glutamate receptor ion channel synaptic plasticity n-methyl-d-aspartate nmda ampa glur2 r2 subunit of ionotropic glutamate receptors (ampa subtype) atd amino-terminal domain iglur ionotropic glutamate receptor α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid bpbp bacterial periplasmic binding proteins lbc ligand binding core hek293s-gnti− human embryonic kidney 293s cells n-acetylglucosaminyltransferase i negative cell surface receptor