Forms pores that allow passive diffusion of cations across the outer membrane. Part of a cation efflux system that mediates resistance to copper and silver. In pathogenic strains it allows the bacteria to invade brain microvascular endothelial cells (BMEC) thus allowing it to cross the blood-brain barrier and cause neonatal meningitis. Homotrimer. Component of the cus efflux system composed of CusA, CusB, CusC and CusF. Transcriptionally regulated by CusR in response to copper and silver ions. The cus system plays an important role in copper tolerance under anaerobic growth and, under extreme copper stress, in aerobic growth. Belongs to the outer membrane factor (OMF) (TC 1.B.17) family.