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Abstract

The structure of the insect flight muscle thin filament has been studied using a Drosophila mutant (Ifm(2)2) which does not contain thick filaments. Thin filaments that are biochemically identical to those of the wild type can be isolated free from thick filament contamination. We show that isolated thin filaments have different symmetries depending upon the calcium concentration. While the filaments mainly contain 13 subunits in six turns of the 5.9 nm genetic helix in the absence of calcium, 50% of the filaments have 28 subunits in 13 turns of the genetic helix at calcium concentrations equivalent to those present during muscle contraction. We also show that the structure (mainly the helical order) of the thin filaments depends on the nature of the nucleotide bound to the actin monomers. Three-dimensional reconstructions of the thin filaments in the presence and absence of calcium show that tropomyosin moves between two different positions on the actin filament. However, in Drosophila the amplitude of the movement as well as the disorder in the positions of the components (tropomyosin, troponin complex) are larger than those generally observed in other species.

Through thick and thin: dual regulation of insect flight muscle and cardiac muscle compared

“…Actin in the thin filaments of IFM has a half pitch of 38.7 nm, compared to 36.0 nm in vertebrate actin (Reedy and Reedy 1985; AL-Khayat et al 2003). The longer half pitch in IFM is not an intrinsic property of the actin, but is likely to be due to the constraining influence of other thin filament proteins (Ruiz et al 1998). The periodicity of troponin (Tn) on the thin filament is also 38.7 nm and all Tns on the filament have the same azimuthal orientation relative to the two neighbouring thick filaments.…”