Discovery and Function of the Very Large G Protein-Coupled Receptor

The very large G protein-coupled receptor 1 (VLGR1), also known as MASS1 or GPR98, is most notable among the family of adhesion GPCR for its size. Encoded by an 18.9 kb open reading frame, the ~700 kDa primary translation product is by far the largest GPCR and, additionally, the largest cell surface protein known to date. The large ectodomain of the protein contains several repeated motifs, including some 35 calcium binding, Calx-β repeats and seven copies of an epitempin repeat thought to be associated with the development of epilepsy. The extreme carboxyl-terminus contains a consensus PDZ ligand sequence, suggesting interactions with other cytosolic or cytoskeletal proteins. At least two spontaneous and two targeted mutant mouse lines are currently known. The mutant mice present with sensitivity to audiogenic seizures but also have cochlear defects and significant progressive hearing impairment. VLGR1 is one of the few adhesion GPCR family members in which mutations have been shown to be responsible for a human malady. Mutations in VLGR1 in humans result in one form (2C) of Usher syndrome, the most common genetic cause of combined blindness and deafness. Consistent with this phenotype, VLGR1 is a major component of the ankle link of stereocilia in cochlear hair cells and the preciliary complex in retinal photoreceptor cells.