DIMENSIONS AND ION SELECTIVITY OF RECOMBINANT AMPA AND KAINATE RECEPTOR CHANNELS AND THEIR DEPENDENCE ON Q R SITE RESIDUES/

1. Recombinant a-amino-3-hydroxy-5-methyl-4-isoxazolepropopionate rece ptor (AMPAR) subunits (GluR-A or GluR-B) and kainate receptor (KAR) su bunit (GluR-6) in their unedited (Q)- and edited (R)-forms were expres sed in HEK 293 cells. To estimate tile dimensions of the narrow portio n of these channels, biionic reversal potentials for organic cations o f different mean diameters were determined with Cs+ as tile internal r eference ion. 2. Homomeric channels assembled from Q-form subunits wer e cation selective. The relation between the relative permeability and the mean size of different organic cations suggests that tile diamete r of the narrow portion of Q-form channels is approximately 0.78 nm fo r AMPAR and 0.75 nm for KAR channels. 3. Homomeric channels assembled from R-form subunits were permeant fur anions and cations. When probed with CsCl gradients the relative chloride permeability (P-Ol/P-Cs) wa s estimated as 0.14 fur GluR-B(R) and 0.74 for GluR-G(R)-subunit chann els. The permeability versus mean size relation for large cations meas ured with the weakly permeant F- as anion, indicates that for the R-fo rm KAR channels thr apparent pore diameter is close to 0.76 nm. 4. Het eromeric AMPAR and KAR channels co-assembled from Q- and R-form subuni ts were cation selective. The diameter of the narrow portion of these channels is estimated to be in the range between 0.70 and 0.74 nm. 5. The results indicate that the diameters of the narrow portion of AMPAR and KAR channels of different subunit composition and of widely diffe rent ion selectivity are comparable. Therefore, the differences in the anion versus cation selectivity, in Ca2+ permeability and in channel conductance are likely to be determined by the difference in charge de nsity of the channel.