The fact magnetosome formation is executed via protein-assistedmechanism was established for the first time by Arakaki and colleagues 46. In this elegantstudy, magnetosomes were sequentially isolated from bacterial cells of Magnetospirillum magneticum strain AMB-1followed by phospholipid bilayer excision. Next, purified nanocrystals weretreated 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), havebeen identified. Further investigation confirmed guide function of Mms proteinson newly synthesized nanocrystals’ morphology in vivo 47-50. Moreover,these proteins are found in Alphaproteobacteria, producing cubo-octahedral crystals, and absent in bullet-shapesynthesizing strains, indicating their specific status 51. Although synthesis in vitro may be performed innon-specific manner in the presence of other proteins executing templatefunction, Mms proteins turned out to alter crystal morphology and homogeneity 50. Table 1 summarizesresults obtained in recent studies aimed to investigate the ability of individual proteinsor peptides to form magnetite nanocrystals invitro.
Further we will focus on Mms6 to uncover some molecular mechanismsunderlying its catalytic functions since this protein is one of most often usedfor magnetite production. Mms6 is a small protein of 6 kDa, known to belocalized in both magnetosome membrane and magnetite nanocrystal. Genetic studies revealed both C- andN-terminal hydrophobic region of Mms6 are required for appropriate proteinconformation and localization on the surface of magnetite crystal. High-resolutionNMR studies revealed C-terminal DEEVE motif undergoing conformational changesupon magnetosome Fe3O4 crystal binding, while hydrophobicpacking of N-terminal region provides appropriate assembly and orientation ofDEEVE motifs crucial for magnetite crystal recognition 52. The key role ofC-terminal region in binding ferrous ion was evaluated 53. Additionally,Asp123, Glu124 and Glu125 resuidies were foundto be core amino acids having direct impact on shape and size of themagnetite 54. Interestingly, that Mms6 was detected alongmagnetosome chain structures under magnetite-forming conditions but wasdispersed in the cell under nonforming conditions, suggesting spatial mechanismof localization control 55. In addition toiron-binding activity, Mms6 protein appeared to be capable of cobaltbiomineralization.
Prozorov and colleagues revealed his-tagged Mms6 as well asits C-terminal domain were both able to produce CoFe2O4 nanocristals56. In this work Mms6was covalently attached to self-assembled polymers and template synthesis ofCoFe2O4 was performed. Intriguingly, synthetic C-terminaldomain of the protein containing only 25 amino acids possessed better catalyticactivity compared to that of full-length Mms6 protein. However,polypeptide appeared not to be necessaryto synthesize cobalt ferrite nanoparticles themselves, but to control their size, shape and phase by changing the kinetics of thenucleation and growth process 57. Indeed,Wolffand colleagues proved cobaltferrite nanocrystal growth is dominated by kinetics, matching oriented attachment model, which has been successfully applied to describe biomineralization previously 58.