As part of a structural genomics project, the crystal structure of a 314-amino-acid protein encoded by Thermus thermophilus HB8 gene TT1099 was solved to 1.75 A using the multiple-wavelength anomalous dispersion (MAD) method and a selenomethionine-incorporated protein. The native protein structure was solved to 1.5 A using the molecular-replacement method. Both structures revealed a bound ligand, L-glutamate or L-glutamine, and a fold related to the periplasmic substrate-binding proteins (PSBP). Further comparative structural analysis with other PSBP-fold proteins revealed the conservation of the predicted membrane permease binding surface area and indicated that the T. thermophilus HB8 molecule is most likely to be an L-glutamate and/or an L-glutamine-binding protein related to the cluster 3 periplasmic receptors. However, the geometry of ligand binding is unique to the T. thermophilus HB8 molecule.