Axoneme-specific β-tubulin specialization:: a conserved C-terminal motif specifies the central pair

Axonemes are ancient organelles that mediate motility of cilia and flagella in animals, plants, and protists, The long evolutionary conservation of axoneme architecture, a cylinder of nine doublet microtubules surrounding a central pair of singlet microtubules, suggests all motile axonemes may share common assembly mechanisms. Consistent with this, alpha- and beta -tubulins utilized in motile axonemes fall among the most conserved tubulin sequences [1, 2], and the beta -tubulins contain a sequence motif at the same position in the carboxyl terminus [3]. Axoneme doublet microtubules are initiated from the corresponding triplet microtubules of the basal body [4], but the large macromolecular ''central apparatus that includes the central pair microtubules and associated structures [5] is a specialization unique to motile axonemes, In Drosophila spermatogenesis, basal bodies and axonemes utilize the same alpha -tubulin but different beta -tubulins [6-13]. beta1 is utilized for the centriole/ basal body, and beta2 is utilized for the motile sperm tail axoneme, beta2 contains the motile axoneme-specific sequence motif, but beta1 does not [3]. Here, we show that the axoneme motif specifies the central pair. beta1 can provide partial function for axoneme assembly but cannot make the central microtubules [14], Introducing the axoneme motif into the pr carboxyl terminus, a two amino acid change, conferred upon beta1 the ability to assemble 9 + 2 axonemes, This finding explains the conservation of the axoneme-specific sequence motif through 1.5 billion years of evolution.