Probably plays a role in biomineralization (Probable). Required for stable accumulation of MamB (By similarity). Probably binds and transports iron. May nucleate iron crystal formation (By similarity). Forms homodimers via its C-terminal domain (CTD) in the presence of metal cations (By similarity). Interacts with MamB via their CTD (By similarity). Part of the probable 18 gene mamAB operon. The C-terminal domain (CTD) is probably responsible for hetero- and homodimerization; it assumes a V-shaped, dimeric metallo-chaperone-like fold that can open and close. Binds up to 3 divalent metal cations (probably iron in vivo); upon binding there is a conformational shift to a tighter dimer. Cells have no magnetic response but still make empty magnetosome membranes; magnetosome proteins MamA and MamE localize normally (PubMed:20212111). Deletion of genes mamH to mamV (amb0961 to amb0978) gives cells with no magnetosomes and no magnetic response (PubMed:20212111). This bacteria makes up to 20 cubo-octahedral magnetosomes of about 45 nm in diameter which contain membrane-bound crystals of magnetite (Fe(3)O(4)). Expression of just the minimal mamAB gene cluster (amb0961 to amb0978), including this gene, is sufficient to form a minimal magnetosome chain with small magnetite particles. Belongs to the cation diffusion facilitator (CDF) transporter (TC 2.A.4) family.