Direction of active sliding of microtubules in Tetrahymena cilia.

Axonemes of protozoan (Tetrahymena thermophila BIII) cilia, isolated by the dibucaine method, were treated briefly with trypsin after removal of the ciliary membranes by treatment with Triton X-100. After attachment to polylysine-coated surfaces, the partially digested axonemes remained mainly intact cylinders. Such attached axonemes can be treated with ATP, which induces microtubles sliding. ATP-treated preparations showed disrupted axonemes in which doublets had telescoped out of the original cylinders. These could be captured in place for electron microscopy after critical point drying. Images of this type were used to determine relative movement between adjacent doublet microtubules. Each doublet actively slid relative to its neighbors in a single direction, in which the polarity of force generation of the dynein arms was from base to tip.