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

The TGFβ superfamily of secreted growth factors comprises more than 30 members including TGFβs, BMPs and Activins. While all TGFβ 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 TGFβ type I and type II receptors. Whereas the Activin and TGFβ 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 TGFβ receptors. Here we present a N-terminally Halo- and SNAP-tagged TGFβ/BMP receptor library to visualize the receptor complexes in dual color. In combination with novel fluorescently labeled TGFβ superfamily ligands, we established a Ligand Surface Binding Assay (LSBA) for optical quantification of receptor-dependent growth factor binding for Activin A, TGFβ1 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.