Polarization of tryptophan fluorescence measurements in muscle. A re-evaluation

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

Cross-bridge movement in muscle and the conformation of the myosin hinge

The force-generating mechanism in muscle is discussed and it is shown that a helix-coil transition in the S-2 link of the cycling cross-bridge is compatible with the physical and chemical properties of this region of the myosin molecule. Thermal melting and temperature-jump experiments are described demonstrating that the light meromyosin-heavy meromyosin (LMM-HMM) hinge domain of S-2 is a segment of low thermal stability. This region can undergo alpha-helix-random coil transitions on a time-scale comparable to the quick-recovery tension transient observed when isometrically contracting muscle is abruptly shortened or stretched. Cross-linking and enzyme probe studies of glycerinated muscle fibres and myofibrils in resting, rigor and activating solvents suggest that the polypeptide chains within the hinge region of S-2 undergo a conformational transition to a more open, proteolytically sensitive structure when the S-2 link is released from the thick filament surface.

Sarcomere dynamics during isotonic velocity transients in single frog muscle fibers

If the load on a tetanized fiber is abruptly changed to a new steady value, the ensuing fiber length change shows the well-known "isotonic velocity transient," in which the velocity oscillates before settling at some steady value. We studied sarcomere dynamics during these transients using two methods: optical diffraction and a segment-length method. Our principal aim was to determine whether these transients might be a reflection of the fact that sarcomere shortening is often found to be stepwise. We found that pauses in sarcomere shortening occurred during the low-velocity phases of the transient and that steps of sarcomere shortening occurred during the high-velocity phases. Thus the isotonic transient appears to arise from the steps. In addition to the isotonic transient, we studied the well-known isometric transient, in which fiber length is abruptly changed, and ensuing tension response is measured. Again, we found that the transient may be a reflection of the stepwise shortening pattern.

Effect of phosphorylation of myosin light chains on interaction of heavy meromyosin with regulated F-actin in ghost fibers

The binding of phosphorylated heavy meromyosin to regulated F-actin in ghost fibers at high Ca2+ concentration increases, and at low Ca2+ concentration decreases, the anisotropy of intrinsic tryptophan fluorescence of F-actin. The effect is opposite to the effect of the binding of dephosphorylated heavy meromyosin.

Perfusion cuvette for the simultaneous measurement of mechanical, optical and energetic parameters of skinned muscle fibres

A micro cuvette is described, which allows to measure mechanical, optical and energetic parameters of single skinned muscle fibres or small fibre bundles. Two examples for possible applications are given: The measurement of ATPase activity of single muscle fibres. The detection of fluorescence changes due to Ca binding to fluorescence labeled TnC, which was incorporated into small bundles of rabbit psoas fibres before the experiment.

The active cross-bridge motions of isolated thick filaments from myosin-regulated muscles detected by quasi-elastic light scattering

Intensity fluctuation spectroscopy has been used successfully as a probe that can detect an increase in high-frequency internal motions of isolated thick filaments of Limulus muscle upon the addition of calcium ions. We have attributed such motions to cross-bridge motion instead of to an increase in the flexibility of the filament backbone. Here we show that after cleavage of the S-1 and then the S-2 moieties with papain, cross-linking the myosin heads to the filament backbone, or heat denaturation (42 degrees C, 10 min), the increase in the high frequency internal motions in the thick filaments no longer occurs. Congo Red, which has been shown to induce shortening of isolated myofibrils, also increases the high-frequency motions of the isolated filaments. Furthermore, the increase is suppressed by treating the filaments with a myosin ATPase inhibitor such as vanadate ions (10 mM) or by replacing ATP with either an equimolar CrADP or the nonhydrolyzable ATP analogue beta, gamma-imido-adenine-5'-triphosphate (AMP-PNP). Calcium ions have a similar effect on isolated thick filaments from scallop muscle, where the myosin is known to be regulatory. Calcium ions have no such effect on thick filaments isolated from frog muscle, which is believed not to be regulated by calcium binding to myosin. These results confirm our earlier supposition that the additional high frequency internal motions of the thick filaments isolated from striated muscle of Limulus are related to the energy dependent, active cross-bridge motions.

