Ion pore properties of ionotropic glutamate receptors are modulated by a transplanted potassium channel selectivity filter

The canonical potassium channel selectivity filter motif TVGYG was transplanted into ionotropic glutamate receptors (iGluRs) of the AMPA and NMDA subtype to test whether it renders the iGluRs K+ selective. The TVGYG motif modulated several ion pore properties of AMPA receptor as well as NMDA receptor mutants, e.g., the intra- and extracellular polyamine block, current/voltage relationships, open channel block by MK801 and Mg2+, and permeability for divalent cations. However, introduction of the selectivity filter failed to increase the K+ selectivity of homomeric AMPA and heteromeric NMDA receptor complexes, which may be due to absence of selectivity filter-stabilizing interaction sites in the iGluR pore domain.Our findings indicate that even if glutamate receptors appear to have the intrinsic capacity for K+ permeability, as is demonstrated by the prokaryotic, glutamate-gated, K+ selective GluR0, the isolated selectivity filter is not able to confer K+ permeability to the relatively unselective iGluR cation pore.