Tails of Tetrahymena

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Abstract
The source of force generation of beating cilia and flagella is an interaction between the doublet microtubules mediated by the dynein-1 arms which caused the doublets to slide relative to one another. Direct sliding of Tetrahymena ciliary axonemes was previously demonstrated by dark field light microscopy. The results of such an experiment were capatured on a polylysine-coated grid surface for whole-mount EM. Images in which sliding between doublets took place were identified. Doublets slide relative to one another with a constant polarity. To produce the observed displacement, the direction of the dynein-1 arm force generation must be from base to tip, so that the doublet (n), to which the arms are attached, pushes the next doublet (n + 1) toward the tip. In addition to the functional polarity, the dynein-1 arms have a structural polarity: they tilt toward the base when viewed along the edges of the A-subfiber. A scheme is presented which reconciles the finding of a single polarity of active sliding with the geometry of microtubule tip displacement of bent cilia.