Cross-bridge states in invertebrate muscles

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.

Binding of ADP and adenosine 5'-[beta, gamma-imido]triphosphate to insect flight muscle fibrils

We have studied the binding of ADP and adenosine 5'-[beta, gamma-imido]triphosphate (AdoPP[NH]P) to insect flight muscle fibrils. We find that 25% of the myosin heads, presumably those which do not interact with actin, bind AdoPP[NH]P with a binding constant greater than 3 X 10(6) M-1, similar to the binding constant of the same compound to the rabbit myosin heads which do not overlap with actin. The remaining heads in insect myofibrils bind AdoPP[NH]P with an association constant of 8 X 10(3) M-1, which is eight times stronger than the affinity of this compound for rabbit myosin heads in overlap with actin. Therefore, in contrast to the situation with rabbit myofibrils where AdoPP[NH]P binds much more weakly than ADP, with insect myofibrils these two adenosine phosphates bind with almost equal affinity. This is consistent with the numerous structural studies on insect flight muscle which were interpreted on the basis that most of the actomyosin sites were saturated with nucleotide at an AdoPP[NH]P concentration of 1 mM.

The structure of insect flight muscle in the presence of AMPPNP

Glycerinated insect (Lethocerus) flight muscle in the presence of the non-hydrolysable ATP-analogue AMPPNP (1 mM at 4 degrees C) has been prepared for electron microscopy using X-ray diffraction monitoring during fixation and embedding. Superior preservation of the original structure has been achieved through use of a fixative which included tannic acid and excess Mg2+. New features have been recognized in single filament layers (myac and actin) and 15 nm cross sections. As previously shown, some aspects of relaxed structure (14.5 nm shelves along thick filaments) and of rigor (38.7 nm angled bridges along thin filaments) are retained in a modified form. New observations include: (1) In 15 nm cross sections that show single 14.5 nm levels: (a) The flared X structure characteristic of rigor is replaced by a straight-X figure in which the crossbridge density is aligned along the myosin-actin plane, rather than skewed across it as in rigor. (b) In AMPPNP, each crossbridge appears to have a separate origin from the thick filament, rather than bifurcating two from one stem as in the flared X of rigor. The separation of crossbridge origins is also evidenced by the loss of 'ladder rungs' in actin layers. (2) A halving (19.3 nm) of the 38.7 nm axial repeat along actin, rather than a thirding (12.9 nm) as in rigor, indicates redistribution of bridge attachments in cold AMPPNP. (3) In AMPPNP, the 14.5 nm shelves of density around the thick filament shaft are thicker but extend to smaller radius than similar shelves in ATP-relaxed muscle. This is shown by a lack of 14.5 nm periodicity and diffraction in actin layers of AMPPNP, in contrast to ATP-relaxed actin layers, in which the 14.5 nm period is present. Our results suggest that attached crossbridges are modified by AMPPNP and that ordered features of the analogue state are not accounted for solely by detached myosin heads or by a mixture of relaxed and rigor crossbridges. A two-domain model for the crossbridge is proposed. Domain 1 binds to the thin filament, and while bound, maintains a constant stereospecificity to actin at low resolution, independent of the type or presence of nucleotide at the myosin active site. Domain 2 is proximal to the thick filament and can exist in two characteristic states. In the absence of nucleotide (rigor), Domain 2 adopts a variable relationship to the thick filament, to accommodate the actin-bound end of the bridge.(ABSTRACT TRUNCATED AT 400 WORDS)

X-ray diffraction studies of the structural state of crossbridges in skinned frog sartorius muscle at low ionic strength

Low-angle X-ray diffraction diagrams were obtained from chemically skinned frog sartorius muscles under low ionic strength relaxing conditions. Experiments on single muscle fibres from rabbit muscle and on muscle proteins in solution have suggested the presence of a 'low ionic strength attached state' of the myosin crossbridges to actin, in which the overall ATP splitting and force-generating cycle is still blocked. This opened up the possibility that structural information about one of the intermediate states in the crossbridge cycle might be obtained under these conditions. Using synchrotron radiation as a high intensity X-ray source we were able to record the appropriate diffraction diagrams with short exposure times and were able to compare the same muscles at normal and at low ionic strength. Changes in the intensities of the equatorial reflections an increase in the 143 A meridional intensity can be interpreted in a similar way. However, these attached bridges do not give rise to changes in the actin-based layer line reflections, nor is their presence associated with a weakening of the myosin layer line pattern. These results provide further evidence for the existence of bound states of crossbridges, in which their orientation relative to actin is not sharply defined.

The effect of the ATP analogue AMPPNP on the structure of crossbridges in vertebrate skeletal muscles: X-ray diffraction and mechanical studies

Adenylylimidodiphosphate (AMPPNP), a nonhydrolysable analogue of ATP, has been used to arrest the crossbridge cycle of muscular contraction in one of its hypothetical intermediate states. Whole frog sartorius muscles were chemically demembranated, and it was found possible to cycle such skinned muscles reversibly between the relaxed and rigor states. The effect of binding of AMPPNP on the structure and spatial arrangement of the crossbridges of such muscles was studied using low-angle X-ray diffraction, with simultaneous recording of the mechanical effects, starting from the rigor state. Saturating concentrations of MgAMPPNP produce a characteristic decrease of about 50% in the original rigor isometric tension with a concomitant increase in muscle length by 0.13%. The equatorial X-ray diffraction pattern is modified in the following way: the lattice dimensions and the intensity of the (10) equatorial reflection do not change, while the intensity of the (11) equatorial reflection increases slightly. These observations of very small equatorial changes could be explained by assuming that in these muscles (as distinct from others such as rabbit psoas) the analogue does not produce a significant degree of detachment of crossbridges; that is, there are only AMPPNP-modified attached ones. The changes in the meridional X-ray diffraction pattern are more pronounced: the meridional reflection at 14.5 nm decreases in intensity, and the meridional reflection at 7.2 nm increases considerably: the intensity of all the actin-based off-meridional layer-lines decreases. There are no signs of the characteristic relaxed layer-lines, and the changes in the layer-line intensities are probably due to there being a single population of AMPPNP-modified attached crossbridges, rather than a mixture of attached and detached crossbridges. Thus the AMPPNP X-ray pattern, both equatorially and meridionally, is somewhat similar to the rigor one, indicating that most of the crossbridges remain attached. On the other hand, the fact that there are some changes in the layer-line intensities of the AMPPNP frog pattern, without the appearance of any signs of a relaxed equatorial pattern, indicates that the attached crossbridges are in a structural state that is different from rigor, one is not seeing, apparently, simply a mixture of rigor and relaxed states. Our tentative interpretation of this result is that there may be a structural change in the crossbridge near to the junction with S2, with less significant changes occurring in the parts of the crossbridge close to actin.