Fig. 1. Schematic representations of the proposed structure of glutamate receptor channel subunits. (  A ) Three transmembrane segment topology model. (  B ) Amino acid sequences of segment M2 of the glutamate receptor channel subunits selective for NMDA (ε1, ε2, ε3, ε4, and ζ1) and those selective for AMPA (α1, α2, α3, and α4). The box indicates the Q (glutamine)/R (arginine)/N (asparagine) site of the glutamate receptor channel subunits, which determines the permeability and block by divalent cations.