Magnetotactic bacteria synthesize highly uniform intracellular magnetite nanoparticles through the action of several key biomineralization proteins. These proteins are present in a unique lipidbound organelle (the magnetosome) that functions as a nanosized reactor in which the particle is formed. A master regulator protein of nanoparticle formation, magnetosome membrane specific F (MmsF), was recently discovered. This predicted integral membrane protein is essential for controlling the monodispersity of the nanoparticles in Magnetospirillum magneticum strain AMB-1. Two MmsF homologs sharing over 60% sequence identity, but showing no apparent impact on particle formation, were also identified in the same organism. We have cloned, expressed, and used these three purified proteins as additives in synthetic magnetite precipitation reactions. Remarkably, these predominantly α-helical membrane spanning proteins are unusually highly stable and water-soluble because they self-assemble into spherical aggregates with an average diameter of 36 nm. The MmsF assembly appears to be responsible for a profound level of control over particle size and iron oxide (magnetite) homogeneity in chemical precipitation reactions, consistent with its indicated role in vivo. The assemblies of its two homologous proteins produce imprecise various iron oxide materials, which is a striking difference for proteins that are so similar to MmsF both in sequence and hierarchical structure. These findings show MmsF is a significant, previously undiscovered, protein additive for precision magnetite nanoparticle production. Furthermore, the self-assembly of these proteins into discrete, soluble, and functional "proteinosome" structures could lead to advances in fields ranging from membrane protein production to drug delivery applications.MmsF | proteinosome | magnetite | magnetosome | in vitro precipitation M agnetic nanoparticles (MNPs) represent an area of intense research due to their diverse and pertinent applications across a range of disciplines and industries. Applications for MNPs include biomedical diagnostics and therapies (1-3), such as MRI contrast reagents, tumor hyperthermia treatments, and magnetically targeted drug delivery, as well as data storage (4) and biotechnology. However, specific magnetic and physical properties of MNPs are critical to the success of each application, with specific size and morphology (with a narrow distribution) being essential considerations. Pure magnetite MNP synthesis under ambient conditions is notoriously difficult to control, with simple precipitations often resulting in a mixture of differently sized and shaped particles with other iron oxide contaminants. This situation can be improved somewhat by using more extreme processes, such as high-temperature incubations or capping surfactants, which favor certain MNP types (5, 6). However, the use of toxic or organic reagents and extreme conditions comes with a high energy and monetary cost, and can limit the biocompatibility of the MNPs for subsequent app...