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 Mg(2+), 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.