Ribbon diagram of the third and fourth domains of β_c with conserved residues shown by CPK spheres. Conserved residues are colored as follows: Cys, yellow; buried hydrophobics, green; Arg, blue; Trp of WSXWS sequence, purple (see cover figure). The two Tyr residues involved in ligand-binding are drawn in red stick form (see cover figure). The strands are labeled in close proximity to the arrow illustrating the direction of the relevant strand.