The 3 (MamC), have been identified. Further investigation

The fact magnetosome formation is executed via protein-assisted
mechanism was established for the first time by Arakaki and colleagues 46. In this elegant
study, magnetosomes were sequentially isolated from bacterial cells of Magnetospirillum magneticum strain AMB-1
followed by phospholipid bilayer excision. Next, purified nanocrystals were
treated with detergent and heat to extract tightly associated proteins. Four of them, namely magnetosome membrane specific (Mms) proteins Mms5 (MamG), Mms6, Mms7 (MamD) and Mms 3 (MamC), have
been identified. Further investigation confirmed guide function of Mms proteins
on newly synthesized nanocrystals’ morphology in vivo 47-50. Moreover,
these proteins are found in Alphaproteobacteria, producing cubo-octahedral crystals, and absent in bullet-shape
synthesizing strains, indicating their specific status 51. Although synthesis in vitro may be performed in
non-specific manner in the presence of other proteins executing template
function, Mms proteins turned out to alter crystal morphology and homogeneity 50. Table 1 summarizes
results obtained in recent studies aimed to investigate the ability of individual proteins
or peptides to form magnetite nanocrystals in
vitro. Further we will focus on Mms6 to uncover some molecular mechanisms
underlying its catalytic functions since this protein is one of most often used
for magnetite production.

Mms6 is a small protein of 6 kDa, known to be
localized in both magnetosome membrane and magnetite nanocrystal. Genetic studies revealed both C- and
N-terminal hydrophobic region of Mms6 are required for appropriate protein
conformation and localization on the surface of magnetite crystal. High-resolution
NMR studies revealed C-terminal DEEVE motif undergoing conformational changes
upon magnetosome Fe3O4 crystal binding, while hydrophobic
packing of N-terminal region provides appropriate assembly and orientation of
DEEVE motifs crucial for magnetite crystal recognition 52. The key role of
C-terminal region in binding ferrous ion was evaluated 53. Additionally,
Asp123, Glu124 and Glu125 resuidies were found
to be core amino acids having direct impact on shape and size of the
magnetite 54. Interestingly, that Mms6 was detected along
magnetosome chain structures under magnetite-forming conditions but was
dispersed in the cell under nonforming conditions, suggesting spatial mechanism
of localization control 55. In addition to
iron-binding activity, Mms6 protein appeared to be capable of cobalt
biomineralization. Prozorov and colleagues revealed his-tagged Mms6 as well as
its C-terminal domain were both able to produce CoFe2O4 nanocristals
56. In this work Mms6
was covalently attached to self-assembled polymers and template synthesis of
CoFe2O4 was performed. Intriguingly, synthetic C-terminal
domain of the protein containing only 25 amino acids possessed better catalytic
activity compared to that of full-length Mms6 protein. However,
polypeptide appeared not to be necessary
to synthesize cobalt ferrite nanoparticles themselves, but to control their size, shape and phase by changing the kinetics of the
nucleation and growth process 57. Indeed,
and colleagues proved cobalt
ferrite nanocrystal growth is dominated by kinetics, matching oriented attachment model, which has been successfully applied to describe biomineralization previously 58.

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