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The establishment of axial polarity in the Drosophila egg and embryo depends on intercellular communication between two cell types in the ovary, the germline, and the soma. The genes gurken and egfr encode two essential players of this communication pathway. Gurken protein, a TGF-alpha-like molecule, is expressed in the germline, while the EGF-receptor homolog, Egfr, is expressed in the somatic cells of the ovary. Using the yeast two-hybrid system we show here, for the first time, that Gurken protein directly binds to the extracellular domain of Egfr. This direct physical association requires the presence of an intact EGF motif within Gurken protein. Furthermore, we provide evidence that this characteristic motif may be sufficient for interaction with the receptor, at list in vitro. Our results firmly establish Gurken as the germline ligand of Drosophila Egfr.

The establishment of anterior-posterior and dorsal-ventral polarity of the Drosophila egg and embryo depends on the function of the genes gurken, cornichon and Egfr (Drosophila epidermal growth factor receptor homolog). These genes encode components of a signal transduction pathway that transmits information between the germline cells and the somatic follicle cells of the ovary. gurken encodes a transforming growth factor-alpha-like protein and is a putative germline ligand of the Egfr present on the follicle cells. In mid-oogenesis the gurken transcript becomes spatially localized to the future dorsal-anterior cortex of the oocyte. To analyze the distribution pattern of Gurken protein we prepared antibodies against Gurken. We describe here the distribution pattern of the Gurken protein in wild-type ovaries and in ovaries from a number of dorsal-ventral patterning mutants. By immunoblotting we detect one major form of the Gurken protein, which likely corresponds to the unprocessed protein.