A crucial aspect of biological signalling is knowing when to stop. The mechanism behind another important aspect — how to stop — has just been discovered in the context of signalling by the epidermal growth factor receptor (EGFR) in Drosophila melanogaster. In Nature, Klein et al. report that the secreted protein Argos functions as an antagonist of EGFR signalling by sequestering the EGF ligand Spitz.

The group had thought that Argos might directly interact with EGFR to stabilize an autoinhibited conformation, so they studied the interactions of Argos, EGFR and Spitz using purified recombinant forms of the proteins in surface-plasmon-resonance studies. Surprisingly, and therefore contrary to their original theory, Argos and EGFR didn't interact. But Argos did interact with Spitz in a 1:1 complex. Argos bound to the EGF-like domain of Spitz, and Spitz bound to the carboxy-terminal part of Argos.

On the basis of these data, Klein et al. proposed that, because Spitz could bind to Argos or EGFR, Argos might function by 'taking' Spitz away from EGFR and blocking its EGFR-binding site — analogous to antagonists of signalling by insulin-like growth factor-1 or bone morphogenetic protein. By adding increasing amounts of Argos to a fixed concentration of free Spitz and testing receptor binding, the authors could show that Argos abolished the Spitz–EGFR interaction. This was most effective when Argos and Spitz were present at the same concentration, consistent with the 1:1 binding ratio.

In a cellular context, adding increasing amounts of Argos progressively decreased the Spitz-induced tyrosine phosphorylation of EGFR. When Klein et al. used fluorescently tagged recombinant proteins to label cell surfaces, they found that Spitz only bound to the surface of cells that were expressing EGFR, and this binding was reduced if Argos was added at the same time or beforehand. However, Argos couldn't displace Spitz from preformed Spitz–EGFR complexes at the cell surface, which led the authors to conclude that Argos can't reverse EGFR activation once it's started.

So these findings go against previous suggestions that Argos is an antagonistic EGFR ligand. Furthermore, this mechanism of inhibition is more consistent with that of other signalling antagonists that sequester stimulatory ligands. As Argos is a Spitz-induced Spitz antagonist, the authors suggest that local production of Argos might effectively limit longer-range signalling by Spitz during D. melanogaster development.