THE ROLE OF CENTRAL APPARATUS COMPONENTS IN FLAGELLAR MOTILITY AND MICROTUBULE ASSEMBLY

In order to generate the complex waveforms typical of beating cilia an d flagella, the action of the dynein arms must be regulated. This regu lation not only depends on the presence of multiple dynein isoforms, b ut also clearly involves other structures in the axoneme such as the r adial spokes and central apparatus; mutants lacking these structures h ave paralyzed flagella. In this article, we review recent progress in identifying protein components of the central apparatus and discuss th e role of these components in regulation of flagellar motility and cen tral apparatus assembly. The central apparatus is composed of two sing le microtubules and their associated structures which include the cent ral pair projections, the central pair bridges linking the two tubules , and the central pair caps which are attached to the distal or plus e nds of the microtubules. To date, the genes encoding four components o f the central apparatus have been cloned, PF15, PF16, PF20 and KLP1. P F16, PF20 and KLP1 have been sequenced and their,gene products localiz ed. Two additional components have been identified immunologically, a 110 kD polypeptide recognized by an antibody generated against highly conserved kinesin peptide sequence, and a 97 kD polypeptide recognized by CREST antisera. Based on a variety of data, one model that has eme rged to explain the role of the central apparatus in flagellar motilit y is that the central apparatus ultimately regulates dynein through in teractions with the radial spokes. The challenge now is to determine t he precise mechanism by which the polypeptides comprising the central apparatus and the radial spokes interact to transduce a regulatory sig nal to the dynein arms. In terms of assembly, the central apparatus mi crotubules assemble with their plus ends distal to the cell body but, unlike the nine doublet microtubules, they are not nucleated from the basal bodies. Since some central apparatus defective mutants fail to a ssemble the entire central apparatus, their gene products may eventual ly prove to have microtubule nucleating or stabilizing properties. By continuing to identify the genes that encode central apparatus compone nts, we will begin to understand the contribution of these microtubule s to flagellar motility and gain insight into their nucleation, assemb ly, and stability. (C) 1997 Wiley-Liss, Inc.