Plays 2 roles; promotes magnetite nucleation/formation and activates the MamE protease (By similarity) (PubMed:20674739). Despite its near conservation of a protease-like sequence, this is probably not a protease (By similarity). Required in conjunction with MamP for proteolysis of at least MamE, itself and MamP (By similarity). May transport a solute that controls MamE's protease activity. May place individual iron atoms into the magnetite lattice (By similarity). One of 7 genes (mamLQBIEMO) able to induce magnetosome membrane biogenesis; coexpression of mamLQRBIEMO in a deletion of the 17 gene mamAB operon restores magnetosome vesicle formation but not magnetite biosynthesis (PubMed:27286560). Part of the probable 17 gene mamAB operon. The N-terminal region is an inactive protease, the C-terminal region may have transporter activity which could contribute to its function in proteolysis by MamE. Subject to proteolytic cleavage by MamE. Single gene deletion, loss of magnetic response, about 30% iron content. Forms magnetosome chains without magnetosome crystals (PubMed:20674739). Deletion of approximately 80 kb of DNA, including this operon, leads to cells that are non-magnetic, lack internal membrane systems, grow poorly, have reduced mobility and take-up and accumulate iron poorly (PubMed:13129949). This bacteria makes up to 60 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 (MGMSRv2__2365 to MGMSRv2__2381), including this gene, is sufficient to form a minimal magnetosome chain with small magnetite particles. In the N-terminal section; belongs to the peptidase S1C family. In the C-terminal section; belongs to the 4-toluene sulfonate uptake permease (TSUP) (TC 2.A.102) family.