HlyD_D23 is the combined domains 2 and 3 of the membrane-fusion proteins CusB and HlyD, which forms a barrel-sandwich. CusB and HlyD proteins are membrane fusion proteins of the CusCFBA copper efflux system in E.coli and related bacteria. The whole molecule hinges between D2 and D3. Efflux systems of this resistance-nodulation-division group - RND - have been developed to excrete poisonous metal ions, and in E.coli the only one that deals with silver and copper is the CusA transporter. The transporter CusA works in conjunction with a periplasmic component that is a membrane fusion protein, eg CusB, and an outer-membrane channel component CusC in a CusABC complex driven by import of protons.

1: Crystal structure of the membrane fusion protein CusB from Escherichia coli. Su CC, Yang F, Long F, Reyon D, Routh MD, Kuo DW, Mokhtari AK, Van Ornam JD, Rabe KL, Hoy JA, Lee YJ, Rajashankar KR, Yu EW; J Mol Biol. 2009;393:342-355. PMID:19695261