Figure 6. Pore loop displacement of the gating protomer suggests power stroke for microtubule severing.

(a) Superposition of the katanin spiral and ring conformations shows the large movement of the gating protomer P1 (green arrow) and the smaller-scale movement of P6 (red arrow). Protomers are colored as in Fig. 2 in the spiral conformation and are gray in the ring conformation. (b) Cartoon depicting the movement of protomer P1 between the spiral and ring conformations. Protomers colored as in Fig. 2. Arrow highlights the translocation of loop 1 of the P1 gating protomer. (c) Cartoon illustrating proposed power stroke that extracts a tubulin dimer and initiates microtubule lattice breakdown and severing. Left panel, Katanin (blue) assembles as a hexamer with a spiral configuration of the AAA domains and the MIT domains emanating from the AAA motor core and making multivalent interactions with the microtubule (green). The flexible tubulin tail is engaged in the axial pore of the katanin hexamer. Right panel, ATP hydrolysis and release in the gating protomer P1 leads to closure of the AAA ring and a ~20Å displacement in the P1 loop that translocates with it the bound C-terminal tail of a tubulin subunit. The cycle is repeated until lattice contacts unravel and the microtubule severs.