Abstract
Motile cilia and flagella are essential, highly conserved organelles, and their motility is driven by the coordinated activities of multiple dynein isoforms. The prevailing "switch-point" hypothesis posits that dyneins are asymmetrically activated to drive flagellar bending. To test this model, we applied cryo-electron tomography to visualize activity states of individual dyneins relative to their locations along beating flagella of sea urchin sperm cells. As predicted, bending was generated by the asymmetric distribution of dynein activity on opposite sides of the flagellum. However, contrary to predictions, most dyneins were in their active state, and the smaller population of conformationally inactive dyneins switched flagellar sides relative to the bending direction. Thus, our data suggest a "switch-inhibition" mechanism in which force imbalance is generated by inhibiting, rather than activating, dyneins on alternating sides of the flagellum.