A model of the MCAK depolymerization reaction. (A) MTs supersaturated with E491A can be rapidly disassembled in ATP. (B) The lower the ratio of E491A to tubulin, the less effective the motor is in disassembling MTs in ATP. (C) E491A is capable of steady catalytic disassembly of MTs in ADP because the motor can release from tubulin and recycle back onto MT lattice. (D) A model for MCAK-dependent disassembly of MTs. Our data suggests that MCAK can completely remove one tubulin dimer before ATP hydrolysis. M:D:Mt represents ADP–MCAK diffusing on MT, M:T:Mt* represents the first ATP–MCAK depolymerization transition state, M:T*:tb represents the ATP hydrolysis transition state/low tubulin affinity transition state, and M:DPi + tb represents ADP-Pi–MCAK dissociated from tubulin dimer. A postulated zero ATP state is also shown. This state exhibits high affinity for the MT lattice (Moore and Wordeman, 2004). (E) MCAK's affinity for tubulin decreases upon reaching the ATP transition state, unlike motile kinesins. (F) Phylogenetic distances based on an alignment of the Switch I, Switch II, and the P-loop of kinesins, myosins, and G proteins. Alignments are shown in Fig. S4 (available).