Adhesion G-protein coupled receptors (ADGRs) have garnered some notoriety as the second largest family of G-protein coupled receptors (GPCRs) with 33 homologues present in the human genome. While they were grouped together based on the phylogeny of their seven transmembrane (7TM) region, they are primarily distinguished from other GPCRs by their large and modular extracellular regions (ECRs) containing a variety of domains implicated in cell adhesion, which gave them their name. Except for GPR123 (ADGRA1, A1), which is missing an ECR entirely, all of them contain a GPCR autoproteolysis inducing (GAIN) domain, immediately adjacent to the 7TM region. The GAIN domain was identified by Araç et al. who showed that it contained the previously identified GPCR proteolysis site (GPS) motif, which is capable of cleaving itself at a conserved HLT/HLS triad. The role of the GAIN domain and its autoproteolysis as well as different modes of signal transduction by ADGRs have been a hot topic of discussion in recent years.
The GAIN domain of human BAI2 (hADGRB2, hB2 did not live up to its name and was expressed and crystallized in an uncleaved state. The crystal structure of hB2-HG provided an as of yet unique perspective on the autoproteolysis behavior of ADGRs, highlighting the importance of properly positioning the nucleophile within the constraints of the catalytic triad. A set of interactions unique among human ADGRs was identified to be responsible for the misalignment of the GPS, hindering both the deprotonation of Ser912’s hydroxyl group and most significantly its subsequent nucleophilic attack at Leu911’s carbonyl carbon position as part of the autoproteolysis mechanism. Besides the unique configuration of its GAIN domain, the ECR of hB2 displayed an interesting dimerization behavior, which was charac-terized by small angle X-ray scattering, indicating a weak transient interaction consistant with a cis-interaction with a KD in the micromolar range.