Gallyas Ferenc, Ball Simon M, Molnar Elek
MRC Centre for Synaptic Plasticity, Department of Anatomy, School of Medical Sciences, University of Bristol, Bristol, UK.
J Neurochem. 2003 Sep;86(6):1414-27. doi: 10.1046/j.1471-4159.2003.01945.x.
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.
海人藻酸受体（KARs）在突触前和突触后位点调节突触传递。KA-2和GluR6/7亚基在几个脑区的分布重叠，表明这些亚基在天然KARs中共同组装。控制KARs组装和表面表达的分子机制尚不清楚。与GluR5-7不同，KA-2亚基不能形成功能性同聚体KAR通道。我们在HEK-293细胞中单独表达KA-2亚基或与其他KAR亚基联合表达。使用生物素化和激动剂刺激的钴摄取分析KAR亚基同聚体和异聚体的细胞表面表达。虽然GluR6或GluR7同聚体在细胞表面表达，但单独的KA-2保留在内质网中。我们发现，与低亲和力KAR亚基GluR5-7中的任何一个共表达，KA-2的细胞表面表达显著增加。然而，与其他相关离子型谷氨酸受体亚基（GluR1和NR1）共表达并不能促进KA-2的细胞表面表达。新皮质亚细胞组分的分析表明，与微粒体相比，突触KARs的KA-2含量相对较高。因此，KA-2可能含有一个内质网保留信号，该信号在与其他KAR亚基组装时被屏蔽。
