Mechanics and Protein Content of Insect Flight Muscles

Abstract

ABSTRACT 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 80×103), 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.