N-terminal phosphorylation of p60 katanin directly regulates microtubule severing.

Proteins of the AAA (ATPases associated with various cellular activities) family often have complex modes of regulation due to their central position in important cellular processes. p60 katanin, an AAA protein that severs and depolymerizes microtubules, is subject to multiple modes of regulation including a phosphorylation in the N-terminal domain involved in mitotic control of severing. Phosphorylation decreases severing activity in Xenopus egg extracts and is involved in controlling spindle length. Here, we show that the evolutionarily divergent N-terminal domains of p60 have maintained hotspots of mitotic kinase regulation. By reconstituting in vitro severing reactions, we show that phosphomimetic modification at amino acid position 131 in Xenopus laevis p60 decreases severing and microtubule-stimulated ATPase activity without affecting the binding affinity of p60 for microtubules. At high concentrations of the phosphomimetic mutant p60, wild-type levels of activity could be observed, indicating a more switch-like threshold of activity that is controlled by regulating oligomerization on the microtubule. This provides a cellular mechanism for high local concentrations of p60, like those found on spindle poles, to maintain severing activity while most of the protein is inhibited. Overall, we have shown that the modular domain architecture of AAA proteins allows for precise control of cellular activities with simple modifications.