Symmetry Breaking in C. elegans: Another Gift from the Sperm

Department of Biology, University of North Carolina at Chapel Hill, North Carolina, United States
Developmental Cell (Impact Factor: 9.71). 10/2006; 11(3):273-4. DOI: 10.1016/j.devcel.2006.08.007
Source: PubMed


Polarization of the C. elegans embryo depends on the sperm-contributed centrosome, which cues a retraction of the actomyosin cortex to the opposite end of the embryo by an unknown mechanism. New evidence reveals that the sperm donates a second polarizing cue that may locally relax the actomyosin cortex near the point of sperm entry.

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Available from: Daniel J Marston, Jan 13, 2014
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    • "The anteroposterior polarity of the embryo is established at fertilization (Goldstein and Hird, 1996). Sperm entering at the posterior pole initiate waves of cortical contraction that move anteriorly; these events result in an anterior cap of contractile, cortical actomyosin, and a non-contractile posterior domain (for reviews, see Cowan and Hyman, 2007; Marston and Goldstein, 2006). A complex of anterior PAR proteins (PAR-3, PAR-6, PKC- 3) accumulates at the anterior cap, at least in part through association with the actomyosin network, while the kinase PAR-1 accumulates in a complementary pattern at the posterior cortex (for reviews, see Goldstein and Macara, 2007; Munro, 2006; Nance, 2005). "
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    ABSTRACT: Anteroposterior polarity in early C. elegans embryos is required for the specification of somatic and germline lineages, and is initiated by a sperm-induced reorganization of the cortical cytoskeleton and PAR polarity proteins. Through mechanisms that are not understood, the kinases PAR-1 and PAR-4, and other PAR proteins cause the cytoplasmic zinc finger protein MEX-5 to accumulate asymmetrically in the anterior half of the one-cell embryo. We show that MEX-5 asymmetry requires neither vectorial transport to the anterior, nor protein degradation in the posterior. MEX-5 has a restricted mobility before fertilization and in the anterior of one-cell embryos. However, MEX-5 mobility in the posterior increases as asymmetry develops, presumably allowing accumulation in the anterior. The MEX-5 zinc fingers and a small, C-terminal domain are essential for asymmetry; the zinc fingers restrict MEX-5 mobility, and the C-terminal domain is required for the increase in posterior mobility. We show that a crucial residue in the C-terminus, Ser 458, is phosphorylated in vivo. PAR-1 and PAR-4 kinase activities are required for the phosphorylation of S458, providing a link between PAR polarity proteins and the cytoplasmic asymmetry of MEX-5.
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