Characterization of a Chlamydomonas insertional mutant that disrupts flagellar central pair microtubule-associated structures

Two alleles at a new locus, central pair-associated complex 1 (CPC1), were selected in a screen for Chlamydomonas flagellar motility mutations. These mutations disrupt structures associated with central pair microtubules and reduce flagellar beat frequency, but do not prevent changes in flagellar ac tivity associated with either photophobic responses or phototactic accumula tion of live cells. Comparison of cpc1 and pf6 axonemes shows that cpc1 aff ects a row of projections along C1 microtubules distinct from those missing in pf6, and a row of thin fibers that form an are between the two central pair microtubules. Electron microscopic images of the central pair in axone mes from radial spoke-defective strains reveal previously undescribed centr al pair structures, including projections extending laterally toward radial spoke heads, and a diagonal link between the C2 microtubule and the cpc1 p rojection. By SDS-PAGE, cpc1 axonemes show reductions of 350-, 265-, and 79 -kD proteins. When extracted from wild-type axonemes, these three proteins cosediment on sucrose gradients with three other central pair proteins (135 , 125, and 56 kD) in a 16S complex. Characterization of cpc1 provides new i nsights into the structure and biochemistry of the central pair apparatus, and into its function as a regulator of dynein-based motility.