The nature of the actin cross-bridge interaction

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.

Visualizing myosin's power stroke in muscle contraction

The long-standing swinging crossbridge or lever arm hypothesis for the motor action of myosin heads finds support in recent results from 3-D tomograms of insect flight muscle (IFM) fast frozen during active contraction and from both fluorescence polarization and X-ray diffraction during rapid stretches or releases of isometrically contracting fibers. The latter provide direct evidence for lever arm movements synchronous with force changes. Rebuilding the atomic model of nucleotide-free subfragment 1 (S1) to fit fast-frozen, active IFM crossbridges suggests a two-stage power stroke in which the catalytic domain rolls on actin from weak to strong binding; this is followed by a 5-nm lever arm swing of the light chain domain, which gives a total interaction distance of approx. 12 nm. Comparison of S1 crystal structures with in situ myosin heads suggests that actin binding may be necessary in order to view the full repertoire of myosin motor action. The differing positions of the catalytic domains of actin-attached myosin heads in contracting IFM suggest that both the actin-myosin binding energy and the hydrolysis of ATP may be used to cock the crossbridge and drive the power stroke.

A comparison of the atomic model of F-actin with cryo-electron micrographs of actin and decorated actin

We compare the atomic model calculated from the crystal structure and the X-ray fiber diagram of orientated F-actin1) with the 3-D reconstructions produced from cryo-electron microscopy of actin2). Out to 30A resolution the two structures are essentially identical. Furthermore, by combining the atomic model with the reconstruction of S1-decorated actin filaments2) one can establish the nature of the actin binding site for myosin in the rigor complex. Each myosin head binds to two actin molecules on two distinct sites. Some of the actin residues involved in each of these binding sites can be identified. Furthermore, the atomic model of actin may be combined with the reconstruction of the S1 decorated thin filament to establish the tropomyosin binding site in the rigor complex. This result is compared with the model of tropomyosin-actin derived from an analysis of the X-ray fibre diagram of a reconstituted thin filament and are shown to be very similar.