An alternative domain near the ATP binding pocket of Drosophila myosin affects muscle fiber kinetics
Douglas M Swank 1*, Joan Braddock 2, Waylon Brown 2, Heather Lesage 2, Sanford I Bernstein 3 and David W Maughan 2
1 Rensselaer Polytechnic Institute
2 University of Vermont
3 San Diego State University
* To whom correspondence should be addressed. E-mail: swankd{at}rpi.edu.
Submitted on September 27, 2005
Revised on November 17, 2005
Accepted on 28 November 2005
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Abstract |
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We examined the importance of alternative versions of a region near the ATP binding site of Drosophila myosin heavy chain (MHC) on muscle mechanical properties. Previously, we exchanged two versions of this region (encoded by alternative exon 7s) between the indirect flight muscle myosin isoform (IFI) and an embryonic myosin isoform (EMB) and found, surprisingly, that in vitro solution actin-activated ATPase rates were increased (higher Vmax) by both exon exchanges. Here we examined the effect of increased ATPase rate on indirect flight muscle (IFM) fiber mechanics and Drosophila locomotion. IFM expressing EMB with the exon 7a domain replaced by the IFM specific exon 7d domain (EMB-7d) exhibited 3.2-fold greater maximum oscillatory power (Pmax) and 1.5-fold greater optimal frequency of power generation (fmax) versus fibers expressing EMB. In contrast, IFM expressing IFI with the exon 7d region replaced by the EMB exon 7a region (IFI-7a), showed no change in Pmax, fmax, step response, or isometric muscle properties compared to native IFI fibers. A slight decrement in IFI-7a flight ability was observed, suggesting a negative influence of the increased ATPase rate on Drosophila locomotion, perhaps due to energy supply constraints. Our results show that exon 7 plays a substantial role in establishing fiber speed and flight performance, and that the limiting step that sets ATPase rate in Drosophila myosin has little to no direct influence in setting fmax for fast muscle fiber types.
Key Words:
actomyosin, cross-bridge, flight, muscle mechanics, wing beat frequency
