An Extracellular Interactome of Immunoglobulin and LRR Proteins Reveals Receptor-Ligand Networks

Extracellular domains of cell surface receptors and ligands mediate cell-cell communication, adhesion, and initiation of signaling events, but most existing protein-protein “interactome” data sets lack information for extracellular interactions. We probed interactions between receptor extracellular domains, focusing on a set of 202 proteins composed of the Drosophila melanogaster immunoglobulin superfamily (IgSF), fibronectin type III (FnIII), and leucine-rich repeat (LRR) families, which are known to be important in neuronal and developmental functions. Out of 20,503 candidate protein pairs tested, we observed 106 interactions, 83 of which were previously unknown. We “deorphanized” the 20 member subfamily of defective-in-proboscis-response IgSF proteins, showing that they selectively interact with an 11 member subfamily of previously uncharacterized IgSF proteins. Both subfamilies interact with a single common “orphan” LRR protein. We also observed interactions between Hedgehog and EGFR pathway components. Several of these interactions could be visualized in live-dissected embryos, demonstrating that this approach can identify physiologically relevant receptor-ligand pairs. [Display omitted] •Avidity-based method detects interactions among Drosophila extracellular proteins•202 immunoglobulin and leucine-rich repeat proteins show 106 interactions•Four families, comprising 53 proteins, create two dense interaction networks•Biophysical methods and in vivo staining experiments validate interactions An oligomerization-based method detects interactions among cell surface receptors and extracellular ligands—interactions that have been difficult to study on a large scale until now, identifying interaction networks among previously uncharacterized protein families and assigning ligands to “orphan” receptors.