Structure of the stand-alone RAM-domain protein from Thermus thermophilus HB8.

The stand-alone RAM (regulation of amino-acid metabolism) domain protein SraA from Thermus thermophilus HB8 (TTHA0845) was crystallized in the presence of zinc ions. The X-ray crystal structure was determined using a multiple-wavelength anomalous dispersion technique and was refined at 2.4 A resolution to a final R factor of 25.0%. The monomeric structure is a betaalphabetabetaalphabeta fold and it dimerizes mainly through interactions between the antiparallel beta-sheets. Furthermore, five SraA dimers form a ring with external and internal diameters of 70 and 20 A, respectively. This decameric structure is unique compared with the octameric and dodecameric structures found for other stand-alone RAM-domain proteins and the C-terminal RAM domains of Lrp/AsnC-family proteins.