A novel pollen-specific alpha-tubulin in sunflower: structure and characterization.

We describe here a new alpha-tubulin isoform from sunflower we named alpha(pi)-tubulin. Alpha(pi)-tubulin is the most divergent higher-plant alpha-tubulin described so far, having an unusual deletion in the H1/B2 loop and a glutamine-rich C-terminus. We constructed a three-dimensional model and discuss its implications. Using specific antibodies, we show that alpha(pi)-tubulin expression is restricted to the male gametophyte. Alpha(pi)-tubulin mRNA represents 90% of alpha-tubulin mRNA and a small percentage of total pollen mRNA. Among the plants tested, alpha(pi)-tubulin was only detected in sunflower and in Cosmos. Since both plants are Asteraceae, we propose that alpha(pi)-tubulin is specific to this family. Our results suggest that alpha(pi)-tubulin can inhibit tubulin assembly in pollen. This hypothesis is reinforced by the fact that alpha(pi)-tubulin is found in a complex with beta-tubulin in mature sunflower pollen.