Muscle fibers of the tarantula femur exhibit structural and biochemical characteristics similar to those of other Iong-sarcomere invertebrate muscles, having long A-bands and long thick filaments. 9-12 thin filaments surround each thick filament. Tarantula muscle has a paramyosin:myosin heavy chain molecular ratio of 0.31 _ 0.079 SD. We studied the myosin cross-bridge arrangement on the surface of tarantula thick filaments on isolated, negatively stained, and unidirectionally metal-shadowed specimens by electron microscopy and optical diffraction and filtering and found it to be similar to that previously described for the thick filaments of muscle of the closely related chelicerate arthropod, Limulus. Cross-bridges are disposed in a four-stranded right-handed helical arrangement, with 14.5-nm axial spacing between successive levels of four bridges, and a helical repeat period every 43.5 nm, The orientation of cross-bridges on the surface of tarantula filaments is also likely to be very similar to that on Limulus filaments as suggested by the similarity between filtered images of the two types of filaments and the radial distance of the centers of mass of the cross-bridges from the surfaces of both types of filaments. Tarantula filaments, however, have smaller diameters than Limulus fi.laments, contain less paramyosin, and display structure that probably reflects the organization of the filament backbone which is not as apparent in images of Limulus filaments. We suggest that the similarities between Limulus and tarantula thick filaments may be governed, in part, by the close evolutionary relationship of the two species.Using a modified isolation procedure and improved electron microscopic techniques together with computer image and optical diffraction analyses, we recently described the structure of the myosin cross-bridge arrangement on the surface of negatively stained thick filaments isolated from unstimulated Limulus telson levator muscle (15,22,30). Four crossbridges extend from the surface of these filaments at each level, with an axial rise of 14.5 nm between adjacent levels and a helical repeat period of 43.5 nm, as predicted by Wray et al. (36) and confirmed by computer image reconstruction (30). Furthermore, by unidirectional metal-shadowing we determined that the myosin cross-bridges are disposed in a major right-handed helix, having 12 subunits per complete turn (175 nm), on the filament surface (16).Wray (35) recently reported that the x ray patterns from glycerinated, relaxed tarantula leg muscle resemble those he earlier obtained from similar preparations of Limulus muscle (36). Here we present the results of an electron microscopic, optical diffraction, and biochemical analysis of the structure and paramyosin content, respectively, of thick filaments from tarantula femur muscle fibers and compare them with those from our previous analysis of Limulus telson muscle thick filaments (15,16,22,30). MATERIALS AND METHODSTarantula (Eurypelma sp.) specimens (sex unknown) were purchased from C...
Abstract. Here we present evidence that strongly suggests that the well-documented phenomenon of A-band shortening in Limulus telson muscle is activation dependent and reflects fragmentation of thick filaments at their ends.Calcium activation of detergent-skinned fiber bundles of Limulus telson muscle results in large decreases in A-band (from 5.1 to 3.3/zm) and thick filament (from 4.1 to 3.3 t~m) lengths and the release of filament end fragments. In activated fibers, maintained stretched beyond overlap of thick and thin filaments, these end fragments are translocated to varying depths within the I-bands. Here they are closely associated with fine filamentous structures that also span the gap between A-and I-bands and attach to the distal one-third of the thick filaments. End-fragments are rarely, if ever, present in similarly stretched and skinned, but unstimulated fibers, although fine "gap filaments" persist. Negatively stained thick filaments, separated from skinned, calcium-activated, fiber bundles, allowed to shorten freely, are significantly shorter than those obtained from unstimulated fibers, but are identical to the latter with respect to both the surface helical array of myosin heads and diameters. Many end-fragments are present on grids containing thick filaments from activated fibers; few, if any, on those from unstimulated fibers. SDS-PAGE shows no evidence of proteolysis due to activation and demonstrates the presence of polypeptides with very high molecular weights in the preparations. We suggest that thick filament shortening is a direct result of activation in Limulus telson muscle and that it occurs largely by breakage within a defined distal region of each polar half of the filament. It is possible that at least some of the fine "gap illaments" are composed of a titin-like protein. They may move the activation-produced, fragmented ends of thick filaments to which they attach, into the 1-bands by elastic recoil, in highly stretched fibers.
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