Biochemical evidence for the co-association of three N-methyl-D-aspartate (NMDA) R2 subunits in recombinant NMDA receptors.

Functional characterization of wild-type and mutant cloned N-methyl-D-aspartate (NMDA) receptors has been used to deduce their subunit stoichiometry and quaternary structure. However, the results reported from different groups have been at variance and are thus inconclusive. This study has employed a biochemical approach to determine the number of NMDA R2 (NR2) subunits/receptor together with the NMDA R1 (NR1)/NR2 subunit ratio of both cloned and native NMDA receptors. Thus, human embryonic kidney 293 cells were transfected with the NR1-1a and NR2A NMDA receptor subunits in combination with both FLAG- and c-Myc epitope-tagged NR2B subunits. The expressed receptors were detergent-extracted and subjected to double immunoaffinity purification using anti-NR2A and anti-FLAG antibody immunoaffinity columns in series. Immunoblotting of the double immunopurified NR2A/NR2B(FLAG)-containing material demonstrated the presence of anti-NR1, anti-NR2A, anti-FLAG, and, more important, anti-c-Myc antibody immunoreactivities. The presence of anti-c-Myc antibody immunoreactivity in the double immunoaffinity-purified material showed the co-assembly of three NR2 subunits, i.e. NR2A/NR2B(FLAG)/NR2B(c-Myc), within the same NMDA receptor complex. Control experiments excluded the possibility that the co-immunopurification of the three NR2 subunits was an artifact of the solubilization procedure. These results, taken together with those previously described that showed two NR1 subunits/oligomer, suggest that the NMDA receptor is at least pentameric.