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Fig. 9. Working model of polarity establishment in C. elegans embryos. (A)
In wild-type meiosis II, a CUL-2-based E3 ligase that also requires
zyg-11 function ubiquitinates a substrate (X) that can induce
polarity when triggered by a surrogate polarity cue, thus targeting the
substrate for degradation by the proteasome. After exit from the meiotic cell
cycle, E3 ligase activity is downregulated, allowing substrate accumulation
and polarity establishment in response to the bona fide centrosome polarity
cue. Green oval, oocyte chromosomes; green disk, centrosomes; blue crescent,
cortical PAR-2; orange disks, P granules. Ubiquitin (Ub, blue circles)
moieties are also shown. (B) In the absence of zyg-11 or
cul-2, substrate accumulation occurs during the meiotic cell cycle,
resulting in polarity establishment in response to a surrogate polarity cue
that correlates with the position of oocyte chromosomes. A related outcome may
ensue when the APC is inactivated at meiosis I (not depicted here). After exit
from the meiotic cell cycle in the absence of zyg-11 or
cul-2, a second focus of polarity is established in response to the
centrosome polarity cue.
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