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Rebecca Cheeks PhD Student, 1997-2002 (in the Goldstein lab 1999-2002) Currently at the American Museum of Natural History How cells divide asymmetrically: PAR proteins play roles in mobilizing and stabilizing protein complexes before asymmetric division  In the past decade, a number of critical developmental factors that are partitioned in asymmetric divisions have been identified in Drosophila, C. elegans, and mammals. Despite this progress, we know very little about how the asymmetric distributions of these gene products are accomplished in any of these systems. The PAR proteins are part of an ancient and widely conserved machinery for polarizing cells during animal development. We have used a combination of genetics and live imaging methods in the model organism Caenorhabditis elegans to dissect the cellular mechanisms by which PAR proteins polarize cells. Our experiments have found two distinct mechanisms by which PAR proteins polarize  the
C. elegans
zygote. First, several components of the PAR pathway
function in intracellular motility, producing a polarized movement of
the cell cortex. We have found evidence that this cortical motility may
drive the movement of cellular components that must become
asymmetrically distributed, including both germline-specific
ribonucleoprotein complexes and cortical domains containing the PAR
proteins themselves. Second, PAR-1 functions to refine the asymmetric
localization of germline ribonucleoprotein complexes by selectively
stabilizing only those complexes that reach the PAR-1-enriched
posterior cell cortex during the period of cortical motility.
These results have identified two cellular mechanisms by which the PAR
proteins polarize the C. elegans zygote, and they suggest
mechanisms by which PAR proteins may polarize cells in diverse animal
systems.
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