Katanin

Katanin is a microtubule-severing AAA protein. It is named after the Japanese sword, katana. Katanin is a heterodimeric protein first discovered in sea urchins. It contains a 60 kDa ATPase subunit, encoded by KATNA1, which functions to sever microtubules. This subunit requires ATP and the presence of microtubules for activation. The second 80 kDA subunit, encoded by KATNB1, regulates the activity of the ATPase and localizes the protein to centrosomes.Electron microscopy shows that katanin forms 14–16 nm rings in its active oligomerized state on the walls of microtubules.

Mechanism and regulation of microtubule length

Structural analysis using electron microscopy has revealed that microtubule protofilaments change from a straight to a curved conformation upon GTP hydrolysis of β-tubulin. However, when these protofilaments are part of a polymerized microtubule, the stabilizing interactions created by the surrounding lattice lock subunits into a straight conformation, even after GTP hydrolysis. In order to disrupt these stable interactions, katanin, once bound to ATP, oligomerizes into a ring structure on the microtubule wall - in some cases oligomerization increases the affinity of katanin for microtubules and stimulates its ATPase activity. Once this structure is formed, katanin hydrolyzes ATP, and likely undergoes a conformational change that puts mechanical strain on the tubulin subunits, which destabilizes their interactions within the microtubule lattice. The predicted conformational change also likely decreases the affinity of katanin for tubulin as well as for other katanin proteins, which leads to disassembly of the katanin ring structure, and recycling of the individual inactivated proteins.