The cation efflux system protein CusB from E. coli can be divided into four different domains, the first three domains of the protein are mostly beta-strands and the fourth forms an all alpha-helical domain. This entry represents the first beta-domain (domain 1) of CusB and it is formed by the N and C-terminal ends of the polypeptide (residues 89-102 and 324-385) [1]. CusB is part of the copper-transporting efflux system CusCFBA [2]. This domain can also be found in other membrane-fusion proteins, such as HlyD, MdtN, MdtE and AaeA. HlyD is a component of the prototypical alpha-haemolysin (HlyA) bacterial type I secretion system, along with the other components HlyB and TolC. HlyD is anchored in the cytoplasmic membrane by a single transmembrane domain and has a large periplasmic domain within the carboxy-terminal 100 amino acids [3], HlyB and HlyD form a stable complex that binds the recombinant protein bearing a C-terminal HlyA signal sequence and ATP in the cytoplasm [4]. HlyD, HlyB and TolC combine to form the three-component ABC transporter complex that forms a trans-membrane channel or pore through which HlyA can be transferred directly to the extracellular medium. Cutinase has been shown to be transported effectively through this pore [5].

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

2: Molecular analysis of the copper-transporting efflux system CusCFBA of Escherichia coli. Franke S, Grass G, Rensing C, Nies DH; J Bacteriol. 2003;185:3804-3812. PMID:12813074

3: The E. coli alpha-hemolysin secretion system and its use in vaccine development. Gentschev I, Dietrich G, Goebel W; Trends Microbiol. 2002;10:39-45. PMID:11755084

4: Recombinant protein secretion in Escherichia coli. Mergulhao FJ, Summers DK, Monteiro GA; Biotechnol Adv. 2005;23:177-202. PMID:15763404

5: Extracellular overexpression of recombinant Thermobifida fusca cutinase by alpha-hemolysin secretion system in E. coli BL21(DE3). Su L, Chen S, Yi L, Woodard RW, Chen J, Wu J; Microb Cell Fact. 2012;11:8. PMID:22239833