THE 3-DIMENSIONAL STRUCTURE OF THE ASPARTATE RECEPTOR FROM ESCHERICHIA-COLI

The crystal structure of the periplasmic domain of the aspartate recep tor from Escherichia coli has been solved and refined to an R factor o f 0.203 at 2.3 Angstrom resolution. The dimeric protein is largely hel ical, with four helices from each monomer forming a four-helix bundle. The dimer interface is constructed from four helices, two from each s ubunit, also packed together in a four-helix bundle arrangement. A sul fate ion occupies the aspartate-binding site. All hydrogen bonds made to aspartate are substituted by direct or water-mediated hydrogen bond s to the sulfate, Comparison of the Escherichia coli aspartate-recepto r structure with that of Salmonella typhimurium [Milburn, Prive, Milli gan, Scott, Yeh, Jancarik, Koshland and Kim (1991), Science, 254, 1342 -1347; Scott, Milligan, Milburn, Prive, Yeh, Koshland & Kim (1993). J. Mol. Biol. 232, 555-573] reveals strong conservation in the structure of the monomer, but more divergence in the orientation of the subunit s with respect to one another. Mutations that render the Escherichia c oli receptor incapable of responding to maltose are either located in spatially conserved sites or in regions of the structures that have hi gh temperature factors and are therefore likely to be quite flexible. The inability of the receptor from Salmonella typhimurium to respond t o maltose may, therefore, be because of differences in amino acids loc ated on the binding surface rather than structural differences.