Alternative N-terminal regions of Drosophila myosin heavy chain tune muscle kinetics for optimal power output.

We assessed the influence of alternative versions of a region near the N-terminus of Drosophila myosin heavy chain on muscle mechanical properties. Previously, we exchanged N-terminal regions (encoded by alternative exon 3s) between an embryonic (EMB) isoform and the indirect flight muscle isoform (IFI) of myosin, and demonstrated that it influences solution ATPase rates and in vitro actin sliding velocity. Because each myosin is expressed in Drosophila indirect flight muscle, in the absence of other myosin isoforms, this allows for muscle mechanical and whole organism locomotion assays. We found that exchanging the flight muscle specific exon 3 region into the embryonic isoform (EMB-3b) increased maximum power generation (P(max)) and optimal frequency of power generation (f(max)) threefold and twofold compared to fibers expressing EMB, whereas exchanging the embryonic exon 3 region into the flight muscle isoform (IFI-3a) decreased P(max) and f(max) to approximately 80% of IFI fiber values. Drosophila expressing IFI-3a exhibited a reduced wing beat frequency compared to flies expressing IFI, which optimized power generation from their kinetically slowed flight muscle. However, the slower wing beat frequency resulted in a substantial loss of aerodynamic power as manifest in decreased flight performance of IFI-3a compared to IFI. Thus the N-terminal region is important in tuning myosin kinetics to match muscle speed for optimal locomotory performance.