Stalk‐dependent and Stalk‐independent Activity of the Adhesion GPCR GPR56/ADGRG1

Abstract

The adhesion G protein‐coupled receptors (aGPCRs) comprise an enigmatic family of 33 receptors in humans. A hallmark of the aGPCRs is the presence of a large extracellular N‐terminal (NT) domain that has been shown to inhibit receptor activity. Yet another defining characteristic of virtually all aGPCRs is the presence of a GPCR Autoproteolysis Inducing (GAIN) domain, which is found on the membrane‐proximal end of the NT and controls the autocatalytic proteolysis of the receptor into an NT protomer and seven‐transmembrane (7TM) protomer, which can stay non‐covalently associated for some time. Recently, great progress has been made to shed light upon a more detailed view of how exactly the NT regions inhibit aGPCR‐mediated signaling. Notably, two separate groups have espoused a “cryptic agonist” model of aGPCR activation wherein GAIN domain cleavage unveils a new NT stalk which serves as the receptor's agonist. In this model, the stalk is a requisite agonist for aGPCR activity and deletion of the stalk result in a signaling‐incompetent receptor. In the work presented here, we show that the stalk of the aGPCR GPR56/ADGRG1 (“G1”) is indeed necessary for activation of the certain downstream readouts (such as activation of SRF luciferase) but dispensable for others (such as the shedding of membrane‐bound TGFalpha and activation of NFAT luciferase). Moreover, a stalkless version of G1 also displays comparable activity to the highly‐active, GAIN‐cleavage‐mimetic form of G1 (G1‐deltaNT) in several measures of activity including beta‐arrestin recruitment, receptor ubiquitination, and association with Galpha13. These lines of evidence suggest that G1 is capable of both stalk‐dependent and stalk‐independent modes of signaling. Based upon our results, we propose a more nuanced model of aGPCR activation in which the NT inhibits activity of the 7TM region in two distinct ways: i) antagonizing stalk‐dependent signaling by masking the cryptic stalk peptide, and ii) directly antagonizing the inherent stalk‐independent activity of the 7TM protomer.Support or Funding InformationThis work was supported by NIH R01 award R01‐NS072394.