Mechanism of Ciliary Movement

Abstract

Eukaryotic cilia and flagella have a common ultrastructure, nine outer doublet microtubules surrounding a pair of central microtubules, although their motility pattern is different from each other. There are water-propelling cilia such as mussel gill cilia and mucus-propelling ones such as mammalian tracheal cilia. The main components of cilia and flagella are an ATPase protein, dynein, and the constituent of microtubule, tubulin. These proteins are different from their counterpart in muscle, myosin and actin, respectively. Two rows of the arms projecting from the A-tubule of each doublet microtubule consist of dynein. Interaction between dynein and tubulin underlies the basic mechanism of ciliary and flagellar movement. The cilia and flagella demembranated with Triton X-100 can be reactivated by adding ATP, showing similar movement to that of intact ones. When the fragments of demembranated cilia and flagella are digested with trypsin to take off the hoops such as the radial spokes and the interdoublet links, adjacent doublets slide with each other one-dimensionally on addition of ATP. The sliding is mediated by the dynein arms, since it is prevented with anti-dynein antibody or vanadate, an inhibitor of dynein ATPase. It has been postulated that one side of cilium, e.g. doublet Nos. 1 to 5, shows active sliding in the effective stroke, whereas the other side, e.g. doublet Nos. 6 to 9, is active in the recovery stroke. How such local sliding is converted to local bending is still an open question. The initiation mechanism of ciliary and flagellar motion is another undissolved problem. In Kartagener's and other immotile-cilia syndrome, aberrant axonemal structures such as the arm- or the radial spoke-central pair complexdeficiency or the dislocation of outer doublets, etc., have been observed. Such defects in the axonemal structure have also been reported in several paralyzed mutants of Chlamydomonas.