The distal ends of ciliary microtubules are attached to the membrane by microtubule-capping structures. The capping structures are located at the sites of tubulin addition and loss in vivo and may be part of the regulatory system that directs ciliary and flagellar microtubule assembly. This study describes conditions for the release and stabilization of microtubule capping structures as a first step in their purification. Two types of capping structures, the distal filaments and the central microtubule caps, are selectively and independently released from the axoneme by CaCl2 and MgCl2 but not by MgSO4, ZnCl2, NaCl, KCl, or KI. The release of the caps and filaments is specific for Ca+2, Mg+2, and Cl- and is not simply a function of ionic strength. The capping structures are released without major disruption of the axonemal structure. In addition to providing a means to purify and identify the cap and filament components, these results suggest ways in which their binding to the axoneme may be modulated during periods of microtubule growth or shortening. This report also reveals that the distal filaments are composed of two separable components, a small bead inserted into the end of each A-tubule and a "Y"-shaped plug and filament that slips through the bead.

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