The fastest muscles are powered by a motor that is comparatively averse to energy-providing ATP, as shown by Douglas Swank (Rensselaer Polytechnic Institute, Troy, NY), Vivek Vishnudas, and David Maughan (University of Vermont, Burlington, VT).
The muscles that power Drosophila flight contract over 200 times per second. To determine what makes their myosin motor work so quickly, Swank et al. compared contraction speeds of flight myosins with a slower embryonic myosin while varying levels of ATP, whose hydrolysis fuels contraction, and its byproducts, Pi and ADP.
Previous studies established that the speeds of slower myosins are limited by how quickly they releases ADP. But Swank found that the flight myosin is instead held back by Pi release. “To increase speed,” Swank reasons, “the ADP release step must be faster. Here, it's gotten so fast that Pi release becomes limiting.”
As ATP is needed for every contraction cycle, “you might think [fast myosin] would want to bind ATP [even tighter],” says Swank. “But that's not the case.” The slow myosin had the higher affinity for ATP. Flight myosin's lower ATP affinity might be a side effect of its faster release of structurally similar ADP.
Flies probably compensate by maintaining very high ATP concentrations. Their flight muscles seem to have enough mitochondria for the task, but Swank plans to measure in vivo ATP levels directly to be sure. By swapping domains of fast and slow myosins, he also hopes to find the structural differences that evolved to power such hustle.