Plant cell biology; intracellular protein trafficking, membrane biogenesis
The
mechanism of cytokinesis in higher plants is distinct from that of
animal and yeast cells. In dividing plant cells, cytosol and organelles
are partitioned between the two daughter cells by the formation of a
new cross wall known as the cell plate. The focus of my lab is to
understand the biochemical mechanisms that direct the formation of this
new membrane compartment. Suspension-cultured tobacco and Arabidopsis
cells are being used as model systems to study cell plate formation
because of their amenability to transgenic, biochemical and
morphological studies. Currently, we are using these cells to identify
and to characterize the transport of several soluble and membrane
secretory proteins to the plate.
Cell plate formation is
initiated when Golgi-derived vesicles are guided by a specialized
cytoskeletal structure called the phragmoplast toward the equatorial
region of the dividing cell, where they fuse. The plate then extends
out centrifugally as more vesicles fuse with it, until it joins with
the original cell plasma membrane, separating the two daughter cells.
Fusion of the transport vesicles may occur by homotypic (self) fusion,
heterotypic fusion, or by a combination of these processes. Recent
studies in yeast and animal cells suggest that these types of
intracellular membrane fusion are mediated by two homologous but
functionally distinct proteins, p97/Cdc48p and NSF. Homologs of these
cytosolic fusogenic factors have also been identified in plants and one
avenue of research in my lab is to examine their role in plate
biogenesis. Additional biochemical studies are being conducted to
identify candidates for other cytosolic and membrane components
involved in the fusion process.
A complementary approach
to the biochemical methodology is to exploit the genetic systems
available in plants, particularly in Arabidopsis. Genetic studies will
help to verify the in vivo role of proteins, identified through
biochemical means in cell plate biogenesis. We have begun to identify
and to characterize Arabidopsis plants containing insertional mutations
in several genes of interest, including CDC48. Future studies will
include genetic screens for mutants defective in cytokinesis in order
to identify other proteins, such as those involved in regulation of
cytokinesis, which may escape detection in a purely biochemical
approach.
Plant Cell Cytokinesis: The
phragmoplast, a unique cytoskeletal structure composed of microtubules
and microfilaments, appears late in mitosis: Golgi-derived vesicles
coalesce within the equatorial region of this structure, to form a
tubular-vesicular network filled with cell wall precursors. The cell
plate expands outward as more vesicles fuse with it until it joins with
the original cell plasma membrane, ultimately separating the daughter
cells. (Figure adapted from Biology of Plants, P. H. Raven, R. F.
Evert, S. E. Eichhorn, 1986 Worth Publishers, Inc.)