N-glycan content modulates kainate receptor functional properties

Key points: Ionotropic glutamate receptor (iGluR) subunits are N-glycosylated at 4-12 sites, and Golgi processing produces mature receptors that contain high-mannose, hybrid and complex oligosaccharides. N-glycosylation is crucial for receptor biogenesis, influences receptor trafficking and provides a binding site for carbohydrate binding proteins. Glycan moieties are large, polar and occasionally charged, and they are attached at sites along iGluRs that position them for involvement in the structural changes underlying gating. Altering glycan content on kainate receptors (KARs), a subfamily of iGluRs, changes functional properties of the receptor, such as desensitization, recovery from desensitization and deactivation. We report the first observation that the charged trisaccharide HNK-1 is conjugated to native KARs, and we find that it substantially alters recombinant KAR functional properties. Our results show that the molecular composition of N-glycans can influence KAR biophysical properties, revealing a potential mechanism for fine-tuning the function of these receptors.

Abstract: Ionotropic glutamate receptors (iGluRs) are tetrameric proteins with between four and 12 consensus sites for N-glycosylation on each subunit, which potentially allows for a high degree of structural diversity conferred by this post-translational modification. N-glycosylation is required for proper folding of iGluRs in mammalian cells, although the impact of oligosaccharides on the function of successfully folded receptors is less clear. Glycan moieties are large, polar, occasionally charged and mediate many protein-protein interactions throughout the nervous system. Additionally, they are attached at sites along iGluR subunits that position them for involvement in the structural changes underlying gating. In the present study, we show that altering glycan content on kainate receptors (KARs) changes the functional properties of the receptors in a manner dependent on the identity of both the modified sugars and the subunit composition of the receptor to which they are attached. We also report that native KARs carry the complex capping oligosaccharide human natural killer-1. Glycosylation patterns probably differ between cell types, across development or with pathologies, and thus our findings reveal a potential mechanism for context-specific fine-tuning of KAR function through diversity in glycan structure.