Understanding the Biomineralization Role of Magnetite-Interacting Components (MICs) From Magnetotactic Bacteria

Abstract

Biomineralization is a process that takes place in all domains of life and which usually helps organisms to harden soft tissues by creating inorganic structures that facilitate their biological functions. It was shown that biominerals are under tight biological control via proteins that are involved in nucleation initiation and/or which act as structural skeletons. Magnetotactic bacteria (MTB) use iron biomineralization to create nano-magnetic particles in a specialized organelle, the magnetosome, to align to the geomagnetic field. A specific set of magnetite-associated proteins (MAPs) is involved in regulating magnetite nucleation, size, and shape. These MAPs are all predicted to contain specific 17–22 residue-long sequences involved in magnetite formation. To understand the mechanism of magnetite formation, we focused on three different MAPs, MamC, Mms6 and Mms7, and studied the predicted iron-binding sequences. Using nuclear magnetic resonance (NMR), we differentiated the recognition mode of each MAP based on ion specificity, affinity, and binding residues. The significance of critical residues in each peptide was evaluated by mutation followed by an iron co-precipitation assay. Among the peptides, MamC showed weak ion binding but created the most significant effect in enhancing magnetite particle size, indicating the potency in controlling magnetite particle shape and size. Alternatively, Mms6 and Mms7 had strong binding affinities but less effect in modulating magnetite particle size, representing their major role potentially in initiating nucleation by increasing local metal concentration. Overall, our results explain how different MAPs affect magnetite synthesis, interact with Fe2+ ions and which residues are important for the MAPs functions.

Tuning properties of biomimetic magnetic nanoparticles by combining magnetosome associated proteins

“…1a). In this case, the percentage of negatively charged amino acids is 17.9% of the total amino acids of the C-terminal and, as proposed by previous authors, those acidic amino acids may bind Fe cation, being Asp12, Glu13, Glu16, and Asp19 (in Mms6-MIC peptide) specially relevant in terms of Kd (0.21 mM ± 0.12 mM for Fe 2+ ) 36 . Such binding results in a local increase of the supersaturation of the system with respect to magnetite and, thus, the nucleation of such a phase is induced in those specific areas due to an ionotropic effect 18,19,22,28,33,37,38,40 .…”

“…Acidic amino acids [Asp123, Glu124, Glu125 38 ] are claimed to be responsible for such a control through iron binding. Some of these amino acids (Asp12 and Glu13) have been identified in Mms6-MIC as strong iron binders with a low dissociation constants (Kd) 36 . Although these studies have been performed with Mms6 proteins from Magnetospirillum magnetotacticum AMB-1 18–22,24,25,27,28 , our multiple sequence analysis show that this C-terminal is relatively conserved into different species (Fig.…”

“…Moreover, Nudelman et al . 36 identified in MamC-MIC two residues with special affinity for Fe 2+ : Asp14, which corresponds to the Asp70 of the full length protein from AMB-1 (NCBI reference: WP_011383388.1), a residue that was already known to play a role in magnetite nucleation and binding 29 , and Gly16. Therefore, as in the case of Mms6, ionotropic effects could induce magnetite nucleation on those specific negatively charged areas.…”

The Biomineralization Proteome: Protein Complexity for a Complex Bioceramic Assembly Process

“…The transcriptome or RNA expression levels in these organisms are also being explored with the goal of understanding the timing of transcription, translation, and protein secretion as it pertains to the events occurring in the biomineralizing matrix . What has emerged from these studies…”

“…Within these genomes lurk mineralization‐associated genes. Since many biomineral‐associated proteins are extracellular and thus subjected to post‐translational modifications, these genomic studies have been augmented by proteomic studies that identify gene products that are phosphorylated, sulfated, glycosylated, enzymatically cleaved, or otherwise chemically modified at side‐chain groups . The challenge that we are now facing is to identify these mineralization‐associated proteins, their structure, synthesis, secretion, and temporal/regional localization, and most importantly, their purpose or function.…”