The x-ray structure of the periplasmic galactose binding protein from Salmonella typhimurium at 3.0-A resolution

The x-ray structure of the periplasmic galactose binding protein from Salmonella typhimurium, the specific receptor for taxis toward, and high-affinity transport of, galactose has been solved at 3.0-A resolution using multiple isomorphous replacement. The path of the polypeptide chain has been traced, and a model structure consisting of 292 amino acids has been fit to the electron density map. The overall shape of the molecule is that of a prolate ellipsoid, with dimensions 35 X 35 X 65 A. The protein consists of two similar domains of roughly equal size, related by an axis of pseudosymmetry, and separated by a deep cleft about 8 A wide. Each domain has a core of parallel beta sheet surrounded by five alpha helices, built by alternating strands of sheet and helix in a repeating pattern. Approximately 36% of the residues are involved in alpha helices, and 27% in beta sheet. The tertiary structure has been compared to that of the Escherichia coli arabinose binding protein (Gilliland, G.L., and Quiocho, F. A. (1981) J. Mol. Biol. 146, 341-362), a periplasmic receptor which is involved in transport, but not in chemotaxis. The overall folding of these two molecules is very similar, with the exception of two areas on the surface of the molecule on the long sides of the prolate ellipsoid. The observed variations are adequate to explain the differences in interaction of L-arabinose binding protein and galactose binding protein with the membrane proteins for transport and chemotaxis.