Export from the endoplasmic reticulum of assembled N-methyl-d-aspartic acid receptors is controlled by a motif in the c terminus of the NR2 subunit.

Functional N-methyl-d-aspartic acid (NMDA) receptors are formed from the assembly of NR1 and NR2 subunits. When expressed alone, the major NR1 splice variant and the NR2 subunits are retained in the endoplasmic reticulum (ER), reflecting a quality control mechanism found in many complex multisubunit proteins to ensure that only fully assembled and properly folded complexes reach the cell surface. Recent studies have identified an RRR motif in the C terminus of the NR1 subunit, which controls the ER retention of the unassembled subunit. Here we investigated the mechanisms controlling the ER retention of the NR2 subunit and the export of the assembled complex from the ER. We found that Tac chimeras of the C terminus of the NR2B subunit show that an ER retention signal is also present in the NR2B subunit. In assembled complexes, ER retention signals on the individual subunits must be overcome to allow the complex to leave the ER. One common mechanism involves mutual masking of the signals on the individual subunits. Our data do not support such a mechanism for regulating the release of assembled NMDA receptors from the ER. We found that the motif, HLFY, immediately following transmembrane domain 4 of the NR2 subunit, is required for the assembled complex to exit from the ER. Mutation of this motif allowed the assembly of NR1 and NR2 subunits into a complex that was functional, based on MK-801 binding, but it is retained in the ER. These results are consistent with HLFY functioning as a signal that is necessary for the release of the assembled functional NMDA receptor complex from the ER.

Appropriate NR1-NR1 disulfide-linked homodimer formation is requisite for efficient expression of functional, cell surface N-methyl-D-aspartate NR1/NR2 receptors.

A c-Myc epitope-tagged N-methyl-D-aspartate receptor NR1-2a subunit was generated, NR1-2a(c-Myc), where the tag was inserted after amino acid 81. NR1-2a(c-Myc) /NR2A receptors when expressed in mammalian cells are not trafficked to the cell surface nor do they yield cell cytotoxicity post-transfection. NR1-2a(c-Myc) was, however, shown to assemble with NR2A subunits by immunoprecipitation and [(3)H]MK801 radioligand binding assays. Immunoblots of cells co-transfected with wild-type NR1-2a/NR2A subunits yielded two NR1-2a immunoreactive species with molecular masses of 115 and 226 kDa. Two-dimensional electrophoresis under non-reducing and reducing conditions revealed that the 226-kDa band contained disulfide-linked NR1-2a subunits. Only the 115-kDa NR1-2a species was detected for NR1-2a(c-Myc)/NR2A. The c-Myc epitope is inserted adjacent to cysteine 79 of the NR1-2a subunit; therefore, it is possible that the tag may prevent the formation of NR1 disulfide bridges. A series of cysteine --> alanine NR1-2a mutants was generated, and the NR1-2a mutants were co-expressed with NR2A or NR2B subunits in mammalian cells and characterized with respect to cell surface expression, cell cytotoxicity post-transfection, co-association by immunoprecipitation, and immunoblotting following SDS-PAGE under both reducing and non-reducing conditions. When co-expressed with NR2A in mammalian cells, NR1-2a(C79A)/NR2A displayed similar properties to NR1-2a(c-Myc)/NR2A in that the 226-kDa NR1 immunoreactive species was not detectable, and trafficking to the cell surface was impaired compared with wild-type NR1/NR2 receptors. These results provide the first biochemical evidence for the formation of NR1-NR1 intersubunit disulfide-linked homodimers involving cysteine 79. They suggest that disulfide bridging and structural integrity within the NR1 N-terminal domain is requisite for cell surface N-methyl-D-aspartate receptor expression.

Relationship between availability of NMDA receptor subunits and their expression at the synapse.

The effect of increasing the expression of NMDA subunits in cerebellar granule cells (CGCs) by transfection was studied to determine how the availability of various NMDA subunits controls both the total pool of functional receptors and the synaptic pool. Overexpression of either NR2A or NR2B, but not splice variants of NR1, by transfection caused a significant increase in the total number of functional NMDA receptors and in surface NR1 subunit cluster density in CGCs in primary culture. These data solidify the central role of NR2 subunit availability in determining the number of cell surface receptors. Overexpression of either NR2A or NR2B significantly altered the deactivation kinetics of NMDA-mediated miniature EPSCs (NMDA-mEPSCs). However, there was no significant effect of NR2 subunit overexpression on the mEPSC amplitude or single-channel conductance. NR2 subunit overexpression did not change the rate of block by MK-801 of NMDA-mediated currents in excised patches from CGCs, indicating that subunit composition does not regulate peak open probability of the channel in CGCs. With the overexpression of a mutant of NR2B lacking the PDZ binding domain, there was an increase in the total number of NMDA receptors without a change in mEPSC kinetics. Therefore, the entry of NMDA receptors into the synapse requires a PDZ binding domain and is limited by means other than receptor subunit availability.