K+ binding in the G-loop and water cavity facilitates Ba2+ movement in the Kir2.1 channel.
K+ are selectively coordinated in the selectivity filter and concerted K+ and water movements in this region ensure high conduction rates in K+ channels. In channels with long pores many K+ binding sites are located intracellular to the selectivity filter (inner vestibule), but their contribution to permeation has not been well studied. We investigated this phenomenon by slowing the ion permeation process via blocking inwardly rectifying Kir2.1 channels with Ba2+ in the selectivity filter and observing the effect of K+ in the inner vestibule on Ba2+ exit. The dose-response effect of the intracellular K+ concentration ([K+]i) on Ba2+ exit was recorded with and without intracellular polyamines, which compete with K+ for binding sites. Ba2+ exit was facilitated by the cooperative binding of at least three K+. Site-directed mutagenesis studies suggest that K+ interacting with Ba2+ bound in the selectivity filter were located in the region between selectivity filter and cytoplasmic pore, i.e. the water cavity and G-loop. One of the K+ binding sites was located at residue D172 and another was possibly at M301. This study provides functional evidence for the three K+ binding sites in the inner vestibule previously identified by crystal structure study.
Chang
Hsueh-Kai
HK
Institute of Biomedical Sciences, Academia Sinica 128 Yen-Chiu Yuan Road, section 2, Taipei 11529, Taiwan.
Marton
Laurence J
LJ
Liang
Kuo Kan
KK
Shieh
Ru-Chi
RC
D000818:Animals; D001464:Barium / Q000378:metabolism*; D002412:Cations; D055724:Electrophysiological Phenomena; D005260:Female; D008958:Models, Molecular; D009154:Mutation / Q000235:genetics; D009865:Oocytes; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000737:chemistry / Q000378:metabolism*; D024661:Potassium Channels, Inwardly Rectifying / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D040681:Structural Homology, Protein; D014867:Water / Q000737:chemistry*; D014982:Xenopus laevis
D002412:Cations; C489461:Kir2.1 channel; D024661:Potassium Channels, Inwardly Rectifying; D014867:Water; D001464:Barium; D011188:Potassium