Evidence for Sequential Ion-binding Loci along the Inner Pore of the IRK1 Inward-rectifier K+ Channel

Steep rectification in IRK1 (Kir2.1) inward-rectifier K(+) channels reflects strong voltage dependence (valence of approximately 5) of channel block by intracellular cationic blockers such as the polyamine spermine. The observed voltage dependence primarily results from displacement, by spermine, of up to five K(+) ions across the narrow K(+) selectivity filter, along which the transmembrane voltage drops steeply. Spermine first binds, with modest voltage dependence, at a shallow site where it encounters the innermost K(+) ion and impedes conduction. From there, spermine can proceed to a deeper site, displacing several more K(+) ions and thereby producing most of the observed voltage dependence. Since in the deeper blocked state the leading amine group of spermine reaches into the cavity region (internal to the selectivity filter) and interacts with residue D172, its trailing end is expected to be near M183. Here, we found that mutation M183A indeed affected the deeper blocked state, which supports the idea that spermine is located in the region lined by the M2 and not deep in the narrow K(+) selectivity filter. As to the shallower site whose location has been unknown, we note that in the crystal structure of homologous GIRK1 (Kir3.1), four aromatic side chains of F255, one from each of the four subunits, constrict the intracellular end of the pore to approximately 10 A. For technical simplicity, we used tetraethylammonium (TEA) as an initial probe to test whether the corresponding residue in IRK1, F254, forms the shallower site. We found that replacing the aromatic side chain with an aliphatic one not only lowered TEA affinity of the shallower site approximately 100-fold but also eliminated the associated voltage dependence and, furthermore, confirmed that similar effects occurred also for spermine. These results establish the evidence for physically separate, sequential ion-binding loci along the long inner pore of IRK1, and strongly suggest that the aromatic side chains of F254 underlie the likely innermost binding locus for both blocker and K(+) ions in the cytoplasmic pore.