Crystal structure of the
crystallographer MamP
An international consortium led by
CEA researchers, in collaboration with
the CNRS, has succeeded in characterizing
the structure and function of a protein
involved in the production of magnetite
nanomagnets in magnetotactic bacteria.
This protein, MamP, is crucial to the
metallurgical activity of the bacterium.
It is this protein that gives the magnetite
its magnetic properties. This work
constitutes an important advance
in the understanding of these bacteria
and the magnetite biomineralization process.
It is expected to result in the development
of additional biotechnological applications
for these nanomagnets, especially
in the fields of medical imaging
and the decontamination of water.
Magnetotactic bacteria have the ability
to synthesize nanocrystals of magnetite
(Fe
3
O
4
) enabling them to align themselves
with the terrestrial magnetic field in order
to find the position in the water column
that is most favorable to their survival.
The alignment of the nanomagnets
is similar to that of a compass needle.
The magnetite crystal synthesis process
is a complex one, and it is little understood
at the present time. Magnetite is a
compound of oxygen and iron in a mixture
of two different oxidation states [Fe(II)
Fe(III)
2
O
4
]. In this study, the researchers
have described the mechanism
by which the bacterium produces
these two states, one of which, Fe(III),
is essentially insoluble.
The determination of the structure
of the protein MamP, using PROXIMA 1
beamline, has shown for the first time
that a section of this protein possesses
an original folding structure known
as a magnetochrome. This structure
is only found in magnetotactic bacteria.
The structure has a crucible-like shape
capable of containing iron. Additional
experiments have shown that MamP has
the ability to oxidize iron from the Fe(II)
state to the Fe(III) state, and to stabilize the
latter in its crucible. Mutagenesis studies
and the phenotyping of magnetotactic
bacteria variants have confirmed the
physiological importance of this crucible.
Finally, a number of
in vitro
experiments
have shown that MamP is capable
of producing a magnetite precursor when
incubated in the presence of Fe(II) alone,
proving that the Fe (III) results from
the activity of this protein.
This fundamental study reveals part of
the process whereby iron is biomineralized
and nanomagnets are synthesized
in magnetotactic bacteria. The potential
applications of these nanomagnets appear
promising. They may, for example,
by used as a contrast agent in magnetic
resonance imaging. Another possible
application relates to the decontamination
of water supplies. Magnetotactic bacteria
carrying an enzyme that breaks down
a contaminant may be used to treat
effluent and may then easily be removed
from the water by means of a magnet.
BIOLOGY AND HEALTH SCIENCES
68
SYNCHROTRON
HIGHLIGHTS
2013
