Self-assembled MmsF proteinosomes control magnetite nanoparticle formation in vitro

Abstract

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...

Magnetita biomimética mediada por proteínas del magnetosoma vs. magnetita del meteorito ALH84001: ¿Son ambas comparables?

“…These particles produced in the presence of MmsF were larger and showed defined crystal faces, as a difference to the particles produced in the absence of the protein. This result is similar to those obtained both with MamC and Mms6, suggesting similar roles in the control of morphology (Rawlings et al, 2014). The magnetite produced in MmsF-bearing experiments had a high saturation magnetization (129 emu/g), which indicates that high-quality magnetite was produced (Rawlings et al, 2014).…”

“…Mms6 and MmsF are encoded in the mms6 operon (Lohβe et al, 2014) and both are believed to play a role controlling shape, and in the case of MmsF, controlling crystal maturation as well Murat et al, 2012). An interesting behaviour is that Mms6 and MmsF have been observed to self-assembly in micelles that have high affinity for iron in aqueous solution (Wang et al, 2012;Rawlings et al, 2014), which has also been observed in MamC (Kashyap et al, 2014). The fact that many different proteins work as a part of a highly coordinated orchestra to build up a chain of magnetosomes and synthetize magnetite in a process that is not fully understood yet is a proof that magnetite crystals are shaped by a genetic code and they play a role that allows MTB survive.…”

“…Although along those experiments crystals that meet the biogenicity criteria have been produced, Jiménez-López et al, (2012) 52 from their carbonate precursor that has not been observed in ALH84001, backing the proposition of Thomas-Keprta et al, (2009), that the majority of magnetite crystals found in ALH84001 are allochthonous. Regarding the sets of magnetite crystals synthetised in different MPMS experiments with different proteins, SD size particles with a better trend to crystallographic perfection have been consistently obtained (Amemiya et al, 2007;Prozorov et al, 2007;Rawlings et al, 2014;Valverde-Tercedor et al, 2015). These characteristics not only resemble those present in the ALH84001 magnetite, but also the variability range is lesser than in the thermal decomposition magnetite.…”

“…Magnetization mesurements of particles synthetized with MamC showed that particles grown at 10 μg/mL of MamC had a high blocking temperature and a slow magnetization increase, thus having a large magnetic moment per particle, even larger than that measured from Mms6-bearing in vitro magnetite (Valverde-Tercedor et al, 2015). Rawlings et al (2014) following the co-precipitation method in the presence of MmsF (10 μg/mL) yielded magnetite particles with an average size of 56nm. These particles produced in the presence of MmsF were larger and showed defined crystal faces, as a difference to the particles produced in the absence of the protein.…”

“…Although some other proteins and operons have been studied in mutants lacking the specific gene (or genes) of interest (Scheffel et al, 2007;Ding et al, 2010;Murat et al, 2010;Murat et al, 2012;Tanaka et al, 2011;Arakaki et al, 2014), the results are not yet conclusive, because there are many other factors that may influence those results. MamC (Valverde-Tercedor et al, 2015), Mms6 (Ameniya et al, 2007;Prozorov et al, 2007;Arakaki et al, 2010;Rawlings et al, 2016) and MmsF (Rawlings et al, 2014) have been expressed as recombi-…”