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Alterations in flight muscle ultrastructure and function in Drosophila tropomyosin mutants

By Andrew J. Kreuz, A Simcox and David Maughan


Abstract. Drosophila indirect flight muscle (IFM) contains two different types of tropomyosin: a standard 284-amino acid muscle tropomyosin, Ifm-TmI, encoded by the TmI gene, and two>400 amino acid tropomyosins, TnH-33 and TnH-34, encoded by TmlI. The two IFM-specific TnH isoforms are unique tropomyosins with a COOH-terminal extension of ~200 residues which is hydrophobic and rich in prolines. Previous analysis of a hypomorphic Tml mutant, Ifm (3)3, demonstrated that Ifm-TmI is necessary for proper myofibrillar assembly, but no null TmI mutant or TmH mutant which affects the TnH isoforms have been reported. In the current report, we show that four flightless mutants (Warmke et al., 1989) are alleles of TmI, and characterize a deficiency which deletes both TrnI and TmlI. We find that haploidy of TmI causes myo-T ROPOMYOSIN is an a-helical protein which is associated with actin filaments and has a regulatory and structural role in muscle and nonmuscle cells. In striated muscle, tropomyosin (Tm) 1 acts with troponin (Tn) to regulate actomyosin interactions in response to changes in Ca 2+ concentration (reviewed in EI-Saleh et al., 1986; Chalovich, 1993). At low Ca 2+ concentrations (<10-7 M), the Tm-Tn complex prevents the formation of strong, force-generating actomyosin cross-bridges, suppressing the actin-activated Mg2+-ATPase activity of myosin. This suppression is relieved when the Ca 2 ÷ concentration rises to ~10-5 M. Models have been proposed to explain the function of tropomyosin. In the steric blocking model (Haselgrov

Year: 1996
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