Figure 1. Selective expression of GluN1/GluN2A/GluN2B triheteromers at the surface of Xenopus oocytes. A, Schematic representation of the engineered NMDAR subunits. NTD, N-terminal domain; ABD, agonist-binding domain; TMD, transmembrane domain; CTD, C-terminal domain. The r1 and r2 tags contain the ER retention/retrieval signals derived from the GABA-B1 and GABA-B2 subunit, respectively. The GluN1-6A subunit has two endogenous ER retention signals (K875KK and R893RR) mutated into alanines. B, Current amplitudes of various combinations of diheteromers and triheteromers. Currents were measured 3–4 d after injection. Bars represent the average value for each condition. C, Immunoblots from oocytes expressing various combinations of GluN1, GluN2A, and/or GluN2B subunits. The anti-GABA-B1 antibody is used to detect GluN2A subunits containing the r1 retention tag. The arrowheads indicate bands corresponding to GluN1wt of GluN1-6A (white, ∼115 kDa), GluN2Ar1 (blue, ∼110 kDa), GluN2B wt (gray, ∼180 kDa), and GluN2Br2 (red, ∼110 kDa). The stars indicate nonspecific bands. α-tub, α-Tubulin; and n.i., noninjected oocytes.