The Drosophila protocadherins Fat (Ft) and Dachsous (Ds) are required for regulating imaginal disc growth, establishing planar cell polarity (PCP), and for the proximodistal (PD) patterning of appendages. Cadherins usually function as adhesion molecules, but Ft and Ds, which interact with each other, could act as a receptor-ligand pair instead. Matakatsu and Blair now report that this seems to happen in the establishment of PCP but that Ds and Ft regulate growth and PD patterning independently (see p. ). The researchers used a structure-function approach to test which domains of Ft and Ds mediate their various activities. They show, for example, that the extracellular domain of Ft is not needed for its functions in growth, PCP establishment and PD patterning but that the extracellular domain of Ds is necessary and sufficient for its effects on PCP. These results suggest a model, in which Ft has a receptor-like function that is mediated by its intracellular domain while Ds has a ligand-like function in PCP.
