The mercury transport membrane protein, MerF has a core helix-loop-helix domain. It has two vicinal pairs of cysteine residues which are involved in the transport of Hg(II) across the membrane and are exposed to the cytoplasm [1].

1: NMR structure determination of a membrane protein with two transmembrane helices in micelles: MerF of the bacterial mercury detoxification system. Howell SC, Mesleh MF, Opella SJ; Biochemistry. 2005;44:5196-5206. PMID:15794657