Program of the 2001 International Worm Meeting - Sternberg Lab ...

1003
1003. The Roles of the par Genes in
Intracellular Motility Before the First
Asymmetric Cell Division in C.
elegans
Rebecca J Cheeks, Bob Goldstein
University of North Carolina-Chapel Hill,
Department of Biology, Chapel Hill, NC
27599-3280
Generation of cell diversity during development
is accomplished either via cell-cell signaling or
asymmetric cell division. In asymmetric cell
division, two distinct daughter cells are born.
Between fertilization and the first cell division
in the C. elegans zygote, central and cortical
cytoplasmic flows are observed. The timing of
these flows coincides with the asymmetric
localization of P granules, RNAs, proteins and
cytoskeletal elements.
To begin to determine whether some of the
genes required for asymmetric cell division play
a role in polarized intracellular motility, we
asked whether the par genes (1) are required for
cytoplasmic flows. Using DIC microscopy, we
filmed embryos with mutations in the par genes
and analyzed rates and duration of flows as well
as distances traveled along the axis of the
embryo, and compared them to wild-type. We
found that strong loss-of-function mutations in
par-2, -3, -4, and -6 prevent cytoplasmic flows,
whereas par-1 mutant embryos (b274, it51, it60)
have flows comparable to wild-type. Kemphues
and colleagues (1) have placed the par genes
into a pathway by analysis of their protein
distributions in various genetic backgrounds.
Our results reveal that, with the exception of
par-1, the par pathway is required to produce
polarized cytoplasmic flows.
P granules are normally redistributed to the
posterior of the embryo before the first cell
division. Hird et al. (2) found that P granules
redistribute primarily by moving to the posterior
at the same time as flows occur, and at a similar
rate, suggesting that P granules might be moved
by bulk flows of cytoplasm. We have filmed
fluorescent P granules and cytoplasmic flows in
the same embryos, and created overlays of these
films. Consistent with the hypothesis that P
granules move by bulk flows of cytoplasm, we
have found that P granules move perfectly
coincident with the flows of yolk granules (n= 6
1003
embryos; 30/30 P granules in the central
cytoplasm, 20/20 P granules in the cortical
cytoplasm). This result suggests that the
mislocalized P granule phenotype in par-2, -3,
-4, and -6 may be a result of failure to produce
cytoplasmic flows. We are currently testing
whether P granules fail to move in these mutants
by filming mutant embryos similarly.
par-1 mutants have cytoplasmic flows
comparable to wild-type embryos but the P
granules do not become asymmetrically
localized (1). These results, along with the
posterior cortical localization of PAR-1 (3),
suggest to us at least two hypotheses for the role
of PAR-1 in localization of P granules. First,
PAR-1 might be part of a complex that anchors
P granules in place once they reach the posterior
cortex, preventing further movement along the
cortex, even as cortical flows continue. This
hypothesis appears unlikely, since our
preliminary evidence from wild-type embryos
suggests that no such anchor exists: P granules
in the cortical cytoplasm stop moving when
cortical flows stop, and not before (3/3 embryos,
9/9 P granules). Second, PAR-1 might be
required to prevent degradation of P granules,
acting as a "safe haven" for P granules that
reach the posterior cortex. Consistent with this,
Hird et al. found that P granules in wild-type
embryos that do not reach the posterior
disappear, suggesting that a P granule degrading
or disassembling activity is present in the
anterior of the embryo (2), and Guo and
Kemphues (3) have noted that P granules are
missing in 2-cell par-1 mutant embryos.
Experiments testing the safe haven hypothesis
are currently underway.
1. Rose & Kemphues (1998) Annu Rev Genet
32:521-45 2. Hird et al (1996) Development
122:1303-12 3. Guo and Kemphues (1995) Cell
81:611-20