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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.〔(McNally, F. & Vale, R. (1993) ''Identification of katanin, an ATPase that severs and disassembles stable microtubules''. )〕 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.〔(Downing, K. & Nogales, E. (1998). ''Tubulin and microtubule structure.'' )〕 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.〔(Hartman, J. & Vale, R. (1999) ''Microtubule Disassembly by ATP-dependent Oligomerization of the AAA Enzyme Katanin'' )〕
The severing of microtubules by katanin is regulated by nucleotide exchange factors, which can exchange ADP with ATP, protective microtubule-associated proteins (MAPs), and the p80 subunit (p60 severs microtubules much better in the presence of p80). These mechanisms have different consequences, depending on where in the cell they are activated or disrupted. For example, allowing katanin-mediated severing at the centrosome releases microtubules for free movement. In one experiment, anti-katanin antibodies were injected into a cell, causing a large accumulation of microtubules around the centrosome and inhibition of microtubule outgrowth.〔(Ahmad, F., Yu, W., McNally, F. & Baas, P. ''An Essential Role for Katanin in Severing Microtubules in the Neuron'' )〕 Therefore, katanin-mediated severing may serve to maintain organization in the cytoplasm by promoting microtubule disassembly and efficient movement. During cell division, severing at the spindle pole produces free microtubule ends and allows poleward flux of tubulin and retraction of the microtubule. Severing microtubules in the cytoplasm facilitates treadmilling and mobility, which is important during development.
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