Cell surface protein-protein interactions encode for connectivity between cells in multicellular organisms. We are working on revealing the set of interactions that build animals and especially the nervous system. One fundamental question we are asking is, how many proteins and interactions are needed to wire the nervous system.
Reference: An extracellular interactome of immunoglobulin and LRR proteins reveals receptor-ligand networks. Özkan E, Carrillo RA, Eastman CL, Weiszmann R, Waghray D, Johnson KG, Zinn K, Celniker SE, Garcia KC. Cell 154, 228 (2013).