X-ray crystal structure of MelB_St. See Protein Data Bank accession no. 4M64. (a) The overall fold of MelB_St in a periplasmic-side-open conformation. Helices in rainbow colors from blue (N terminus) to red (C terminus). (b) Cosubstrate-binding sites. The helices are labeled in roman numerals, and side chains potentially involved in the cation binding (residues D55, D59, D124, N58, and T121) or in the galactoside binding (residues D19, R149, Y120, D124, W128, W342, and K377) are highlighted as sticks. Residues Y120, D124, and K373 may be involved in both sites.