Structure of the Insect Thick Filaments

Abstract

Myosin filaments of insect indirect flight muscles (IFM) are 17 to 19 nm thick and 1.9 to 3.6 μm long structures with probably 4 cross-bridges per level (= crown). These crowns repeat in periods of 14.5 nm along the longitudinal axis of the filament. The cross-bridges are located at 4 helical tracks with axial spacings of 38.7 nm and a true axial repeat of 116 nm on the surface of the filaments. Twelve myosin subfilaments, arranged in pairs, run parallel to the longitudinal filament axis and form a wall around a myosin-free filament core. The core may be filled by additional elements, the number of which is related to the paramyosin content of the filaments.

Aggregates of peptide fragments representing the C-terminal two-thirds of Drosophila light meromyosin with the exon-19 encoded C-terminus display 116 nm repeating units with substructured elements of 43.5-29-14.5-29 nm in width. Optical diffraction patterns of these aggregates show layer line characteristics that resemble those of negatively stained, isolated thick filaments. These results suggest that the IFM specific cross-bridge pattern results from the aggregation properties of insect myosin. Filament models, based on these properties are consistent with electron microscopy and X-ray diffraction data.