Activation of cytomegalovirus-encoded G protein-coupled receptor UL33 by an innate N-terminal peptide.

Human cytomegalovirus (HCMV) encodes the orphan G protein-coupled receptor (GPCR) UL33, which exhibits constitutive activity that disrupts host G protein signalling, facilitating efficient viral replication and pathogenesis. The cryo-electron microscopy (cryo-EM) structure of UL33 bound to the Gs subtype of G protein reveals the N-terminal peptide as a tethered ligand reminiscent of the protease-activated receptors and adhesion GPCRs. This self-agonism induces a non-canonical active state that facilitates promiscuous G protein coupling, a plausible viral strategy for fine-tuning host signalling. Structure-guided mutagenesis disrupting key interactions between the N-terminus and its binding pocket abolishes G protein-mediated signalling, confirming the role of the N-terminus as a self-agonist. Our findings elucidate the structural basis for this activation mechanism and highlight the strategies employed by HCMV to hijack host G protein signalling.