as an anchor for magnetosome vesicles to the MamK filament (Scheffel et al. 2006;
Scheffel and Sch€ uler 2007). However, this proposed model is partly contradicted by
the different phenotypes of DmamK and DmamJ mutants, as one would expect
similarities between both, assuming an anchoring function on a filamentous structure. It is unclear whether discrepancies are due to experimental differences,
species-dependent differences, or because the proposed model is oversimplified
(Sch€ uler 2008). Furthermore, MamJ has so far only been identified in magnetospirilla, leaving open the question of how other magnetotactic organisms achieve
chain formation.
1.4 Applications of Magnetosomes
Synthetic magnetic iron oxide nanoparticles are used or in development for a
variety of technical applications, particularly in the biotechnological and medical
field. Examples are the magnetic separation of various biomolecules, magnetic
resonance imaging (MRI), hyperthermia treatment of cancer cells and other possible applications (Laurent et al. 2008). Due to their monodisperse size in the
magnetic single domain range, magnetosomes are particularly interesting with
respect to these potential applications.
Synthetic particles can easily be produced in larger quantities, but for a wide
array of applications, a coating of the particle surface is necessary to obtain stable
colloidal solutions in water and to provide an anchor to add further functionality.
Several synthetic routes have been developed to address these issues, e.g., coating
with charged, aliphatic molecules or SiO 2 .
Magnetosomes are themselves provided with colloidal stability in aqueous
solution as they possess a lipid membrane preventing crystallite aggregation and
transmembrane proteins which can be used as anchors for diverse modifications,
introduced either chemically or genetically (Lang et al. 2007). Functionalizations
have been chemically introduced using standard methods for covalent
modifications of proteins by, e.g., glutaraldehyde cross-linking or NHS-esters
(Matsunaga and Kamiya 1987). Secondary modifications have been produced
using biotin–streptavidin links (Amemiya et al. 2005; Ceyhan et al. 2006). Genetically introduced modifications have been obtained by fusing proteins of interest to
magnetosome membrane proteins.
First attempts were carried out with the MagA protein and the myristoylanchored Mms16 in M. magneticum (Nakamura et al. 1995; Matsunaga et al.
2000; Yoshino et al. 2004). A luciferase-based assay later identified Mms13 as
better anchor in magnetosomes of this organism, putatively by a crystal-binding
mechanism (Yoshino and Matsunaga 2006). More recently, to detect the best
potential membrane proteins as anchors in M. gryphiswaldense, several candidates
were fused to the enhanced green fluorescence protein (EGFP) and analyzed
according to resulting magnetosome particle fluorescence by flow cytometry and
fluorescence microscopy. As biomineralization of magnetite is favored under
20
J. Baumgartner and D. Faivre
Scheffel and Sch€ uler 2007). However, this proposed model is partly contradicted by
the different phenotypes of DmamK and DmamJ mutants, as one would expect
similarities between both, assuming an anchoring function on a filamentous structure. It is unclear whether discrepancies are due to experimental differences,
species-dependent differences, or because the proposed model is oversimplified
(Sch€ uler 2008). Furthermore, MamJ has so far only been identified in magnetospirilla, leaving open the question of how other magnetotactic organisms achieve
chain formation.
1.4 Applications of Magnetosomes
Synthetic magnetic iron oxide nanoparticles are used or in development for a
variety of technical applications, particularly in the biotechnological and medical
field. Examples are the magnetic separation of various biomolecules, magnetic
resonance imaging (MRI), hyperthermia treatment of cancer cells and other possible applications (Laurent et al. 2008). Due to their monodisperse size in the
magnetic single domain range, magnetosomes are particularly interesting with
respect to these potential applications.
Synthetic particles can easily be produced in larger quantities, but for a wide
array of applications, a coating of the particle surface is necessary to obtain stable
colloidal solutions in water and to provide an anchor to add further functionality.
Several synthetic routes have been developed to address these issues, e.g., coating
with charged, aliphatic molecules or SiO 2 .
Magnetosomes are themselves provided with colloidal stability in aqueous
solution as they possess a lipid membrane preventing crystallite aggregation and
transmembrane proteins which can be used as anchors for diverse modifications,
introduced either chemically or genetically (Lang et al. 2007). Functionalizations
have been chemically introduced using standard methods for covalent
modifications of proteins by, e.g., glutaraldehyde cross-linking or NHS-esters
(Matsunaga and Kamiya 1987). Secondary modifications have been produced
using biotin–streptavidin links (Amemiya et al. 2005; Ceyhan et al. 2006). Genetically introduced modifications have been obtained by fusing proteins of interest to
magnetosome membrane proteins.
First attempts were carried out with the MagA protein and the myristoylanchored Mms16 in M. magneticum (Nakamura et al. 1995; Matsunaga et al.
2000; Yoshino et al. 2004). A luciferase-based assay later identified Mms13 as
better anchor in magnetosomes of this organism, putatively by a crystal-binding
mechanism (Yoshino and Matsunaga 2006). More recently, to detect the best
potential membrane proteins as anchors in M. gryphiswaldense, several candidates
were fused to the enhanced green fluorescence protein (EGFP) and analyzed
according to resulting magnetosome particle fluorescence by flow cytometry and
fluorescence microscopy. As biomineralization of magnetite is favored under
20
J. Baumgartner and D. Faivre