Long alpha hairpin domain of cation efflux system protein, CusB
HlyD_D4 is the long alpha-hairpin domain in the centre of CusB or HlyD proteins. CusB and HlyD proteins are membrane fusion proteins of the CusCFBA copper efflux system in E.coli and related bacteria. 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. HlyD_D4 is thought to interact with the alpha-helical tunnels of the corresponding outer-membrane channels, ie the periplasmic domain of CusC.