N-linked glycosylation of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-selective glutamate receptor channel alpha 2 subunit is essential for the acquisition of ligand-binding activity

Abstract

The N-linked glycosylation of the alpha 2 subunit of the mouse alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-selective glutamate receptor (GluR) channel was characterized. The receptor subunit protein has five putative N-glycosylation sites. The recombinant receptor proteins were identified by [35S]methionine/[35S]cysteine metabolic labeling, western blot analysis, immunocytochemical detection, and [3H]AMPA binding experiments when expressed in insect Spodoptera frugiperda cells using a baculovirus system. The effect of tunicamycin on the metabolic labeling and immunoblots suggested that the two products, a major protein species of approximately 102 kDa and a minor species of approximately 98 kDa, correspond to glycosylated and unglycosylated forms, respectively, which was also supported by the enzymic deglycosylation experiments. Immunofluorescence staining of tunicamycin-treated cells expressing only the unglycosylated form differed little from that of tunicamycin-nontreated cells expressing both glycosylated and unglycosylated forms. The lack of AMPA-binding activity of the unglycosylated form expressed in the presence of tunicamycin suggested that N-glycosylation is required, directly or indirectly, for functional expression in insect cells for ligand binding. These results demonstrate that occupancy of at least one N-glycosylation site is required for the formation and maintenance of the GluR alpha 2 subunit protein in an active conformation for ligand binding. Possible roles of N-glycosylation of GluR alpha 2 subunit protein are discussed.