Part of a cation efflux system that mediates resistance to copper and silver. Binds one copper per polypeptide (PubMed:11399769, PubMed:24917681). The cus efflux system is composed of CusA, CusB, CusC and CusF. Interacts with copper-exporting P-type ATPase CopA; when this protein is precharged with copper it binds very little CopA (PubMed:24917681). 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.