¹ European Molecular Biology Laboratory, Meyerhofstrasse 1, 69117 Heidelberg, Germany.
² Department of Biology, University of York, York Y01 5DD, UK.
³ Current address: Hamon Center for Basic Cancer Research, University of Texas Southwestern Medical Center at Dallas, Dallas, TX 75235-9148, USA.

^* Author to whom all correspondence should be addressed.

Tel: + 49 6221 387 241
Fax: + 49 6221 387 306
email: clayton@embl-heidelberg.de

Keywords: Glutathione transferase, Troponin, Drosophila, flight, muscle, genetics

Summary

Drosophila indirect flight muscles (IFMs) contain a 35 kDa protein which cross-reacts with antibodies to the IFM specific protein TnH-34. Peptide finger-printing and peptide sequencing showed that this 35 kDa protein is glutathione S-transferase-2 (GST-2). GST-2 is present in the asynchronous indirect flight muscles but not in the synchronous tergal depressor of the trochanter (jump muscle). Genetic dissection of the sarcomere showed that GST-2 is stably associated with the thin filaments but the presence of myosin is required to achieve the correct stoichiometry, suggesting that there is also an interaction with the thick filament.

The two Drosophila TnHs (isoforms -33 and -34) are naturally occurring fusion proteins in which a proline-rich extension of ~ 250 a.a. replaces the 27 C-terminal residues of the muscle specific tropomyosin II isoform. The proteolytic enzyme, Ig-ase, cleaves the hydrophobic C-terminal sequence of TnH-34 at three sites and TnH-33 at one site. This results in the release of GST-2 from the myofibril. The amount of GST-2 stably bound to the myofibril is directly proportional to the total amount of undigested TnH. It is concluded that GST-2 in the thin filament is stabilised there by interaction with TnH. We speculate that the hydrophobic N-terminal region of GST-2 interacts with the hydrophobic C-terminal extension of TnH and that both are close to a myosin crossbridge.