| Abstract: | Summary
Drosophila indirect flight muscles (IFMs) contain a 35 kDa protein which cross-reacts with antibodies to the IFM specific
protein troponin-H isoform 34 (TnH-34). Peptide fingerprinting 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 amino acids replaces the 27 C-terminal residues of the muscle-specific
tropomyosin II isoform. The proteolytic enzyme, Igase, 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
stabilized 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 cross-bridge.
This revised version was published online in August 2006 with corrections to the Cover Date. |
