Most functional ABC transporters are composed of at least four sub-units: two trans-membrane (TM) domains where the transport process takes place and two cytoplasmic nucleotide binding domains (NBDs) providing the energy required for active transport [1]. This entry is one of the two NBDs found at the the C-terminal domain of FbpC, ferric iron uptake transporter, from the Neisseria gonorrhoeae. The C-terminal regulatory domain adopts two OB-folds per monomer. These are similar in topology to those seen in the NBD (nucleotide binding domain) from the maltose uptake ABC transporter, MalK. However, FbpC does not open as far as MalK when ATP is removed from their respective closed structures. This difference was suggested to be due to the substantial domain swap in the regulatory domain of FbpC [2].

1: Visualizing functional motions of membrane transporters with molecular dynamics simulations. Shaikh SA, Li J, Enkavi G, Wen PC, Huang Z, Tajkhorshid E; Biochemistry. 2013;52:569-587. PMID:23298176

2: Insights into how nucleotide-binding domains power ABC transport. Newstead S, Fowler PW, Bilton P, Carpenter EP, Sadler PJ, Campopiano DJ, Sansom MS, Iwata S; Structure. 2009;17:1213-1222. PMID:19748342