comparison to the rates of ATP-induced dissociation of actomyosin in solution, reported in the literature, the rate of relaxation from rigor is slower. This may reflect a slow rigor cross-bridge dissociation or mechanical interactions not associated with cross-bridges in the muscle fibre.(ABSTRACT TRUNCATED AT 250 WORDS)