Axoneme-specific beta-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]. beta 1 is utilized for the centriole/basal body, and beta 2 is utilized for the motile sperm tail axoneme. beta 2 contains the motile axoneme-specific sequence motif, but beta 1 does not 3]. Here, we show that the "axoneme motif" specifies the central pair. beta 1 can provide partial function for axoneme assembly but cannot make the central microtubules 14]. Introducing the axoneme motif into the beta 1 carboxyl terminus, a two amino acid change, conferred upon beta 1 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.