Assembly and cell surface expression of KA-2 subunit-containing kainate receptors

Kainate receptors (KARs) modulate synaptic transmission at both pre-synaptic and post-synaptic sites. The overlap in the distribution of KA-2 and GluR6/7 subunits in several brain regions suggests the co-assembly of these subunits in native KARs. The molecular mechanisms that control the assembly and surface expression of KARs are unknown. Unlike GluR5-7, the KA-2 subunit is unable to form functional homomeric KAR channels. We expressed the KA-2 subunit alone or in combination with other KAR subunits in HEK-293 cells. The cell surface expression of the KAR subunit homo- and heteromers were analysed using biotinylation and agonist-stimulated cobalt uptake. While GluR6 or GluR7 homomers were expressed on the cell surface, KA-2 alone was retained within the endoplasmic reticulum. We found that the cell surface expression of KA-2 was dramatically increased by co-expression with either of the low-affinity KAR subunits GluR5-7. However, co-expression with other related ionotropic glutamate receptor subunits (GluR1 and NR1) does not facilitate the cell surface expression of KA-2. The analysis of subcellular fractions of neocortex revealed that synaptic KARs have a relatively high KA-2 content compared to microsomal ones. Thus, KA-2 is likely to contain an endoplasmic reticulum retention signal that is shielded on assembly with other KAR subunits.