in vitro with magnetotactic bacterial proteins such as Mms6 or peptides which
selectively recognize crystal faces and thus influence crystal morphology during
growth. An amorphous precursor which is later transformed into magnetite would
more probably allow shaping of more extraordinary crystals forms such as bullet or
tooth shapes, a phenomenon known from other biomineralizing systems such as
calcium carbonate in various higher organisms (Politi et al. 2008). So far,
explanations for non-isometric crystal forms in magnetosomes have remained
purely speculative and will need verification by experimental evidence in vivo
and vitro.
1.3.2.4 Magnetosome Chain Assembly and Chain Function
Magnetosome crystals are arranged in chain structures within the bacterial cell to
increase the magnetic dipole for enhanced functionality (Dunin-Borkowski et al.
1998). By assembly, the individual magnetic dipoles of magnetosome particles
are summed up, providing cells with the ability to orient in a magnetic field.
Magnetospirilla contain a single chain that is oriented along the longitudinal
cell axis. Some uncultured magnetotactic bacteria possess multiple chains with
sometimes more complex assemblies (Sch€ uler 2008). Colloidal magnetic particles
tend to aggregate or form so-called flux closure rings, a feature which can also be
observed with isolated magnetosomes (Philipse and Maas 2002; Xiong et al. 2007).
Induction experiments in M. gryphiswaldense have shown that magnetite particles
form at various sites and start to align during further growth at midcell (Faivre et al.
2007). A filamentous structure that would prevent collapse of magnetosome chains
into clusters or rings by connecting it to cellular structures had been proposed and
was later confirmed by cryo-electron tomography in M. gryphiswaldense and
M. magneticum (Komeili et al. 2006; Scheffel et al. 2006). The two proteins
MamK and MamJ have been shown to be involved in this chain assembly.
MamK is a bacterial actin-like protein assembling into a filamentous structure
along the magnetosome chain as visualized by fluorescent microscopy with a
GFP-fusion protein. The phenotype of a M. magneticum MamK deletion mutant
shows magnetosomes dispersed within the bacterial cell (Komeili et al. 2006).
Recombinant expression of MamK in E. coli led to the formation of detectable
filaments within this host and the isolated protein has also been polymerized in vitro
into filamentous bundles of 100 mm length and around 100 nm width, with single
filaments of around 6 nm in width (Pradel et al. 2006; Taoka et al. 2007). The
polymerization has been shown to be ATP dependent and could possibly drive
magnetosome arrangement by a treadmilling-like mechanism known from actin
and other filamentous protein polymers. Furthermore, the acidic protein MamJ,
which is only found in magnetospirilla, is involved in magnetosome chain formation in these species. A M. gryphiswaldense deletion mutant (DmamJ) no longer
produced chains of magnetosomes but clustered aggregates. Complementation of
the mamJ gene in the deletion mutant could restore chain formation. Two-hybrid
experiments in E. coli suggest that both proteins interact and that MamJ might serve
1 Magnetite Biomineralization in Bacteria
19
selectively recognize crystal faces and thus influence crystal morphology during
growth. An amorphous precursor which is later transformed into magnetite would
more probably allow shaping of more extraordinary crystals forms such as bullet or
tooth shapes, a phenomenon known from other biomineralizing systems such as
calcium carbonate in various higher organisms (Politi et al. 2008). So far,
explanations for non-isometric crystal forms in magnetosomes have remained
purely speculative and will need verification by experimental evidence in vivo
and vitro.
1.3.2.4 Magnetosome Chain Assembly and Chain Function
Magnetosome crystals are arranged in chain structures within the bacterial cell to
increase the magnetic dipole for enhanced functionality (Dunin-Borkowski et al.
1998). By assembly, the individual magnetic dipoles of magnetosome particles
are summed up, providing cells with the ability to orient in a magnetic field.
Magnetospirilla contain a single chain that is oriented along the longitudinal
cell axis. Some uncultured magnetotactic bacteria possess multiple chains with
sometimes more complex assemblies (Sch€ uler 2008). Colloidal magnetic particles
tend to aggregate or form so-called flux closure rings, a feature which can also be
observed with isolated magnetosomes (Philipse and Maas 2002; Xiong et al. 2007).
Induction experiments in M. gryphiswaldense have shown that magnetite particles
form at various sites and start to align during further growth at midcell (Faivre et al.
2007). A filamentous structure that would prevent collapse of magnetosome chains
into clusters or rings by connecting it to cellular structures had been proposed and
was later confirmed by cryo-electron tomography in M. gryphiswaldense and
M. magneticum (Komeili et al. 2006; Scheffel et al. 2006). The two proteins
MamK and MamJ have been shown to be involved in this chain assembly.
MamK is a bacterial actin-like protein assembling into a filamentous structure
along the magnetosome chain as visualized by fluorescent microscopy with a
GFP-fusion protein. The phenotype of a M. magneticum MamK deletion mutant
shows magnetosomes dispersed within the bacterial cell (Komeili et al. 2006).
Recombinant expression of MamK in E. coli led to the formation of detectable
filaments within this host and the isolated protein has also been polymerized in vitro
into filamentous bundles of 100 mm length and around 100 nm width, with single
filaments of around 6 nm in width (Pradel et al. 2006; Taoka et al. 2007). The
polymerization has been shown to be ATP dependent and could possibly drive
magnetosome arrangement by a treadmilling-like mechanism known from actin
and other filamentous protein polymers. Furthermore, the acidic protein MamJ,
which is only found in magnetospirilla, is involved in magnetosome chain formation in these species. A M. gryphiswaldense deletion mutant (DmamJ) no longer
produced chains of magnetosomes but clustered aggregates. Complementation of
the mamJ gene in the deletion mutant could restore chain formation. Two-hybrid
experiments in E. coli suggest that both proteins interact and that MamJ might serve
1 Magnetite Biomineralization in Bacteria
19