The orientation and configuration of the central-pair microtubules in cilia were studied by serial thin-section analysis of "instantaneously fixed" paramecia. Cilia were frozen in various positions in metachronal waves by such a fixation . The spatial sequence of these positions across the wave represents the temporal sequence of the positions during the active beat cycle of a cilium . Systematic shifts of central-pair orientation across the wave indicate that the central pair rotates 360°counterclockwise (viewed from outside) with each ciliary beat cycle (C . K. Omoto, 1979, Thesis, University of Wisconsin, Madison; C. K. Omoto and C. Kung, 1979, Nature [Lond .] 279:532-534) . This is true even for paramecia with different directions of effective stroke as in forward-or backward-swimming cells. The systematic shifts of centralpair orientation cannot be seen in Ni"-paralyzed cells or sluggish mutants which do not have metachronal waves. Both serial thin-section and thick-section high-voltage electron microscopy show that whenever a twist in the central pair is seen, it is always left-handed . This twist is consistent with the hypothesis that the central pair continuously rotates counterclockwise with the rotation originating at the base of the cilium . That the rotation of the central pair is most likely with respect to the peripheral tubules as well as the cell surface is discussed . These results are incorporated into a model in which the central-pair complex is a component in the regulation of the mechanism needed for three-dimensional ciliary movement .Cilia (eucaryotic flagella and sperm tails) have been studied to determine the function of the components responsible for generating motion (reviewed in references 4, 14, and 40) . The sliding-microtubule model is now accepted as the fundamental motile mechanism for cilia . In the late 1960's Satir (39) produced the first evidence for sliding rather than contraction as the basis for ciliary motion. The sliding-microtubule hypothesis was given support by the direct demonstration of sliding disintegration in trypsin-treated axonemes by Summers and Gibbons (46) . The force that causes the ciliary microtubules to slide is generated by the pair of dynein arms that project out periodically along the length of each peripheral microtubule doublet (11). Sliding was shown to be unipolar by Sale and Satir (37) in Tetrahymena cilia .Studies indicate some interaction between the radial spokes of the peripheral doublets and the projections from the central pair (47,51) . It is suggested that such an interaction converts sliding to bending, but how sliding of the peripheral doublets is transduced into active coordinated beating of cilia is not known . The functions in motility of such components as the radial spokes and the central-tubule complex are largely un-
We investigated the swimming patterns of trout sperm using computer-assisted analyses of video microscopy. Under full activation conditions, in which 80-100% of sperm activate their motility, sperm swim in circular paths for 2-5 sec, followed by 30-60 sec of a more linear swimming, and, finally, cessation of movement, with a straightening of the flagella. Threshold activation, in which 50% of the sperm activate, is characterized by circular patterns of swimming for less than 20 sec, with straightened flagella upon cessation. Full activation and threshold activation are observed in low-K' solution or in an Mg+ +-supplemented K + solution. Similarities in swimming patterns in low-K+ solution and in a Mg + +-supplemented K' solution suggest a common underlying mechanism of activation. Initiation of movement in solutions with high C a t ' to K + ratio is similar to activation in K'-free solution. However, sperm in C a t + -supplemented media resume circular swimming within 20-25 sec after activation, and, upon cessation of movement, the flagella are frequently cane shaped or bent. Differences in swimming patterns upon activation by high Cat
Abstract. Using the CHO2 monoclonal antibody raised against CHO spindles (Sellitto, C., M. Kimble, and R. Kuriyama. 1992. Cell Motil. Cytoskeleton.22:7-24) we identified a 66-kD protein located at the interphase centrosome and mitotic spindle. Isolated cDNAs for the antigen encode a 622-amino acid polypeptide. Sequence analysis revealed the presence of 340-amino acid residues in the COOH terminus, which is homologous to the motor domain conserved among other members of the kinesin superfamily. The protein is composed of a central u-helical portion with globular domains at both NH2 and COOH termini, and the epitope to the monoclonal antibody resides in the central a-helical stalk. A series of deletion constructs were created for in vitro analysis of microtubule interactions. While the microtubule binding and bundling activities require both the presence of the COOH terminus and the o~-helical domain, the NH2-
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