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First published online 28 March 2006
doi: 10.1242/jcs.02873

Journal of Cell Science 119, 1592-1603 (2006)
Published by The Company of Biologists 2006
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Research Article

The aPKC–PAR-6–PAR-3 cell polarity complex localizes to the centrosome attracting body, a macroscopic cortical structure responsible for asymmetric divisions in the early ascidian embryo

Solenn Patalano*, Gérard Prulière, François Prodon, Alexandre Paix, Philippe Dru, Christian Sardet and Janet Chenevert{ddagger}

BioMarCell, Laboratoire de Biologie de Developpement, UMR 7009 CNRS, University Pierre and Marie Curie, Observatoire, Villefranche-sur-mer 06230, France

{ddagger} Author for correspondence (e-mail: chenevert{at}obs-vlfr.fr)

Accepted 4 January 2006

Posterior blastomeres of 8-cell stage ascidian embryos undergo a series of asymmetric divisions that generate cells of unequal sizes and segregate muscle from germ cell fates. These divisions are orchestrated by a macroscopic cortical structure, the `centrosome attracting body' (CAB) which controls spindle positioning and distribution of mRNA determinants. The CAB is composed of a mass of cortical endoplasmic reticulum containing mRNAs (the cER-mRNA domain) and an electron dense matrix, but little is known about its precise structure and functions. We have examined the ascidian homologues of PAR proteins, known to regulate polarity in many cell types. We found that aPKC, PAR-6 and PAR-3 proteins, but not their mRNAs, localize to the CAB during the series of asymmetric divisions. Surface particles rich in aPKC concentrate in the CAB at the level of cortical actin microfilaments and form a localized patch sandwiched between the plasma membrane and the cER-mRNA domain. Localization of aPKC to the CAB is dependent on actin but not microtubules. Both the aPKC layer and cER-mRNA domain adhere to cortical fragments prepared from 8-cell stage embryos. Astral microtubules emanating from the proximal centrosome contact the aPKC-rich cortical domain. Our observations indicate that asymmetric division involves the accumulation of the aPKC–PAR-6–PAR-3 complex at the cortical position beneath the pre-existing cER-mRNA domain.

Key words: Cell polarity, Unequal cleavage, PAR proteins, Ascidian embryo, Cytoskeleton, Cortex


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