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Figure 6.
Figure 6. Structurally implied mechanism for branch
migration. Illustration of a mechanism for RuvB branch migration
involving a rotation of the RuvB hexamer (green, cyan, and blue
subunits) relative to the RuvA tetramer (yellow bar). Stepwise
migration of the DNA is indicated by motion of the circled
numbers through the center of the hexamer, although the
fundamental translocation step size is unknown. The 2-fold
symmetry of the loading of the nucleotide binding sites is based
on pre-steady state kinetics of RuvB, which hydrolyzes two ATP
molecules per hexamer.[38 and 45] The starting state (a) with
two ATP and two ADP molecules is inferred from the optimal
nucleotide ratio (2 ATPgS:1 ATP) for forming topologically
underwound DNA, [21, 38 and 49] equivalent to step (b), and the
productive arginine finger geometry observed in the AMP-PNP
bound NSF-D2. [41] ATP hydrolysis in step (b) may drive rotation
of the RuvB hexamer (c) by opening of the ADP-bound state along
DNA as well as through interactions with RuvA. ATP serves as an
allosteric effector for ADP release, [45] which may be driven by
interface changes between subunits that may be released after
rotation (d) or during rotation. Hydrolysis of ATP by RuvB is
kinetically rapid and ADP release is slow. [45]
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