Phone: (301) 435-2814
Fax: 301-402-3095
Biography:
Research:
The G1 phase of the cell cycle represents a critical stage where
cells can respond to extracellular cues either to commit to another round
of cell division, to withdraw temporarily from the cell cycle, or to terminally
differentiate. Studies from both yeast and mammalian systems suggest
that progression through G1 is regulated in response to extra- and intracellular
signals which act directly on the cell cycle machinery. I am interested
in exploring the mechanisms involved in regulating G1 progression in vivo,
and the requirement for G1 in the developmental decision to proliferate
or to differentiate. To this end, I have been studying a gene, roughex
(rux), that is required to arrest cells in G1 in the developing compound
eye of Drosophila.
A striking feature of development in the Drosophila eye is the simultaneous
synchronization of cell cycle progression in G1 and the onset of pattern
formation mediated by intercellular signaling molecules. The adult
eye of Drosophila develops from a tissue called the eye imaginal disc,
which is formed during embryogenesis from a small group of cells that are
determined to form eye tissue. These cells proliferate steadily during
the first two stages of larval growth; differenitation initiates during
the third and final stage of larval development within a physical constriction
in the eye disc epithelium called the morphogenetic furrow (MF). The onset
of differenitation in the MF is marked by a synchronization in cell cycle
progression and arrest in the G1 phase of the cell cycle. Thus, anterior
to the MF cells are undifferentiated and cycle asynchronously, while posterior
to the MF cells begin to differentiate and undergo a single, synchronous
cell division. We have previously shown that rux mutants fail to arrest
in G1 in the MF, and instead all cells ectopically re-enter S phase. The
loss of G1 leads to subsequent defects in pattern formation and cell fate
determination. This suggests that G1 must be actively established
and maintained during development by a pathway that requires rux, and that
cell fate determination is dependent on G1 arrest.
The rux locus encodes a polypeptide of 335 amino acids with an N-terminal
cyclin-binding motif and a C-terminal bipartite NLS. Previous genetic studies
indicated that Rux is required to inhibit the kinase activity associated
with the G2 cyclin, Cyclin A (CycA). Molecular experiments indicate that
Rux functions by binding to and inhibiting CycA-dependent kinase activity,
and may function in part by dissociating the cyclin from its kinase partner.
Further, in vivo studies show that CycA protein becomes mislocalized to
nucleus and is degraded when Rux is overexpressed. We are currently exploring
the idea that Rux may target CycA for destruction in G1 cells.
Interestingly, Rux protein itself is degraded in cells that normally
re-enter S phase for a final wave of cell division behind the MF. S phase
in higher eukaryotes is marked by the expression and activation of a cyclin
complex containing the G1 cyclin, CycE. We have shown that Rux also binds
CycE, and we propose that Rux is targeted for destruction by a CycE-dependent
kinase activity in cells that re-enter S phase. In support of this notion,
the mislocalization and subsequent degradation of CycA resulting from Rux
overexpression is reversed in cells that also overexpress CycE. This effect
is dependent on four consensus sites for phosphorylation by cyclin-dependent
kinases present in the Rux protein and a mutant derivative of Rux lacking
these sites is stabilized in S phase cells. Interestingly, the CycA-binding-defective
version of Rux is also stable in S phase cells, suggesting that this site
may also mediate binding to CycE.
A large-scale screen to identify new dominant suppressers of rux
has identified a number of novel loci as well as the Drosophila patched
(ptc) gene. Ptc functions in the Hedgehog signaling cascade, suggesting
that this approach will be useful to identify genes that are required for
regulating cell cycle progression in response to developmental signals.
The high degree of conservation of cell cycle components between species
makes it likely that many of the pathways for regulating cell cycle progression
during development will be conserved during evolution.
Recent Publications:
I also have a listing on the Drosophila Interest Group web site.
Last revised on June 6, 2001, by Zoraida S. Villadiego
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