Sandwiching of cholic acid between protomers stabilizes the superhelix. The cross-eyed stereo image shows density-modified but unbiased (i.e. before building of the cholic acid) electron density at the 2 r.m.s.d. level as a grey mesh. The density for the two N-terminal residues Glu1 and Lys2 reveals the error in the sequence of the synthetic peptide (red labels). The hydrophobic concave surface into which cholic acid binds is lined by Glu1, Ala4, Leu5 and Tyr8.