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Journal of Experimental Biology 168,57-76 (1992)
Published by Company of Biologists 1992

Mechanics and Protein Content of Insect Flight Muscles

MICHELLE PECKHAM ¹, RICHARD CRIPPS ², DAVID WHITE ³, and BELINDA BULLARD ⁴

¹ Department of Biology, University of York York, Y01 5DD; Department of Biophysics, Kings College London, 26-29 Drury Lane, London WC2B 5RL.
² Department of Biology, University of York York, Y01 5DD; Department of Biology and Molecular Biology Institute, San Diego State University, San Diego, CA 92182-0057, USA.
³ Department of Biology, University of York York, Y01 5DD
⁴ EMBL Postfach 10.2209, Heidelberg, Germany, D-6900

In asynchronous insect flight muscles, stretch activation may arise from a matching of the helix periodicities of actin target sites to myosin heads and/or a special form of troponin subunit called troponin-H (Tn-H, relative molecular mass 80x10³), which has so far only been found in the asynchronous flight muscles of Drosophila (Diptera) and Lethocerus (Hemiptera). The sequence of Tn-H in Drosophila shows it to be a fusion protein of tropomyosin and a hydrophobic proline-rich sequence. Tn-H in Lethocerus is immunologically similar. From immunoblots of synchronous (non-stretch-activated) and asynchronous flight muscles from a wide range of insects, using antibodies against tropomyosin and the hydrophobic sequence of Tn-H, raised against Lethocerus proteins, we found two forms of Tn-H. One, found in most flight muscles, only reacted with antibodies to the hydrophobic sequence. The other, found in asynchronous flight muscles from Diptera and Hemiptera, reacted with antibodies to both the hydrophobic sequence and to tropomyosin, although in the Hemiptera the reaction of Tn-H with anti-tropomyosin was weak. When we compared the mechanics of most of the asynchronous flight muscles used in this study, we found that in the Diptera Ca²⁺-activated tension was much lower at rest length and stretch-activated tension was more highly dependent on muscle length than in other orders of insects. This suggests that, when Tn-H is in the form found in Diptera, it may be able to modulate Ca²⁺-activated and stretch-activated tension. We conclude that Tn-H is not sufficient for stretch-activation, but it may enhance stretch-activation, particularly when it is in the form found in the Diptera.

Key words: muscle, insect, troponin-H, stretch-activation

Accepted on March 31, 1992

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