Effect of troponin-tropomyosin complex and Ca2+ on conformational changes in F-actin induced by myosin subfragment-1

The interaction of regulated and unregulated actin of myosin-free ghost single fibre with myosin subfragment-1 free of 5,5'-dithiobis(2-nitrobenzoic acid) light chains was investigated by polarized microphotometry. The anisotropy of intrinsic tryptophan fluorescence of regulated actin is Ca2+-dependent and has a maximal value at low (pCa greater than or equal to 7) and a minimal value at high (pCa less than or equal to 6) concentrations of Ca2+. The interaction of myosin subfragment-1 with actin induces cooperative changes in actin structure, which manifest themselves in a decrease in the anisotropy of tryptophan fluorescence. The cooperativity of conformational changes in actin, induced by myosin subfragment-1, is high for regulated actin in the absence of Ca2+ and low for unregulated and regulated actin in the presence of Ca2+. The data obtained suggest that the decrease of the flexibility of actin filaments, induced by tropomyosin or by Ca-unsaturated troponin-tropomyosin complex, results in increased cooperativity of conformational changes of actin induced by myosin subfragment-1.

Effect of Ca2+ binding to 5,5'-dithiobis(2-nitrobenzoic acid) light chains on conformational changes of F-actin caused by myosin subfragment-1

The fluorescent ADP analogue, 1:N6-ethenoadenosine 5'-diphosphate, was incorporated into F-actin in a myosin-free ghost single fibre. Polarized fluorescence measurements of tryptophan residues and 1:N6-ethenoadenosine 5'-diphosphate were performed under a microspectrophotometer to investigate the conformation of F-actin and the changes induced in it by myosin subfragment-1 with 5,5'-dithiobis(2-nitrobenzoic acid) light chains and without them. A relation was found between the conformational state of F-actin and the presence of 5,5'-dithiobis(2-nitrobenzoic acid) light chains. The conformational changes were shown to be controlled by Ca2+ in the presence of 5,5'-dithiobis(2-nitrobenzoic acid) light chains.

Conformational transition in the myosin hinge upon activation of muscle

We have determined the rates of chymotryptic proteolysis of the myosin hinge region in glycerinated rabbit psoas fibers and myofibrils in in rigor-inducing, activating, and relaxing buffers. The time course of formation of light meromyosin (LMM) provides a specific probe for cleavage within the hinge domain. In rigor-inducing and relaxing buffers proteolysis within the hinge is depressed, but on activation LMM is formed at a markedly increased rate, which is dependent on the concentration of MgATP. Peptide bond cleavage occurs at four widely separated sites spanning the length of the hinge domain. Only a trivial amount of proteolysis occurs at the head--rod swivel or within the heavy chain of the head itself (S-1 subunit) in rigor-inducing and relaxing solvents, and we find no significant change on activation. The rate of formation of LMM in rigor-inducing buffer is unchanged by addition of MgADP, Pi, or magnesium adenosine 5'-[beta, gamma-imido]triphosphate or in activating solvent at zero overlap between thick and thin filaments. These results provide evidence for a conformational (helix--coil) transition within the myosin hinge upon activation of skeletal muscle.

ATPase activity in rapidly activated skinned muscle fibres

The ATPase activity of single mechanically skinned fibres from semitendinosus muscle of Rana esculenta was measured. The method was based on the fluorimetrically detected consumption of NADH inside the fibre. NADH breakdown is caused by the reaction of an ATP regenerating system coupled to an NADH oxidising system. The time resolution was about 5 s. The initial rise in ATPase activity was delayed when a fibre was activated by Ca, and ATPase activity reached its plateau at the same time as fibre tension. An activated fibre that was isolated from the incubation solution by immersing it into oil showed a temporary decrease in tension without corresponding decrease in the ATPase activity.