In the Drosophila ovary, different levels of Egfr signalling establish the axis of the egg and the future embryo. A dorsal-ventral gradient of the morphogen Gurken, an Egfr ligand, is thought to control Egfr activation; but what is the precise shape of this gradient and how is it regulated? Using a horseradish peroxidase (HRP)-Gurken fusion protein, Chang and colleagues now report that the gradient of Gurken is directly regulated by Cbl, a protein that downregulates Egfr signalling by mediating its endocytosis(see ). They show that HRP-Gurken is internalised with Egfr into follicle cells and passes through the Rab5/7-associated endocytic pathway to the lysosome for degradation. Loss-of-function and overexpression studies show that Cbl facilitates this internalisation. Finally, the researchers show for the first time that the Gurken gradient extends from its source at the anterior/dorsal side of the egg to the ventral follicle cells, which suggests that Gurken is a long-range morphogen that directly determines the fate of these cells.
