Functional Roles of the Edited Isoform of GluA2 in GluA2-Containing AMPA Receptor Channels.

GluA2R is the edited isoform at the Q/R site of the AMPA receptor GluA2 subunit. In vivo, GluA2R assembles with other AMPA receptor subunits into a high-conductance, tetrameric complex. Here we study the functional role of GluA2R in the context of GluA1/2R and GluA2Q/2R heteromeric receptors, using whole-cell recording and a laser-pulse photolysis technique. We find the incorporation of GluA2R with GluA1 or GluA2Q slows the channel-opening and channel-closing rates; for GluA2Q/2R, it further lowers EC50, compared to the value of the GluA2Q homomeric channels. When a conserved leucine-to-tyrosine (L → Y) mutation is "placed" in each of these subunits, the L → Y mutation exhibits a greater effect in GluA2R than in GluA1. Together, our results show GluA2R is the dominating isoform that shapes the overall functional properties of the GluA2R-containing channels. Our results further suggest that glutamate most likely binds to GluA2R with an EC50 of ∼0.5 mM, rather than a non-GluA2R subunit in a heteromeric AMPA receptor, and binding of two glutamate molecules to the two GluA2R subunits is sufficient to initiate a conformational change, leading to the opening of the channel.