Plays a major role in synthesis of cubooctahedral magnetite crystals by controlling crystal growth and morphology after nucleation (PubMed:22716969). Has a partially redundant function with MamF (By similarity). When overexpressed in E.coli the soluble protein self assembles into shells of about 36 nm. This protein mediates the formation of magnetite nanoparticles from a solution of Fe(2+) and Fe(3+) sulfate; the crystals are larger and lack alternative iron oxide/oxyhydroxide species seen in the protein's absence (PubMed:25349410). Its function may be negatively regulated by one of the MamGFDC proteins. May oligomerize. Tagged protein forms straight lines with a punctate pattern extending along most of the cell associated with its inner curvature, in the correct position to be associated with magnetosomes (PubMed:22716969, PubMed:27481925, PubMed:26884433). During magnetosome formation protein is first dispersed in cell inner membrane, the gradually forms foci which align and form lines as the magnetic response increases (PubMed:26884433). Single non-polar gene mutation and deletion of the probable mms6 operon (amb0955, mms6 and mmsF) leads to weak magnetic response and much smaller, elongated magnetite crystals (PubMed:22716969). Vesicles that are probably empty precursor magnetosomes are present and can grow to wild-type size (PubMed:26884433). May be useful for the manufacture of precision magnetite nanoparticles of uniform mineral type, and consistent morphology, with high magnetization. Addition to an Fe(2+) and Fe(3+) sulfate solution increases the quality of the magnetite crystals. 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)). Belongs to the magnetosome MamF/MmsF protein family.