CALCIUM ions are probably a general regulator of the assembly state and function of most cytoplasmic microtubules, the organelles upon which processes such as cell division, transport and secretion, as well as ciliary motility, may finally depend. Ciliary activity is the best model we have of microtubule function and the cilium is the only microtubule-based system allowing extensive systematic dissection at present. We are therefore interested in the mechanism by which increasing cytoplasmic Ca2+ causes changes in ciliary beat parameters, particularly an arrest response in the mussel (Elliptio or Mytilus) gill1,2. It is well known that the beat in cilia is the result of a sliding interaction between doublet microtubules in the axoneme3,4, as can be demonstrated directly when axonemes are treated briefly with trypsin5. On addition of ATP the trypsin-treated axonemes no longer reactivate to beat normally; instead, they telescope apart. We demonstrate here that, although all sliding of the ciliary microtubules stops during ciliary arrest in vivo, the sliding interaction seen after trypsin treatment is not directly inhibited by Ca2+.
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