Rational Design of Nanobody80 Loop Peptidomimetics: Towards Biased β2 Adrenergic Receptor Ligands

G protein-coupled receptors (GPCRs) play an important role for many cellular responses, and as such their mechanism of action is of utmost interest. To gain insight into the active conformation of GPCRs, the X-ray crystal structures of Nanobody(Nb)-stabilized β2-adrenergic receptor (β2AR) have been reported. Nb80 in particular is able to bind the intracellular G protein binding site of β2AR and stabilize the receptors in an active conformation. Within Nb80, the complementarity-determining region 3 (CDR3) is responsible for most of the binding interactions. Hence we hypothesized that peptidomimetics of the CDR3 loop might be sufficient for binding to the receptor, inhibiting the interaction of β2AR with intracellular GPCR interacting proteins (e.g. G proteins). Based on previous crystallographic data, a set of peptidomimetics were synthesized which, similarly to the Nb80 CDR3 loop, adopt a -hairpin conformation. Syntheses, conformational analysis, binding and functional in vitro assays, as well as internalization experiments were performed. We demonstrate that peptidomimetics can structurally mimic the CDR3 loop of a Nanobody and its function by inhibiting G protein coupling as measured by partial inhibition of cAMP production.