Ligand-induced receptor dimerization is the first functional step in receptor activation, representing the most proximal,
functional read-out for receptor activation. Here we present a novel application of the Enzyme Fragment Complementation system to monitor receptor-receptor interactions at the surface of intact cells, applicable to diverse receptor types such as Interleukin receptors, BMP receptors and cytokine receptors. For the purpose of this poster, we will focus on the HER family of receptors. It is well understood that the HER family proteins can dimerize with the other members of its family leading to a complicated oncogenic signaling cascade. Surprisingly, existing cellular assays have been unable to faithfully monitor these interactions proximally in a drug discovery setting. We present EFC-based cellular assays that monitor EGFR homodimerization, EGFR-ErbB2 and ErbB2-ErbB3 heterodimerization events, amongst others. The high signal to noise ratio, serum tolerance and reproducibility make these assays applicable to a diverse range of applications for the identification and development of therapeutic small molecules and biologics, including screening, functional characterization, QC lot release assays and neutralizing antibody studies. Additionally, some data will be presented on how these assays can be used to determine potency of bi-specific antibodies.