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Figure 2.
Fig. 2. Visualization of the permeation pathways of HI1470/1
and BtuCD with the program HOLE (12). (A) The permeation pathway
generated by the two HI1471 subunits is narrow at the
periplasmic surface and open to the cytoplasm, which are located
toward the top and bottom of the figure, respectively. (B) In
contrast, the pathway for BtuC is closed at the cytoplasm and
open to the periplasm. The HOLE representation of the pore
surface is shown in a multicolored form that was displayed and
rendered with the program VMD (33). Red, green, and blue
surfaces designate regions of the permeation pathway with
effective radii <0.6, 0.6 to 1.15, and >1.15 Å,
respectively. The calculated diameters at the widest part of the
pathways illustrated for HI1470/1 and BtuCD are  11
and 9 Å, respectively. The permeation pathway in BtuCD is
of sufficient size to accommodate a corrin ring but not the
entire B[12] molecule (9); the ligand for HI1470/1 has not been
identified.
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