Axoneme-specific beta-tubulin specialization: a conserved C-terminal motifspecifies the central pair

Axonemes are ancient organelles that mediate motility of cilia and flagella in animals, plants, and protists, The long evolutionary conservation of ax oneme architecture, a cylinder of nine doublet microtubules surrounding a c entral pair of singlet microtubules, suggests all motile axonemes may share common assembly mechanisms. Consistent with this, alpha- and beta -tubulin s utilized in motile axonemes fall among the most conserved tubulin sequenc es [1, 2], and the beta -tubulins contain a sequence motif at the same posi tion in the carboxyl terminus [3]. Axoneme doublet microtubules are initiat ed from the corresponding triplet microtubules of the basal body [4], but t he large macromolecular ''central apparatus" that includes the central pair microtubules and associated structures [5] is a specialization unique to m otile axonemes, In Drosophila spermatogenesis, basal bodies and axonemes ut ilize the same alpha -tubulin but different beta -tubulins [6-13]. beta1 is utilized for the centriole/ basal body, and beta2 is utilized for the moti le sperm tail axoneme, beta2 contains the motile axoneme-specific sequence motif, but beta1 does not [3]. Here, we show that the "axoneme motif" speci fies the central pair. beta1 can provide partial function for axoneme assem bly but cannot make the central microtubules [14], Introducing the axoneme motif into the pr carboxyl terminus, a two amino acid change, conferred upo n beta1 the ability to assemble 9 + 2 axonemes, This finding explains the c onservation of the axoneme-specific sequence motif through 1.5 billion year s of evolution.