Tripartite ATP-independent periplasmic (TRAP) transporters are a large family of membrane-embedded proteins that transport nutrients into bacterial and archaeal cells. They employ a substrate-binding protein that captures the substrate with high affinity before delivering it to the membrane domain for transport across the cytoplasmic membrane. The TRAP-associated extra-cytoplasmic immunogenic (TAXI) transporters are a subfamily of TRAP proteins that have additional domains in the membrane subunit. The structural and functional characteristics of TAXI transporters are poorly understood compared to other TRAPs. TAXI transporters are found in both archaeal and bacterial genomes and are considered an ancestral form of the TRAP transporter.
Here, we report the crystal structure of the glutamate-bound TAXI-TRAP substrate domain TtGluP from Thermus thermophilus at 1.11 Å resolution. TtGluP consists of two domains separated by a deep substrate-binding cleft, akin to a Venus fly trap. Both domains exhibit a similar secondary structure, with internal β-sheets surrounded by α-helices. Both glutamate and glutamine fit the electron density in the ligand binding site perfectly. The protein-ligand interactions are stabilized by the hydrogen bonds and hydrophobic interactions. Water molecules stabilize the glutamate in its substrate binding site and allow interaction with both domains. Biophysical characterization data suggest TtGluP exists in a concentration-dependent monomer-dimer equilibrium. A limited number of studies provide evidence regarding the functional and structural properties of TAXI proteins. A full structural characterization of the complete TAXI complex that includes the membrane domain will help in unravelling the potential of TAXI transporters as future drug targets for antimicrobials.