Part of a cation efflux system that mediates resistance to copper and silver. The cus efflux system is 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 resistance-nodulation-cell division (RND) (TC 2.A.6) family.