Chaperone-mediated copper handling in the periplasm

Metal transport systems are broadly utilized to maintain low levels of metals to prevent cellular malfunction caused by an overabundance of metals. The CusCFBA Cu(I)/Ag(I) resistance system, commonly found in Gram-negative organisms, typically consists of a tripartite CBA transport complex that spans both the inner and outer membranes as well as a small periplasmic protein, CusF. In the CusCFBA system, CusF functions as a metallochaperone which transfers metal to the tripartite complex to aid in metal resistance. However, CusF-like proteins have also been observed in genomic contexts apart from the CBA-type transport systems, suggesting it could either play a role as a metallochaperone to other systems or have other roles than that of a metallochaperone. In this review, we focus on the molecular function of CusF in the CusCFBA transport system and discuss the metal transport pathway through this system. In addition we briefly discuss the potential functions of CusF-like proteins in other contexts.