The GPR56 N‐Terminus Controls Receptor Signaling Activity

GPR56 is an adhesion G protein‐coupled receptor (GPCR) that plays a key role in cortical development. Mutations to GPR56 in humans cause malformations of the cerebral cortex, but little is known about the normal function of the receptor. We found that the large N‐terminus (NT) of GPR56 is cleaved from the rest of the receptor during processing but remains non‐covalently associated with the receptor's seven‐transmembrane region. We also found that truncations to the GPR56‐NT result in constitutive activation of receptor signaling, as revealed by desensitization of GPR56‐stimulated signaling pathways induced by transfection of HEK‐293 cells with truncated GPR56, enhanced binding of beta‐arrestins by truncated GPR56 relative to the full‐length receptor, extensive ubiquitination of truncated GPR56, and cytotoxicity induced by truncated GPR56 that could be rescued by co‐transfection of cells with beta‐arrestin 2. Furthermore, we found that the GPR56‐NT is capable of homophilic trans‐trans interactions that enhance receptor signaling activity. Based on these findings, we suggest a model of receptor activation in which the large N‐terminus of GPR56 constrains receptor activity, but N‐terminal interactions can remove this inhibitory influence of the N‐terminus to activate receptor signaling.