PTK7 is a positive allosteric modulator of GPR133 (ADGRD1) signaling in GBM

GPR133 (ADGRD1), an adhesion G protein-coupled receptor, supports growth of glioblastoma, a brain malignancy. We demonstrated that GPR133 is intramolecularly cleaved, and that dissociation of its N-terminal and C-terminal fragments (NTF and CTF) at the plasma membrane correlates with increased receptor signaling. However, how the extracellular interactome of GPR133 in glioblastoma modulates signaling remains unknown. Here, we use affinity purification and mass spectrometry to identify extracellular binding partners of GPR133 in patient-derived glioblastoma cells. We show that the transmembrane protein PTK7 binds the GPR133 NTF and its expression in trans increases GPR133 signaling. This effect requires the intramolecular cleavage of GPR133 and PTK7s anchoring in the plasma membrane. The GPR133-PTK7 interaction facilitates orthosteric activation of GPR133 by soluble peptide mimicking the endogenous tethered Stachel agonist, suggesting PTK7 binding allosterically enhances accessibility of GPR133s orthosteric Stachel binding pocket. GPR133 and PTK7 are expressed in adjacent cells in glioblastoma, where their knockdown phenocopies each other. We propose that this novel ligand-receptor interaction is relevant to the pathogenesis of glioblastoma, as well as physiological processes in several tissues.