The establishment of cell polarity is crucial for building and maintaining multicellular organisms. In Drosophila, the protein kinase Par-1 is thought to be critically involved in establishing polarity but, because par-1 mutants die early in development, the function of the kinase is poorly characterized. Now, on p. 711, Richard Carthew and colleagues use RNAi to characterize the role of Par-1 in Drosophila embryos and eye imaginal discs. By depleting maternal and zygotic Par-1, they discover that it restricts the adherens junctions that link neighbouring cells to an apical position within blastoderm cells, thus revealing a role for it in establishment of cell polarity. Other RNAi experiments indicate that Par-1 is not essential for maintaining epithelial cell polarity once it is established. However, since Par-1 overexpression disrupts polarity, Par-1 must play some role in maintenance of polarity. Finally, the authors use immunostaining and epistasis analysis to uncover a novel role for Par-1: it regulates Notch signalling during embryonic neurogenesis and retina determination, possibly by ensuring that the Notch ligand Delta is correctly localized.
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