Crystal structure of a predicted phosphoribosyltransferase (TT1426) from Thermus thermophilus HB8 at 2.01 A resolution

Abstract

TT1426, from Thermus thermophilus HB8, is a conserved hypothetical protein with a predicted phosphoribosyltransferase (PRTase) domain, as revealed by a Pfam database search. The 2.01 A crystal structure of TT1426 has been determined by the multiwavelength anomalous dispersion (MAD) method. TT1426 comprises a core domain consisting of a central five-stranded beta sheet surrounded by four alpha-helices, and a subdomain in the C terminus. The core domain structure resembles those of the type I PRTase family proteins, although a significant structural difference exists in an inserted 43-residue region. The C-terminal subdomain corresponds to the "hood," which contains a substrate-binding site in the type I PRTases. The hood structure of TT1426 differs from those of the other type I PRTases, suggesting the possibility that TT1426 binds an unknown substrate. The structure-based sequence alignment provides clues about the amino acid residues involved in catalysis and substrate binding.

Figure 1.

(A) Ribbon representation of TT1426 (stereo view). In the core domain, the PRPP-binding motif is shown in magenta, the flexible loop is yellow, and the inserted region is green. The rest of the core domain is shown in red, and the hood is orange. (B) Sequence alignment of TT1426 and its homologs. The secondary structure for TT1426, shown above the sequences, is colored as in A. Filled magenta circles indicate the conserved PRPP-binding motif. RS00882 indicates Ralstonia solanacearum GMI1000; SMc01463, Sinorhizobium meliloti 1021; mll6596, Mesorhizobium loti MAFF303099; MA2685, Methanosarcina acetivorans C2A; aq_059, Aquifex aeolicus VF5; TVN1369, Thermoplasma volcanium GSS1; and Ta0227, Thermoplasma acidophilum DSM 1728.

Figure 2.

(A) Structure-based sequence alignment of TT1426 and the T. gondii UPRTase. The secondary structures for TT1426 and the T. gondii UPRTase (PDB 1BD3) are shown above and below the sequences, respectively. Filled magenta circles indicate the PRPP-binding motif. Residues involved in sulfate binding (S) and uracil binding (U) in the T. gondii UPRTase structure are labeled below the sequences. (B) Electrostatic surface representation of TT1426. Blue and red surfaces represent positive and negative potentials, respectively. The locations of the conserved hydrophilic residues around the putative active site are labeled.