Chemistry Reference
In-Depth Information
it was the most abundant of 5 proteins present in the vesicle. Cells grown in the presence
of AlF
4
−
, a GTPase inhibitor, showed less overall magnetism and produced fewer
magnetosomes, suggesting that GTPase activity is required for magnetosome synthesis
and involved in vesicle formation.
Iron transport into the magnetosome membrane vesicle.
Electron microscopy from
early on has shown magnetite crystals in various stages of maturity and that the crystals
increase in size within magnetosome vesicles. Thus, regardless of when the magnetosome
membrane vesicle is formed, additional iron must be transported through the
magnetosome vesicle for the crystal to grow. It is not known which redox forms of iron
are transported into the magnetosome vesicle in most magnetotactic bacteria but there is
evidence that Fe
2+
is transported into vesicles of
Magnetospirillum magneticum
strain
AMB-1 (Nakamura et al. 1995a).
The
magA
gene was identified through transposon mutagenesis that encodes for a
protein with significant homology to the cation efflux proteins, KefC, a K
+
translocating
protein in
Escherichia coli
and NapA, a putative Na
+
/H
+
antiporter from
Enterococcus
hirae
(Nakamura et al. 1995a,b). The MagA protein is present in both the cytoplasmic
and magnetosome membranes of
Magnetospirillum magneticum
strain AMB-1. Inverted
membrane vesicles prepared from
E. coli
cells that expressed
MagA
transported Fe
2+
in
an energy-dependent manner into the vesicle suggesting that MagA functions as a
H
+
/Fe
2+
antiporter in
M. magneticum
strain AMB-1. However,
magA
was expressed to a
much greater degree when wild-type
M. magneticum
strain AMB-1 cells were grown
under iron-limited conditions than under iron-sufficient conditions where they produce
more magnetosomes. Moreover, cells of the non-magnetotactic Tn5 transposon mutant
over-expressed
magA
under iron-limited conditions although they did not make
magnetosomes. Although MagA appears to be involved in iron transport, it alone is not
responsible for magnetosome synthesis. Genes sharing significant homology with
magA
are present in other magnetotactic bacteria including
M. magnetotacticum
and the
unnamed magnetotactic coccus, strain MC-1 (Grünberg et al. 2001).
Controlled Fe
3
O
4
biomineralization within the magnetosome vesicle.
Frankel et al.
(1983) proposed a model in which Fe
3+
is taken up by the cell, reduced to Fe
2+
, and
transported to the magnetosome membrane vesicle. It is then presumably reoxidized in
the magnetosome vesicle to form hydrous Fe
3+
oxides similar to the mineral ferrihydrite.
One-third of the Fe
3+
ions in ferrihydrite are reduced and with further dehydration,
magnetite is produced. Contrary to this study, cells of
M. gryphiswaldense
take up Fe
3+
and rapidly use it in the formation of magnetite without any apparent delay (Schüler and
Bäuerlein 1998), suggesting that significant accumulation of a precursor to magnetite
inside the cell does not occur, at least under conditions that appeared to be optimal for
magnetite production by that organism.
The size and morphology of the magnetosome mineral crystal is thought to be
controlled by the magnetosome membrane although how it does this is unclear. Specific
proteins are perhaps distributed asymmetrically in the magnetosome membrane
facilitating crystal growth in certain crystallographic directions but retarding it in others.
In addition, it is possible that the magnetosome membrane vesicle places physical
constraints on the growing crystal thereby limiting the size of the mineral crystal to the
single-magnetic-domain size range. Arakaki et al. (2003) partially characterized a number
of magnetosome membrane proteins that were tightly bound to the magnetite crystals in
Magnetospirillum magneticum
strain AMB-1. These proteins included Mms5, Mms6,
Mms7, and Mms13. Mms6 was overexpressed in
Escherichia coli,
purified, and found to
bind iron. More importantly, magnetite crystals, formed chemically in the presence of
Mms6, had a size range of about 20 to 30 nm and a cuboidal morphology like those
