We have used a heterologous system of expression of N-methyl-d-aspartate (NMDA) receptors based on the use of vaccinia virus to analyse the maturation, transport, assembly and differential expression of the NR1 and NR2A subunits of the receptors. We have demonstrated that the NR1 subunit is efficiently transported to the plasma membrane in cells expressing NR1 alone, similarly to cells producing NR1 and NR2A together. In contrast, NR2A requires NR1 expression to be located at the cell surface. The stability of both receptor subunits expressed alone is similar to that obtained in cells producing NR1 and NR2A. In pulse–chase experiments, the NR1 subunit displays a biphasic decay, with a fraction of the protein having a half-life of only 1h and the remaining presenting a turnover longer than 24h, similar to values obtained for the NR2A subunit. Our results also show a maturation process affecting the carbohydrate moiety in the NR1 subunit, such that immature NR1has a much shorter half-life than the mature form or the NR2A subunit. Finally, we show that only a fraction of mature NR1 interacts with NR2A to form multimeric functional complexes.
Abbreviations used: DL-AP5, 2-aminophosphonopentanoic acid; ER, endoplasmic reticulum; NHS-SS-biotin, sulphosuccinimidyl-2-(biotinamido)ethyl-1,3-dithiopropionate; NMDA, N-methyl-d-aspartate; VV, vaccinia virus.