LAUSR

and negative interactions can occur between core protein complexes, and are necessary to establish their cellular asymmetric localisation. However, the roles of individual core proteins in mediating such interactions are not well understood. Here we present experimental evidence for both positive and negative interactions regulating the stability of the core protein Frizzled at cell junctions. To assay in vivo protein dynamics we are using fluorescence recovery after photobleaching (FRAP) of Frizzled-EGFP in theDrosophila pupalwing.Wemeasure Frizzled-EGFP stability either in the absence of a core protein or after acutely inducing expression of a core protein at physiological levels to analyse its immediate effect on Frizzled stability. Here we show that acute expression of the core protein Prickle destabilises Frizzled at cell junctions. This interaction occurs cellautonomously within a Prickle-expressing cell, whilst Frizzled in the neighbouring cell is not destabilised. The destabilisation of Frizzled via Prickle does not require the presence of Prickle’s binding partner Strabismus, but does require Dishevelled. We also show that acute expression of the core proteins Strabismus and Dishevelled stabilise Frizzled, and these stabilising interactions are dependent on the other core proteins. Therefore, positive and negative interactions between the core polarity proteins are required to localise junctional Frizzled into locally stable, asymmetric membrane subdomains.