E. In the stalk mutant, the spring between AAA4 / AAA5 does not close upon ATP binding / hydrolysis presumably due to a disruption of the stalk / buttress interface. Moreover, the gap at the AAA4 / AAA5 interface is larger for the stalk mutant in both states, AMPPNP and ADP-vi, than for wild-type in the AMPPNP state. This "loose spring" at AAA4 / AAA5 uncouples these domains from the closure of the AAA1 / AAA2 interface, and this accounts for microtubule independent hydrolysis. 