2002 ASCB Annual Meeting Press Book - page 10

December 14-18, 2002, San Francisco, CA
The Geologist’s Delight,
the Biologist’s Dilemma
There is nothing new under the sun, or for that matter
under the cell membrane. Bacteria invented the magnetic
compass billions of years before any human being sailed the
ocean blue, eyes on the binnacle. Magnetotactic bacteria are
an ancient and diverse group of prokaryotes with the unique
ability to align themselves in the earth’s magnetic field, ap-
parently to guide themselves to low oxygen microenviron-
ments. Their compass is an organelle called the magnetosome
that consists of an inorganic magnetite crystal surrounded by
a lipid bilayer. Magnetosomes usually form a single chain
near the cell membrane of the bacterium, creating a larger
magnet that is sensitive to external magnetic fields.
For geologists, magnetosomes are a gift. Their shape
and magnetic properties are so distinctive that geologists can
identify them in samples that are billions of years old. These
“magnetofossils” have been used to trace the history of life
on earth and the evolution of the earth’s magnetic field over
For cell biologists, the magnetosomes are a problem.
Prokaryotes are not supposed to have organelles. Membrane-
bounded organelles, such as the nucleus and endoplasmic
reticulum, are supposed to be strictly the province of eukary-
otes. Yet magnetosomes, which are a distinctly prokaryotic
organelle, share many features of eukaryotic organelles. For
example, each magnetite crystal is formed within a membra-
nous vesicle made of a lipid bilayer. This vesicle contains
unique proteins that are not found in any other part of the cell
while the number of vesicles and even their position within
the cell is tightly regulated. Clearly magnetotactic bacteria,
like eukaryotic cells, possess mechanisms for the biogenesis,
assembly and maintenance of their organelles. Being so an-
cient, could magnetotactic bacteria offer insights into organelle
biogenesis in eukaryotes?
To answer these and other questions, Arash Komeili and
Dianne Newman of the Geological and Planetary Sciences
Department at Caltech have been applying genetic, cell bio-
logical, and biochemical approaches to magnetotactic bacte-
ria. They have identified several mutants that cannot form
magnetite, and are now looking for the specific defects that
interfere with vesicle formation or protein targeting. The genes
affected in these mutants localize to a previously identified
cluster that seems to be unique to magnetotactic bacteria.
This gene cluster codes for several proteins that have inter-
esting homologies to serine proteases, PDZ proteins, and even
actin. These are all players in some of the most fundamental
processes in eukaroytic cells, and genomic connections at
this level could, indeed, throw light on the common origins of
Contact: ArashKomeili, California Institute of Technology, Geologi-
cal and Planetary Sciences, 1200 E. California Street, N. Mudd 009,
Pasadena, CA 91125, (626) 395-8590,
Cell biology of the bacterial magnetosome.
A. Komeili, D. K.
Newman; Geological and Planetary Sciences, California In-
stitute of Technology, Pasadena, CA
At the ASCB Meeting: Presentation 1594, Minisymposium
21: Organelle Biogenesis and Inheritance. Author presents:
Tuesday, December 17, 5:05 —5:25 PM.
Thin section electron micrographs of wildtype and
magnetite mutants (mnm1 and mnm 2) of
Magnetospirillum magnetotacticum sp. AMB-1. The two
mutants have distinct phenotypes; mnm2 is devoid of any
minerals wheras mnm1 contains small mineral inclusions
arranged in a chain within the cell.
1,2,3,4,5,6,7,8,9 11,12,13,14,15,16,17,18,19,...20
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