Axoneme specialization embedded in a "generalist" beta-tubulin.

The relationship between the primary structure of the beta-tubulin C-terminal tail (CTT) and axoneme structure and function is explored using the spermatogenesis-specific beta2-tubulin of Drosophila. We previously showed that all beta-tubulins used for motile 9 + 2 axonemes contain a conserved sequence motif in the proximal part of the CTT, the beta-tubulin axoneme motif. The differential ability of tubulin isoforms and abilities of beta2-tubulin C-terminal truncations to form axonemes led us to hypothesize that the axoneme motif is essential for axoneme formation and the distal half of the CTT was less important. The studies we report here indicate that it is not that simple. Unexpectedly, some changes in the core sequence of the axoneme motif did not disrupt formation of motile axonemes. And, while deletion of the distal CTT did not disrupt the ability to produce functional sperm [Popodi et al., Cell Motil Cytoskeleton 2005;62:48-64], changing the amino acid sequence in this region can. Thus both regions are important. The deep conservation of the axoneme motif in all eukaryotic groups implies that the presence of the sequence motif confers a functional advantage. The central pair is the axoneme structure most sensitive to perturbations in tubulin molecules; we hypothesize central pair assembly is facilitated by the presence of this motif. Our data reveal that beta2-tubulin has robust properties for axoneme assembly, and that axonemal specializations are embedded in both the CTT and the body of the beta2 molecule.