Adsorption of Biomineralization Protein Mms6 on Magnetite (Fe3O4) Nanoparticles

Abstract

Biomineralization is an elaborate process that controls the deposition of inorganic materials in living organisms with the aid of associated proteins. Magnetotactic bacteria mineralize magnetite (Fe₃O₄) nanoparticles with finely tuned morphologies in their cells. Mms6, a magnetosome membrane specific (Mms) protein isolated from the surfaces of bacterial magnetite nanoparticles, plays an important role in regulating the magnetite crystal morphology. Although the binding ability of Mms6 to magnetite nanoparticles has been speculated, the interactions between Mms6 and magnetite crystals have not been elucidated thus far. Here, we show a direct adsorption ability of Mms6 on magnetite nanoparticles in vitro. An adsorption isotherm indicates that Mms6 has a high adsorption affinity (Kd = 9.52 µM) to magnetite nanoparticles. In addition, Mms6 also demonstrated adsorption on other inorganic nanoparticles such as titanium oxide, zinc oxide, and hydroxyapatite. Therefore, Mms6 can potentially be utilized for the bioconjugation of functional proteins to inorganic material surfaces to modulate inorganic nanoparticles for biomedical and medicinal applications.

Keywords: biomineralization; magnetotactic bacteria; metal oxide; nanoparticle; protein adsorption.

Figure 1

Characteristic amino acid sequences in the Mms proteins. The structure prediction of Mms6 (A) and Mms7 (B) by RaptorX software. (C) The sequence alignment of the N-terminal hydrophobic region. (D) The sequence alignment of the C-terminal hydrophilic region; acidic amino acids are highlighted in yellow.

Figure 2

The adsorption assay of different proteins on magnetite nanoparticles. Adsorbed (black bar) or unadsorbed (white bar) protein amounts of His-Mms6, His-Mms7, His-α synuclein, BSA, cytochrome c, and lysozyme. A solution containing 40 µg of protein was added to 2.5 mg of the particles. N.D.: not detected.

Figure 3

Adsorption isotherm for the His-Mms6 protein interacting with the magnetite nanoparticles. The dissociation constant obtained from the curve fitting was Kd = 9.52 µM.

Figure 4

The TEM image of the magnetite nanoparticles in the absence (A) or presence (C) of the His-Mms6 protein. A single magnetite nanoparticle observed using HR-TEM in the absence (B) or presence (D) of the His-Mms6 protein. The white arrow indicates the protein corona.

Figure 5

The TEM images of (A) SP35, (B) OP177, (C) SP214, and (D) OP200. (E) Adsorbed (black bar) or unadsorbed (white bar) protein amounts of His-Mms6 to different magnetite nanoparticles (SP35, OP177, SP214, and OO200). A solution containing 40 µg of protein was added to 2.5 mg of the particles.

Figure 6

The binding assay of the His-Mms6 protein to nanoparticles. The TEM image of the (A) titanium oxide nanoparticles, (B) zinc oxide nanoparticles, and (C) hydroxyapatite nanoparticles. (D) Adsorbed (black bar) or unadsorbed (white bar) protein amounts of His-Mms6 to each nanoparticle. A solution containing 40 µg of protein was added to 2.5 mg of the particles.