Plays 2 roles; promotes magnetite nucleation/formation and activates the MamE protease (Probable). Despite its near conservation of a protease-like sequence, this is probably not a protease (PubMed:26981620) (Probable). Required in conjunction with MamP for proteolysis of at least MamE, itself and MamP (PubMed:26981620). May transport a solute that controls MamE's protease activity. May place individual iron atoms into the magnetite lattice (Probable). Binds to transition metal ions via His-148 and His-263, but also binds metal via other residues. Part of the probable 18 gene mamAB operon. The N-terminal region is an inactive protease, the C-terminal region may have transporter activity which could activate the MamE protease. Subject to proteolytic cleavage by MamE. 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)). There is a paralogous gene with partially overlapping function in the genome (amb1004, called limO, AC Q2W8L7). LimO does not encode the C-terminal TSUP domain and does not complement a mamO deletion. 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. 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. Initally suggested to be a protease, mutation of the protease domain active site residues does not alter its in vivo function in magnetosome formation.