Figure 7. Combining structural data of GspD and GspC and the exoprotein cholera toxin.

(A) Two perpendicular views of the dodecameric ring of GspD^N0-N1 (blue) obtained from crystallographic and electron microscopy studies [20], [24] with a dodecameric ring of GspC^HR (green) assembled as described in the text. (B) Two perpendicular views of the same dodecameric ring of GspD^N0-N1 (blue) shown in (A) with six GspC^HR subunits (green) binding to alternating GspD subunits as described in the text. (C) Two perpendicular views of the exoprotein cholera toxin (B pentamer in gold, A subunit in yellow) positioned inside the dodecameric GspC^HR–GspD^N0-N1 ring depicted in (A). The five-fold axis of the B-pentamer is aligned by hand with the twelve-fold axis of the ring. The orientation of the AB₅ hexamer with respect to the dodecamer is otherwise arbitrary. The cross-section of the double dodecamer of GspD^N0-N1 and GspC^HR is just sufficient to permit binding of the toxin heterohexamer. Obviously, the alternative assembly shown in (B) would also provide sufficient space for the toxin to bind at this location.