Fig. 4.

Shrinking of the central cavity. (A) In the T state (shown as molecular surface), the central channel along the C2-symmetry axis is fully open (same view and coloring as Fig. 1B). (B) In the intermediate structure (λ = 0.5, viewed from the opposite end of the channel), the channel is nearly closed between the α-subunits (also shown as licorice in A) but still open between the β-subunit. When reaching the R state (shown as licorice in B), the channel closes also between the β-subunits.