Molecular dissection of Drosophila Prickle isoforms distinguishes their essential and overlapping roles in planar cell polarity

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Prickle-Spiny-Legs (Pk) is an essential component of the planar cell polarity (PCP) pathway, together with Frizzled (Fz) and Dishevelled (Dsh). A role for Pk was proposed to mediate feedback amplification of asymmetric Fz/Dsh activity across cell boundaries, ensuring a single prehair initiates at each distal vertex. Here we show that apical localisation of Pk Pk and Pk Sple isoforms are mutually independent and regulated by the C-terminal domain. The N-terminus of Pk Pk is dispensable for PCP, whereas the unique N-terminal domain of Pk Sple contains an additional localisation function, which confers a qualitatively different activity. Our results suggest that endogenous Pk Pk and Pk Sple can affect each other's function via the C-terminal domain, yet may not form heteromeric complexes. Overexpressing PET domain-deleted Pk variants interferes with a branch of Fz/Dsh signalling that regulates the number of wing hairs, and blocks non-cell-autonomous repolarisation. We infer that Pk Pk is sufficient to mediate the intercellular feedback signalling. Significantly, Pk Pk but not Pk Sple is required for hexagonal cell packing in the pupal wing. We propose that Fz-dependent PCP readout reflects short-range, cell-contact based, interactions between hexagonal cells, rather than a direct response to an as yet unidentified diffusible ligand.