Abstract
Several members of the adhesion subfamily of G protein-coupled receptors (aGPCRs) are capable of self-activation by an internal agonist sequence (aka the Stachel) that's exposed upon removal or conformational changes of the N-terminal fragment of the receptor. Synthetic peptides derived from the Stachel sequence can be used as exogenous agonists. In the inactive form, the Stachel is sequestered as the β13-strand within the GPCR Autoproteolysis-INducing (GAIN) domain, but it engages the seven transmembrane region as a helix when it is either an intramolecular sequence or a synthetic peptide. Little is known about the molecular details underlying this transition, but we hypothesize that a disordered conformation is central to this intermediate state in receptor activation. Despite the primarily helical Stachel AlphaFold3 and Pepfold4 models predicted with high confidence for the entire aGPCR subfamily, disorder predictions and biophysical experiments reveal a predominantly disordered conformation in solution. Investigating the ADGRG6/GPR126 Stachel peptide, circular dichroism (CD) and nuclear magnetic resonance (NMR) experiments reveal a predominantly random coil conformation in aqueous buffer, polar detergent micelles, and zwitterionic lipids. Titration of trifluoroethanol uncovered a two-state equilibrium between an unfolded and helix-containing conformation with NMR localizing a single-turn helix to residues L846-L849. Taken together, these data indicate the ADGRG6/GPR126 Stachel peptide is primarily disordered, but small populations may adopt a helix-containing conformer that seems to support a conformational-selection activation mechanism.