ATP-binding cassette (ABC) transporters are a major family of small molecule transporter proteins, and their deregulation is associated with several diseases, including cancer. Here, we report the crystal structure of the nucleotide binding domain (NBD) of an amino acid ABC transporter from Thermus thermophilus (TTHA1159) in its apo form and as a complex with ADP along with functional studies. TTHA1159 is a putative arginine ABC transporter. The apo-TTHA1159 was crystallized in dimeric form, a hitherto unreported form of an apo NBD. Structural comparison of the apo and ADP-Mg2+ complexes revealed that Phe14 of TTHA1159 undergoes a significant conformational change to accommodate ADP, and that the bound ADP interacts with the P-loop (Gly40-Thr45). Modeling of ATP-Mg2+:TTHA1159 complex revealed that GIn86 and Glul 64 are involved in water-mediated hydrogen bonding contacts and Asp163 in Mg2+ ion-mediated hydrogen bonding contacts with the gamma-phosphate of ATP, consistent with the findings of other ABC transporters. Mutational studies confirmed the necessity of each of these residues, and a comparison of the apo/ADP Mg2+:TTHA1159 with its ATP-complex model suggests the likelihood of a key conformational change to the GIn86 side chain for ATP hydrolysis. (C) 2015 Elsevier Inc. All rights reserved.