Attachment of the cap to the central microtubules of Tetrahymena cilia.

The central microtubule cap is bound to the ends of the two central microtubules in Tetrahymena thermophila cilia by plug-shaped structures similar in appearance to the distal filament plugs attached to the ends of the A-microtubules. The caps have been separated from the microtubules and are composed of a bead, two plates, and two peg-like plugs to which the microtubules are attached. The structure of the cap is discussed in relation to the directionality of microtubule assembly in vivo.