Sharma et al. The slicer, called Katanin, builds up microtubules in cilia while it cuts down those in the cell body.
All microtubules are not created equal. In Tetrahymena, for example, the polymers come in many flavors, including ciliary extensions, an internal network, and cortical bundles. These subsets, the new results indicate, are differentially affected by katanin activity.
After knocking out Tetrahymena katanin, the authors found that cilia were missing their central microtubules and had shorter outer doublets. Cortical bundles and internal microtubules, by contrast, were more abundant and unusually stable. These inner polymers were more heavily laden with posttranslational modifications, including acetylation, glutamylation, and glycylation.
These modifications normally increase as microtubules age, so their accumulation might be a simple byproduct of the loss in severing activity, which may keep the polymer dynamic. The katanin mutants, however, resembled a β-tubulin mutant lacking the glutamate and glycyl projections. The similarity suggests that microtubule modifications might activate katanin, thereby focusing its activity on long, old filaments.
Why cilial microtubules were shorter or missing is unclear. Cilia that were lopped off regrew to a similarly stunted length, suggesting that free tubulin subunits were available. Perhaps katanin cuts microtubules to a particular length that can be transported into or along the cilia, as has been suggested for axons.