Structural basis of GAIN domain autoproteolysis and cleavage-resistance in the adhesion G-protein coupled receptors.

The GAIN domain is a hallmark of adhesion G protein-coupled receptors (aGPCRs), as this extracellular domain contains an integral agonistic sequence (Stachel) that activates the receptor by binding to its 7-transmembrane helix (7TM) domain. Many aGPCRs undergo autoproteolytic cleavage at the GPCR proteolysis site (GPS), which has a canonical H-X-S/T sequence motif located within the GPCR autoproteolysis-inducing (GAIN) domain. Here we present the crystal structure of the Hormone Receptor (HormR) and GAIN domains of ADGRB2/BAI2. The protein was not cleaved at the GPS, despite possessing an HLS sequence. Through structural comparisons and molecular dynamics (MD) simulations, we identify determinants that contribute to autoproteolytic activity beyond the H-X-S/T motif. Specifically, we highlight a T-shaped π-π interaction between the histidine base of the H-X-S/T motif and a phenylalanine residue that is highly conserved in cleavage-competent aGPCRs. This interaction is critical for properly positioning the imidazole group of the histidine to deprotonate the alcohol nucleophile. Disruption of this interaction reduces autoproteolytic activity in the ADGRL1 and introduction of the phenylalanine restores cleavage competence in the otherwise non-cleavable ADGRB3 upon expression in HEK293 cells. In addition, the poorly conserved and flexible flap regions flanking the GPS also contribute to full autocleavage activity.