An important way of controlling certain signalling pathways, such as the Wingless (Wg) pathway, is through the endocytic degradation of extracellular ligands. Now, Piddini et al. report that, in Drosophila wing imaginal discs, the degradation of Wg (which is required to form a gradient of this morphogen) is cooperatively controlled by Arrow and Frizzled2, two receptors involved in Wg signal transduction (see ). The researchers show that although Frizzled2, a seven-transmembrane receptor,stimulates the internalisation of Wg in wing imaginal discs and cultured cells, it does not take Wg to a degradative compartment. This requires the LDL receptor-related protein Arrow, which stimulates the targeting of the Frizzled2-Wg complex to lysosomes for degradation. The separation of ligand capture and degradation between two receptors, the researchers suggest, could help to generate a reliable concentration gradient of Wg and other morphogens during development.
