Except for twinning in some cases, only single crystals are found within
magnetosomes, indicating a single nucleation event (Devouard et al. 1998; Faivre
and Sch€ uler 2008). This finding suggests the existence of a specific nucleation site
putatively provided by a single protein or complex. Several proteins have been
proposed to play roles in the nucleation and later growth process, but in vivo proof
remains sparse. The small Mms6 protein was described in M. magneticum as tightly
bound to the magnetite crystal (Arakaki et al. 2003). The protein is processed
proteolytically and bears a leucine-glycine (LG)-rich motif which is also conserved
in other magnetosome proteins and might mediate its aggregation. Its C-terminus is
acidic and thought to provide its iron-binding functionality as shown in vitro;
however, thus far the lack of mms6 mutants leaves open the question of whether
it acts as a nucleator in vivo or performs other necessary functions. The absence
of mms6 in the genome of D. magneticus also suggests that this gene might not be
essential for magnetite formation in MTB (Nakazawa et al. 2009).
During the formation of magnetite, protons are released and have to be
translocated from the magnetosome vesicles to maintain pH within the stable
regime for the magnetite phase. It has been speculated that MamN might provide
this H
+ efflux function in Magnetospirillum because it shows similarities to respective known proteins (Jogler and Sch€ uler 2007). MamT, thought to be a cytochrome
c heme-binding protein, has been speculated to be involved in possible redox
reactions involved in magnetite formation (Jogler and Sch€ uler 2007). However,
for both proteins no experimental data is available that proves or disproves these
postulated roles.
1.3.2 Why Are Magnetosomes an Extraordinary Material?
MTB have evolved means to optimize the structure of magnetosomes to work as a
magnetic field actuator. This optimization can be observed on at least three hierarchical levels from the A ˚ ngstr€ om to the submicrometer length scale, i.e., from the
atomic structure up to filaments within the bacterial cell. In the following sections,
we describe these levels of structural optimization with respect to function and how
they are achieved.
1.3.2.1 Magnetosome Structure
Magnetite (Fe
III
[Fe
II
Fe
III
]O 4 ) can easily be oxidized to maghemite (Fe
III
[Fe 5/3
III
[ ] 1/3 ]O 4 )
even at low temperatures, and naturally occurring magnetite is typically partially
oxidized in its equilibrium state. This oxidation from stoichiometric magnetite to
maghemite leads to a decrease in magnetic saturation moment that is disadvantageous for its magnetic properties required for magnetotaxis. The transformation
between both iron oxide phases is indicated by a gradual decrease in its lattice
parameter a from 8.397 A ˚ to 8.347 A ˚ , respectively. Recent findings in our group
1 Magnetite Biomineralization in Bacteria
15
magnetosomes, indicating a single nucleation event (Devouard et al. 1998; Faivre
and Sch€ uler 2008). This finding suggests the existence of a specific nucleation site
putatively provided by a single protein or complex. Several proteins have been
proposed to play roles in the nucleation and later growth process, but in vivo proof
remains sparse. The small Mms6 protein was described in M. magneticum as tightly
bound to the magnetite crystal (Arakaki et al. 2003). The protein is processed
proteolytically and bears a leucine-glycine (LG)-rich motif which is also conserved
in other magnetosome proteins and might mediate its aggregation. Its C-terminus is
acidic and thought to provide its iron-binding functionality as shown in vitro;
however, thus far the lack of mms6 mutants leaves open the question of whether
it acts as a nucleator in vivo or performs other necessary functions. The absence
of mms6 in the genome of D. magneticus also suggests that this gene might not be
essential for magnetite formation in MTB (Nakazawa et al. 2009).
During the formation of magnetite, protons are released and have to be
translocated from the magnetosome vesicles to maintain pH within the stable
regime for the magnetite phase. It has been speculated that MamN might provide
this H
+ efflux function in Magnetospirillum because it shows similarities to respective known proteins (Jogler and Sch€ uler 2007). MamT, thought to be a cytochrome
c heme-binding protein, has been speculated to be involved in possible redox
reactions involved in magnetite formation (Jogler and Sch€ uler 2007). However,
for both proteins no experimental data is available that proves or disproves these
postulated roles.
1.3.2 Why Are Magnetosomes an Extraordinary Material?
MTB have evolved means to optimize the structure of magnetosomes to work as a
magnetic field actuator. This optimization can be observed on at least three hierarchical levels from the A ˚ ngstr€ om to the submicrometer length scale, i.e., from the
atomic structure up to filaments within the bacterial cell. In the following sections,
we describe these levels of structural optimization with respect to function and how
they are achieved.
1.3.2.1 Magnetosome Structure
Magnetite (Fe
III
[Fe
II
Fe
III
]O 4 ) can easily be oxidized to maghemite (Fe
III
[Fe 5/3
III
[ ] 1/3 ]O 4 )
even at low temperatures, and naturally occurring magnetite is typically partially
oxidized in its equilibrium state. This oxidation from stoichiometric magnetite to
maghemite leads to a decrease in magnetic saturation moment that is disadvantageous for its magnetic properties required for magnetotaxis. The transformation
between both iron oxide phases is indicated by a gradual decrease in its lattice
parameter a from 8.397 A ˚ to 8.347 A ˚ , respectively. Recent findings in our group
1 Magnetite Biomineralization in Bacteria
15