Abstract

The biologic activity induced by ligand binding to orthosteric or allosteric sites on a G protein–coupled receptor (GPCR) is mediated by stabilization of specific receptor conformations. In the case of the β₂ adrenergic receptor, these ligands are generally small-molecule agonists or antagonists. However, a monomeric single-domain antibody (nanobody) from the Camelid family was recently found to allosterically bind and stabilize an active conformation of the β₂-adrenergic receptor (β₂AR). Here, we set out to study the functional interaction of 18 related nanobodies with the β₂AR to investigate their roles as novel tools for studying GPCR biology. Our studies revealed several sequence-related nanobody families with preferences for active (agonist-occupied) or inactive (antagonist-occupied) receptors. Flow cytometry analysis indicates that all nanobodies bind to epitopes displayed on the intracellular receptor surface; therefore, we transiently expressed them intracellularly as “intrabodies” to test their effects on β₂AR-dependent signaling. Conformational specificity was preserved after intrabody conversion as demonstrated by the ability for the intracellularly expressed nanobodies to selectively bind agonist- or antagonist-occupied receptors. When expressed as intrabodies, they inhibited G protein activation (cyclic AMP accumulation), G protein–coupled receptor kinase (GRK)–mediated receptor phosphorylation, β-arrestin recruitment, and receptor internalization to varying extents. These functional effects were likely due to either steric blockade of downstream effector (G_s, β-arrestin, GRK) interactions or stabilization of specific receptor conformations which do not support effector coupling. Together, these findings strongly implicate nanobody-derived intrabodies as novel tools to study GPCR biology.