Tethered Agonist- and GAIN-Domain Independent Signaling of an Adhesion-GPCR

In transfected cells, adhesion-GPCRs are activated by tethered agonists that are embedded in their canonical autoproteolytic GAIN-domain. It is unknown, however, whether this tethered agonist-dependent activation mechanism generally mediates the physiological functions of adhesion-GPCRs. Here, we show that G-protein signaling by BAI3 (Adgrb3), a brain-specific adhesion-GPCR, is essential for its function in controlling axon and dendrite growth and promoting synapse formation. Moreover, signal transduction assays confirmed that constitutive exposure of BAI3’s tethered agonist massively stimulated (~5-fold) its GPCR activity. However, the same constitutive exposure of BAI3’s tethered agonist by deletion of its extracellular domains blocked, instead of activating, BAI3’s function in regulating axon and dendrite growth and synapse formation. Moreover, inactivating mutations of BAI3’s tethered agonist or deletion of BAI3’s GAIN domain containing the tethered agonist did not detectably impair BAI3’s physiological functions. Thus, the GPCR activity of BAI3 is functionally required, whereas tethered agonist-mediated stimulation of GPCR signaling is not.