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Drosophila Muscle Regulation Characterized by Electron Microscopy and Three-Dimensional Reconstruction of Thin Filament Mutants

Anthony Cammarato ^*, Victoria Hatch ^*, Judith Saide ^*, Roger Craig , John C. Sparrow , Larry S. Tobacman ^¶ and William Lehman ^*

^* Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts; Department of Cell Biology, University of Massachusetts Medical School, Worcester, Massachusetts; Department of Biology, University of York, York, United Kingdom; and ^¶ Departments of Biochemistry and Internal Medicine, University of Iowa College of Medicine, Iowa City, Iowa

Correspondence: Address reprint requests to William Lehman, Dept. of Physiology and Biophysics, Boston University School of Medicine, 715 Albany St., Boston, MA 02118-2526. Tel.: 617-638-4397; Fax: 617-638-4273; E-mail: wlehman{at}bu.edu.

Wild-type and mutant thin filaments were isolated directly from "myosinless" Drosophila indirect flight muscles to study the structural basis of muscle regulation genetically. Negatively stained filaments showed tropomyosin with periodically arranged troponin complexes in electron micrographs. Three-dimensional helical reconstruction of wild-type filaments indicated that the positions of tropomyosin on actin in the presence and absence of Ca²⁺ were indistinguishable from those in vertebrate striated muscle and consistent with a steric mechanism of regulation by troponin-tropomyosin in Drosophila muscles. Thus, the Drosophila model can be used to study steric regulation. Thin filaments from the Drosophila mutant heldup², which possesses a single amino acid conversion in troponin I, were similarly analyzed to assess the Drosophila model genetically. The positions of tropomyosin in the mutant filaments, in both the Ca²⁺-free and the Ca²⁺-induced states, were the same, and identical to that of wild-type filaments in the presence of Ca²⁺. Thus, cross-bridge cycling would be expected to proceed uninhibited in these fibers, even in relaxing conditions, and this would account for the dramatic hypercontraction characteristic of these mutant muscles. The interaction of mutant troponin I with Drosophila troponin C is discussed, along with functional differences between troponin C from Drosophila and vertebrates.

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