EFFECTS OF CALCIUM AND NUCLEOTIDES ON THE STRUCTURE OF INSECT FLIGHT-MUSCLE THIN-FILAMENTS

The structure of the insect flight muscle thin filament has been studi ed using a Drosophila mutant (Ifm(2)2) which does not contain thick fi laments. Thin filaments that are biochemically identical to those of t he wild type can be isolated free from thick filament contamination. W e show that isolated thin filaments have different symmetries dependin g upon the calcium concentration. While the filaments mainly contain 1 3 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 gene tic helix at calcium concentrations equivalent to those present during muscle contraction. We also show that the structure (mainly the helic al order) of the thin filaments depends on the nature of the nucleotid e bound to the actin monomers. Three-dimensional reconstructions of th e thin filaments in the presence and absence of calcium show that trop omyosin moves between two different positions on the actin filament. H owever, in Drosophila the amplitude of the movement as well as the dis order in the positions of the components (tropomyosin, troponin comple x) are larger than those generally observed in other species. (C) Chap man & Hall Ltd.