Comparison of nucleotide free-Pgp (nf-Pgp) and Pgp-AMP-PNP (Pgp-AMP-PNP) three-dimensional structures. A, stereo pair of the nf-Pgp three-dimensional structure, displayed using netting at 1.0 σ (red) and 1.5 σ (yellow) above the mean density level and viewed perpendicular to the crystal plane from the more heavily stained side (corresponding to the extracellular surface). B, equivalent views of the Pgp-AMP-PNP structure. The arrow indicates the gap along one side of the central pore. The locations of the three discrete densities A, B, and C are indicated. C, stereo pair of a side view of Pgp-AMP-PNP with the same color scheme as above. The directions of the principle crystallographic axes a and b are shown. Scale bar = 2.2 nm. AMP-PNP, non-hydralizable ATP analogue .