The extracellular domain of mGluR6 regulates targeting to the conventional secretion pathway

In the retina, rod and cone photoreceptors relay information to bipolar cells at glutamatergic synapses. At dendritic tips of ON-type bipolar cells, which depolarize in response to light, the metabotropic glutamate receptor mGluR6 is required for neurotransmitter detection. mGluR6 also has a critical interaction with the presynaptic cell adhesion molecule ELFN1, and N-linked glycosylation of mGluR6 is required for this interaction. In the retina and in heterologous cells, mGluR6 undergoes conventional secretory trafficking with complex glycosylation acquired in the Golgi. However, the mechanisms regulating mGluR6 secretory trafficking are poorly understood. Like other class C GPCRs, mGluR6 has a large extracellular domain, which includes a bi-lobed ligand binding domain. We show that a series of small deletions in the upper lobe of the ligand-binding domain led to exclusive use of unconventional secretion and plasma membrane insertion of immature core-glycosylated protein in heterologous cells. Deletion of larger regions partially restored Golgi trafficking and complex glycosylation. The mutants with large deletions also exhibited dramatically increased plasma membrane localization, which was not recapitulated in the panel of mutants with small deletions. A large deletion did not prevent constitutive internalization, suggesting the increase in plasma membrane protein is due to forward trafficking flux. The results indicate an important role of the upper lobe of the ligand binding domain in regulating mGluR6 secretory trafficking, and suggest that disruption of the structure of this domain leads to unconventional trafficking. These findings are consistent with an intraluminal interaction regulating mGluR6 sorting within the endoplasmic reticulum.