A versatile Halo- and SNAP-tagged BMP/TGFβ receptor library for quantification of cell surface ligand binding

The TGF{beta} superfamily of secreted growth factors comprises more than 30 members including TGF{beta}s, BMPs and Activins. While all TGF{beta} superfamily members signal through heteromeric receptor complexes to regulate a plethora of developmental and homeostatic processes, each ligand possesses a unique affinity towards a subset of BMP and TGF{beta} type I and type II receptors. Whereas the Activin and TGF{beta} class display a higher affinity towards type II receptors, BMPs and GDFs preferentially bind to type I receptors. Sofar, the lack of specific antibodies and chemical biology tools hampered simultaneous testing of ligand binding towards all BMP and TGF{beta} receptors. Here we present a N-terminally Halo- and SNAP-tagged TGF{beta}/BMP receptor library to visualize the receptor complexes in dual color. In combination with novel fluorescently labeled TGF{beta} superfamily ligands, we established a Ligand Surface Binding Assay (LSBA) for optical quantification of receptor-dependent growth factor binding for Activin A, TGF{beta}1 and BMP9 in a cellular context. We confirm ligand-receptor interface specificity by identifying BMPR2- or ALK2-mutants that switch from a low-affinity Activin A- or BMP9-receptor to a high-affinity receptor, respectively.