To elucidate the mechanism of oscillatory bending in cilia and flagella, we studied the effect of protease digestion on the response of axonemes to localized application of ATP. When the axonemes were treated with elastase and then reactivated locally by ATP iontophoresis, a pair of local bends were formed due to localized unidirectional sliding in the vicinity of the ATP pipette. Upon repeated application of ATP, the direction of bending with respect to the sperm head axis changed cyclically from side to side over several cycles. The bends were planar and similar to those observed in axonemes that had not been treated with elastase. In trypsin-treated axonemes, in contrast, repetitive local reactivation did not induce such cyclical bending; instead, it induced a bend that grew only in one direction upon repeated application of ATP. Moreover, the bends were not planar. Electron microscopy of these protease-digested axonemes showed that both the interdoublet (nexin) links and the radial spokes were disrupted, but the effects of these proteases were different; trypsin disrupted 60–70% of these structures whereas elastase disrupted 20–30% of them. In both cases, spokes no. 3 and no. 8 (and no. 7) were more resistant to digestion than the others, although they tended to be more resistant to elastase than to trypsin. The importance of radial spokes and interdoublet links in the generation of cyclical bending and the determination of the bending plane is discussed.

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