Functional expression of recombinant N-methyl-D-aspartate receptors in the yeast Saccharomyces cerevisiae--localization and pharmacological characterization.

The yeast Saccharomyces cerevisiae was used for expressing the genes encoding the ionotropic N-methyl-D-aspartate (NMDA) receptor subunits from rats (NR1a, NR2A, NR2C) and mice (NR2B). Four plasmids were constructed by cloning the different NMDA receptor genes in the two multi-copy yeast-Escherichia coli shuttle vectors pMB01 (--> NR1a gene) and pMB02 (--> NR2A-2C genes). The protease-deficient S. cerevisiae strain cI3-ABYS-86 (leu-, ura-) was transformed or co-transformed with the resulting plasmids pMBNR1a (leu+) or pMBNR1a/pMBNR2A-C (ura+) respectively. Western blotting analysis with antibodies raised against amino acid sequences at the C-termini of the respective subunits revealed that the recombinant receptor proteins were differently expressed and only partially glycosylated in the cell membranes of the recombinant yeast strains. The expression and localization of the recombinant NMDA receptor proteins were also proved by immunofluorescence microscopy which indicated a distinct expression of the different NMDA receptor subunits in the plasma membrane of the transformed yeast cells. Pharmacological characterization of crude membrane preparations of the recombinant yeast cells showed saturable binding of the glycine antagonist [3H]MDL105,519 with Kd values of 56.9 +/- 6.19 nM (NR1a/NR2A), 26.72 +/- 2.13 nM (NR1a/NR2B), and 21.22 +/- 1.64 nM (NR1a/NR2C), and bound capacities of 17.94 +/- 1.24 pmol/mg membrane protein (NR1a/NR2A), 11.45 +/- 0.67 pmol/mg (NR1a/NR2B), and 16.15 +/- 0.86 (NR1a/NR2C) pmol/mg. The [3H]MDL105,519 binding was inhibited by the glycine antagonist 5,7-dichlorokynurenate, ethyl-2-carboxy-4,6-dichloro-3-indoleacetate, and itself, but not by glycine, D-serine or 1-amino-cyclopropanecarboxylic acid. Specific binding of [3H]glycine or the NMDA channel blocker [3H]dizolcipine were not observed.