Structure–function studies of the magnetite-biomineralizing magnetosome-associated protein MamC

Abstract

Magnetotactic bacteria are Gram-negative bacteria that navigate along geomagnetic fields using the magnetosome, an organelle that consists of a membrane-enveloped magnetic nanoparticle. Magnetite formation and its properties are controlled by a specific set of proteins. MamC is a small magnetosome-membrane protein that is known to be active in iron biomineralization but its mechanism has yet to be clarified. Here, we studied the relationship between the MamC magnetite-interaction loop (MIL) structure and its magnetite interaction using an inert biomineralization protein-MamC chimera. Our determined structure shows an alpha-helical fold for MamC-MIL with highly charged surfaces. Additionally, the MamC-MIL induces the formation of larger magnetite crystals compared to protein-free and inert biomineralization protein control experiments. We suggest that the connection between the MamC-MIL structure and the protein's charged surfaces is crucial for magnetite binding and thus for the size control of the magnetite nanoparticles.

Functionalized Biomimetic Magnetic Nanoparticles as Effective Nanocarriers for Targeted Chemotherapy

“…The extremely low yields of MNPs from magnetotactic bacteria cultures prevent their use in biomedicine. However, BMNPs production can be scaled up in vitro in eco‐friendly, cost‐effective magnetite precipitation experiments run at room temperature and 1 atm total pressure by simple addition of the recombinant MamC protein from Magnetococcus marinus MC‐1 . These BMNPs i) are super‐paramagnetic at room and body temperature while they present a saturation magnetization of 55 emu g −1 (61 emu g −1 corresponding to the magnetic core, at 500 Oe, 25 °C); ii) are larger than most commercial SPION and/or other biomimetic magnetites, which makes them to be single magnetic domain, showing higher blocking temperature and slower magnetization increase, and thus, larger magnetic moment per particle; iii) contain up to 4.5 wt% of MamC that gives them novel surface properties and provides functional groups that allow functionalization; iv) have an isoelectric point (iep) of pH 4.4, and are strongly negatively charged at physiological pH (pH 7.4).…”

The importance of the helical structure of a MamC-derived magnetite-interacting peptide for its function in magnetite formation

“…MamC from Magnetospirillum magneticum AMB-1 is a small integral membrane protein (12.4 kDa) with two transmembrane helices connected by a small peptide of 21 residues in length (MamC magnetite-interacting component; MamC-MIC) that is directed into the magnetosome lumen. In in vitro experiments, MamC, expressed as a recombinant protein, was shown to affect the size of magnetite crystals, with larger crystals growing in the presence of the protein compared with those grown in the absence of MamC (Valverde-Tercedor et al, 2015;Nudelman et al, 2016).…”

Membrane protein engineering to the rescue

“…Some of these proteins are found tightly bound to the crystal, interacting via the loops between transmembrane spanning helices. In one example the loop region from MamC was transplanted onto maltose binding protein (MBP) [73] . This allowed the crystal structure of the loop to be obtained that revealed a helical conformation, from which a proposed magnetite interaction site was derived [73] .…”

“…Protein produced using this approach was tested in in vitro magnetite formation reactions to ascertain whether this construct retained their native functionality. Inspection of the synthesised magnetite nanocrystals showed these proteins were able to influence the properties of synthetic magnetite nanoparticles [73] .…”