A new trifunctional reagent allows researchers to identify ligand-receptor interactions in living cells and tissues, reports a Swiss team led by Erick Carreira and Bernd Woll-scheid of ETH Zurich (Nat. Biotechnol., DOI: 10.1038/nbt.2354). The TRICEPS reagent contains an N-hydroxysuccinimide ester that reacts with primary amines on ligands, a protected hydrazine that reacts with aldehydes in carbohydrates of glycoprotein receptors, and a biotin group that serves as a handle for affinity purification. These reagents enable ligand-based tagging of cell surface glycoproteins under near-physiological conditions and mass spectrometric identification of specific receptors. The researchers demonstrated the applicability of this ligand-based receptor-capture technology using insulin, transferrin, apelin, epidermal growth factor, the antibody trastuzumab, and two antibody mimetics to capture their respective cell surface receptors and to identify approximate ligand-binding sites. In collaboration with Jason Mercer from ETH Zurich, the researchers used intact vaccinia viruses as ligands to identify a binding factor panel consisting of five cell surface proteins for these highly complex viruses.