Structural insight into Ca2+ specificity in tetrameric cation channels.
Apparent blockage of monovalent cation currents by the permeating blocker Ca(2+) is a physiologically essential phenomenon relevant to cyclic nucleotide-gated (CNG) channels. The recently determined crystal structure of a bacterial homolog of CNG channel pores, the NaK channel, revealed a Ca(2+) binding site at the extracellular entrance to the selectivity filter. This site is not formed by the side-chain carboxylate groups from the conserved acidic residue, Asp-66 in NaK, conventionally thought to directly chelate Ca(2+) in CNG channels, but rather by the backbone carbonyl groups of residue Gly-67. Here we present a detailed structural analysis of the NaK channel with a focus on Ca(2+) permeability and blockage. Our results confirm that the Asp-66 residue, although not involved in direct chelation of Ca(2+), plays an essential role in external Ca(2+) binding. Furthermore, we give evidence for the presence of a second Ca(2+) binding site within the NaK selectivity filter where monovalent cations also bind, providing a structural basis for Ca(2+) permeation through the NaK pore. Compared with other Ca(2+)-binding proteins, both sites in NaK present a novel mode of Ca(2+) chelation, using only backbone carbonyl oxygen atoms from residues in the selectivity filter. The external site is under indirect control by an acidic residue (Asp-66), making it Ca(2+)-specific. These findings give us a glimpse of the possible underlying mechanisms allowing Ca(2+) to act both as a permeating ion and blocker of CNG channels and raise the possibility of a similar chemistry governing Ca(2+) chelation in Ca(2+) channels.
Alam
Amer
A
Department of Physiology, University of Texas Southwestern Medical Center, Dallas, TX 75390-9040, USA.
Shi
Ning
N
Jiang
Youxing
Y
D001426:Bacterial Proteins / Q000737:chemistry; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry*; D002132:Calcium Radioisotopes / Q000737:chemistry; D002244:Carbon / Q000737:chemistry; D002412:Cations / Q000737:chemistry*; D003460:Crystallization; D004926:Escherichia coli / Q000378:metabolism; D007473:Ion Channels / Q000737:chemistry*; D008956:Models, Chemical; D008968:Molecular Conformation; D009154:Mutation; D010100:Oxygen / Q000737:chemistry; D010539:Permeability; D015221:Potassium Channels / Q000737:chemistry; D012414:Rubidium Radioisotopes / Q000737:chemistry
D001426:Bacterial Proteins; D002132:Calcium Radioisotopes; D002412:Cations; D007473:Ion Channels; D015221:Potassium Channels; D012414:Rubidium Radioisotopes; C107790:prokaryotic potassium channel; D002244:Carbon; D010100:Oxygen; D002118:Calcium