The Chlamydomonas PF6 locus encodes a large alanine/proline-rich polypeptide that is required for assembly of a central pair projection and regulatesflagellar motility

Efficient motility of the eukaryotic flagellum requires precise temporal an d spatial control of its constituent dynein motors. The central pair and it s associated structures have been implicated as important members of a sign al transduction cascade that ultimately regulates dynein arm activity. To i dentify central pair components involved in this process, we characterized a Chlamydomonas motility mutant (pf6-2) obtained by insertional mutagenesis . pf6-2 flagella twitch ineffectively and lack the 1a projection on the C1 microtubule of the central pair. Transformation with constructs containing a full-length, wild-type copy of the PF6 gene rescues the functional, struc tural, and biochemical defects associated with the pf6 mutation. Sequence a nalysis indicates that the PF6 gene encodes a large polypeptide that contai ns numerous alanine-rich, proline-rich, and basic domains and has limited h omology to an expressed sequence tag derived from a human testis cDNA libra ry. Biochemical analysis of an epitope-tagged PF6 construct demonstrates th at the PF6 polypeptide is an axonemal component that cosediments at 12.6S w ith several other polypeptides. The PF6 protein appears to be an essential component required for assembly of some of these polypeptides into the Cl-1 a projection.