Epithelial cilia, whether motile or primary, often display an off-centered planar localization within the apical cell surface. This form of planar cell polarity (PCP) involves the asymmetric positioning of the ciliary basal body (BB). Using the mono-ciliated epithelium of the embryonic zebrafish floor-plate, we investigated the dynamics and mechanisms of BB polarization by live-imaging. BBs were highly motile, making back-and-forth movements along the antero-posterior axis and contacting both the anterior and posterior membranes. Contacts exclusively occurred at junctional Par3 patches and were often preceded by membrane digitations extending towards the BB, suggesting focused cortical pulling forces. Accordingly, BBs and Par3 patches were linked by dynamic microtubules. Later, BBs became less motile and eventually settled at posterior apical junctions enriched in Par3. BB posterior positioning followed Par3 posterior enrichment and was impaired upon Par3 depletion or disorganization of Par3 patches. In the PCP mutant Vangl2, BBs were still motile but displayed poorly-oriented membrane contacts that correlated with Par3 patch fragmentation and lateral spreading. We propose an unexpected function for posterior Par3 enrichment in controlling BB positioning downstream of the PCP pathway.

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