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Figure 8.
Figure 8. Model for Concerted PilT Motions
(A) The
quasi-two-fold symmetric C2 crystal structure has two peripheral
wide-open subunits (B, E; blue), two central “active”
subunits (C, F; orange), and two central “resting” subunits
(A, D; green). Four CTD:CTD interfaces are engaged (double
lines). The remaining two are disengaged (zig-zag). Subunit F is
clamped around bound nucleotide.
(B) When ATP (red) binds
in the E cleft, the two domains close around the ligand (short
black arrows), causing the β5/β6 arginines to approach the
ATP. Because of the extensive CTD[D]:NTD[E] interface, the
motion of NTD[E] forces the swiveling of CTD[D](in particular
the C-terminal helices) toward the periphery of the hexamer
(long gray arrow). Consequently, the D arginine fingers approach
the E active site (double lines). On the other side of CTD[D],
the interface likewise rearranges, disengaging CTD[C] from the D
active site (zig-zag).
(C) Subunit D is now poised as the
most peripheral, wide-open subunit and ready to bind nucleotide;
E is clamped around nucleotide and contributing to an engaged
CTD:CTD interface on either side.
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