Limulus striated muscle provides an unusual model for muscle contraction

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.

A-band shortening in single fibers of frog skeletal muscle

The question of whether A-bands shorten during contraction was investigated using two methods: high-resolution polarization microscopy and electron microscopy. During shortening from extended sarcomere lengths in the passive state, sarcomere-length changes were essentially accounted for by I-band shortening. During active shortening under otherwise identical conditions, the sarcomere length change was taken up approximately equally by A- and I-bands. Several potential artifacts that could give rise to apparent A-band shortening were considered and judged unlikely. Results obtained with polarization microscopy were similar to those obtained with electron microscopy. Thus, modest but significant thick filament shortening appears to occur during active sarcomere shortening under physiological conditions.

Paramyosin and myosin content of the thick filament in the striated muscle of Limulus

The thick myofilaments of the striated muscle from the horseshoe crab (Limulus) are composed of the proteins paramyosin and myosin. Using quantitative gel electrophoresis we find that there are 1.02 paramyosin molecules for every myosin. This protein composition is consistent with scanning transmission electron microscopy measurements of isolated thick filament mass and the four-stranded helical filament structure reported previously by others if one assumes one myosin molecule per surface subunit as has been suggested. Polyclonal antibodies were used to distinguish myosin and paramyosin bands from others of similar electrophoretic mobility as well as to detect aggregates and breakdown products by examination of gel patterns transferred to nitrocellulose membranes. Two-dimensional gel maps were utilized as a check for coelectrophoresing bands that might go undetected in one dimension. Coomassie Blue binding per mg of purified myosin or paramyosin was found to differ by less than 5%. Extinction coefficients for Limulus myosin and paramyosin as calibrated to dry weight measurements are also reported.