In addition, a dramatic widening of the TMDs into the extracellular space occurs, which is very clearly observed when the fully opened outward transition state is adopted.
Finally, as the transition state conformation with fully opened outward DBD was reached (frame 26 of the figures), the cross-linked residues once again separated to large and in some cases near maximal values.
In the Pgp conformations that approached the fully opened outward ADP-Vi transition state, ligand binding in the extracellular half of the TMDs was not favored, with preferred docking found in the cytoplasmic half of the membrane regions.
Then as the transition state conformation with the fully opened outward DBD was reached, the cross-linked residues on helices 1 and 11 and helices 6 and 12 separated widely again.
This smooth closure between residues 556 and 1201 continued through to the final "transition state" targeted structure with a fully opened outward DBD where the minimal separation of 15 Å between these catalytically important residues was observed.
It seems noteworthy that the fully opened outward conformations did not show preferential docking in the extracellular half of the transmembrane regions, potentially reflecting structural destabilization of ligand binding to the DBS as Pgp adopts the opened outward conformation (see below).
The only known mechanism applicable in this situation is the one shown in Fig. 5: H+ transport out of the T tubular lumen is achieved by an inward diffusion of HCO3− (which will partly or fully electrically compensate outward lactate diffusion) in combination with an outward diffusion of CO2, i.e. a CO2-HCO3− shuttle for H+ transport.
Subsequent simulations sequentially targeted the "partially opened inward" MsbA structure (PDB entry 3B5X), the Sav1866 partially opened outward, fully engaged NBDs conformation (PDB entry 2HYD 18)), and finally the MsbA transition state structure with fully engaged NBDs and "fully opened outward" DBS PDBB entry 3B5Z).
During the next transitions between the partially opened inward and fully engaged NBDs partially opened outward conformations and finally the transition state fully opened outward conformations, the extracellular ends of helices 1/7 and helices 2/8 continued the relatively smooth trend of opening outward all the way through to the final transition state conformation of the simulation.
They also can hang by the feet, swinging beneath a branch using their rear toe, which can be rotated outward and fully forward, to obtain a strong clinging grip.
