The glycine-tyrosine-glycine (GYG) sequence in the ploop of K[sup +] channel subunits lines a narrow pore through which K[sup +] ions pass in single file intercalated by water molecules. Mutation of the motif can give rise to nonselective channels, but it is clear that other structural features are also required for selectivity because, for instance, a recently identified class of cyclic nucleotide gated pacemaker channels has the GYG motif but are poorly K[sup +] selective. We show that mutation of charged glutamate and arginine residues behind the selectivity filter in the Kir3.1/Kir3.4 K[sup +] channel reduces or abolishes K[sup +] selectivity, comparable with previously reported effects in the Kit2.1 K[sup +] channel. It has been suggested that a salt bridge exists between the glutamate-arginine residue pair. Molecular modeling indicates that the salt bridge does exist, and that it acts as a "bowstring" to maintain the rigid bow-like structure of the selectivity filter and restrict selectivity to K[sup +]. The modeling shows that relaxation of the bowstring by mutation of the residue pair leads to enhanced flexibility of the p-loop, allowlng permeation of other cations, including polyamines. In experiments, mutation of the residue pair can also abolish polyamine-induced inward rectification. The latter effect occurs because polyamines now permeate rather than block the channel, to the remarkable extent that large polyamine currents can be measured. [ABSTRACT FROM AUTHOR]