Planar cell polarity depends on the asymmetric localisation of core planar polarity proteins at apicolateral junctions. This asymmetric distribution probably develops through amplification of an initial asymmetry and seems to require the regulation of core protein levels. Now, Helen Strutt, Elizabeth Searle and co-workers () show that two distinct ubiquitylation pathways regulate the junctional levels and asymmetry of core planar polarity proteins in Drosophila. The researchers report that a Cullin-3-Diablo/Kelch ubiquitin ligase complex and the deubiquitylating enzyme Fat facets regulate the levels of the core planar polarity proteins Dishevelled and Flamingo, respectively, at apicolateral junctions but have no effect on the total cellular levels of Dishevelled and Flamingo. Notably, both increases and decreases in the junctional levels of core proteins caused by disruption of the ubiquitylation machinery reduce core protein asymmetry and disrupt planar cell polarity. Thus, the researchers suggest, ubiquitylation maximises the asymmetric localisation of core planar polarity proteins by fine-tuning their levels at junctions.
