title: Revisiting inward rectification: K ions permeate through Kir2.1 channels during high-affinity block by spermidine.
abstract: Outward currents through Kir2.1 channels play crucial roles in controlling the electrical properties of excitable cells, and such currents are subjected to voltage-dependent block by intracellular Mg(2+) and polyamines that bind to both high- and low-affinity sites on the channels. Under physiological conditions, high-affinity block is saturated and yet outward Kir2.1 currents can still occur, implying that high-affinity polyamine block cannot completely eliminate outward Kir2.1 currents. However, the underlying molecular mechanism remains unknown. Here, we show that high-affinity spermidine block, rather than completely occluding the single-channel pore, induces a subconducting state in which conductance is 20% that of the fully open channel. In a D172N mutant lacking the high-affinity polyamine-binding site, spermidine does not induce such a substate. However, the kinetics for the transitions between the substate and zero-current state in wild-type channels is the same as that of low-affinity block in the D172N mutant, supporting the notion that these are identical molecular events. Thus, the residual outward current after high-affinity spermidine block is susceptible to low-affinity block, which determines the final amplitude of the outward current. This study provides a detailed insight into the mechanism underlying the emergence of outward Kir2.1 currents regulated by inward rectification attributed to high- and low-affinity polyamine blocks.
authors.affiliation: Institute of Biomedical Sciences, Academia Sinica, Taipei 11529, Taiwan.
authors.forename: Tai-An
authors.initials: TA
authors.lastname: Liu
authors.forename: Hsueh-Kai
authors.initials: HK
authors.lastname: Chang
authors.forename: Ru-Chi
authors.initials: RC
authors.lastname: Shieh
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D001665:Binding Sites / Q000235:genetics; D055724:Electrophysiological Phenomena; D005260:Female; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials; D008954:Models, Biological; D016297:Mutagenesis, Site-Directed; D009865:Oocytes / Q000378:metabolism; D024661:Potassium Channels, Inwardly Rectifying / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D013095:Spermidine / Q000494:pharmacology*; D013096:Spermine / Q000494:pharmacology; D014982:Xenopus laevis
chemical_list: C489461:Kir2.1 channel; D024661:Potassium Channels, Inwardly Rectifying; D011994:Recombinant Proteins; D013096:Spermine; D013095:Spermidine