|
Cell
cycle regulation and cellular differentiation.
Our research program addresses the principles that govern temporally and spatially
controlled-dynamics of protein localization, and the mechanisms that underlie
cell cycle control and the acquisition and propagation of asymmetry. To do this,
we use a simple prokaryotic model system, Caulobacter crescentus, whose
life cycle depends on obligate steps of cell differentiation and asymmetric cell
division. This simple bacterial system provides sophisticated genetics and biochemistry,
ease of obtaining synchronized cell cycle cultures, new cytology tools to study
protein dynamics in live cells, and post-genomic DNA microarray techniques that
can systematically monitor the expression of the 3,700 C. crescentus genes. |
|
 |
|
Each cell division in C. crescentus is
asymmetric and gives rise to a swarmer (SW) progeny and a stalked (ST)
progeny (Fig.
1). The motile but replication inert swarmer cell (SW) has first
to differentiate into a non-motile stalked cell (ST) before it can initiate DNA replication. During the
swarmer-to-stalked (G1-S) cell transition, the flagellum and
pili are replaced by a polar stalk. The stalked cell elongates into
a predivisional
cell which builds a new flagellum at the pole opposite the stalk.
Each morphogenetic event during the cell cycle relies on the completion
of
a specific step of the cell cycle. Several two-component signal
transduction proteins are involved in controlling differentiation and
cell cycle progression
in this organism. We and others discovered that several essential
signal transduction proteins change their subcellular location in a
cell cycle-dependent
fashion (movies), adding an unexpected layer of spatial regulation.
We have recently shown that asymmetric spatial distribution of cell
cycle
regulators is not limited to intrinsically polarized bacteria
but is also employed by bacteria with no apparent asymmetry or polarity
(movie
3). This suggests that protein localization may provide a general
mechanism of prokaryotic regulation to control signal transduction.
Using a combination
of genetic, biochemistry, and innovative cell imaging approaches,
we are investigating the significance and the temporal and spatial
mechanisms
of protein localization in bacteria.
|
|
Cell
shape and the bacterial cytoskeleton.
The eukaryotic cytoskeleton, which consists of microtubules,
actin microfilaments and intermediate filaments, plays
a major role in maintaining the various cell
shapes of higher organisms. The prokaryotic world is also rich in cell shapes
whose conservation for a given species highlights its importance for fitness.
Yet, little is known about how bacteria achieve their diverse, often asymmetric
cell shapes. In most bacteria, cell shape maintenance requires the integrityof
the exocellular cell wall. It was recently discovered that
bacteria  possess functional and structural homologs of
actin (MreB and
relatives) that support the shape by forming helical, actin-like
cables that encircle the cell beneath the cytoplasmic membrane.
This actin-like cytoskeleton plays a major role in determining
the rod shape of Escherichia coli, Bacillus subtilis,
C. crescentus, and presumably all non- spherical bacteria.
Lately, we have discovered that, in addition to previously
characterized actin-like and tubulin-like proteins (the cell
division protein, FtsZ, being the bacterial counterpart of
tubulin), bacteria can also possess an intermediate filament-like
cytoskeleton.
|
We have shown that an intermediate filament-like protein,
crescentin (encoded by creS) is essential for the
vibroid (curved rod)
and helical shapes of C. crescentus. In its absence, the cells adopt
a straight-rod morphology (Fig. 2). Crescentin and animal intermediate filament
 proteins share many characteristic features, including the ability to assemble
into filaments in vitro without energy or cofactor
requirements. In
vivo, crescentin forms filamentous structures that colocalize with the inner
cell curvatures (Fig. 3), suggesting that the laterally asymmetric localization
of crescentin causes cell curvature. Using cell imaging techniques, genetics,
and biochemistry, we are now 1) investigating the mechanism by which crescentin
filaments cause cell curvature, 2) characterizing the assembly properties of
crescentin filaments, in comparison with those of animal intermediate filaments,
3) identifying other cytoskeletal and cytoskeleton-associated factors involved
in bacterial cell shape. |
|
| |
|
|
