Cs+ block of the cardiac muscarinic K+ channel, GIRK1/GIRK4, is not dependent on the aspartate residue at position 173.
Cs+ block of GIRK1/GIRK4 expressed in Xenopus oocytes has been investigated. It has been reported that a negatively charged aspartate residue at position 172 in IRK1 is responsible for Cs+ block of the channel. IRK1, a homotetramer, has four aspartate residues at this position. GIRK1/GIRK4 is a heterotetramer and has two aspartate residues at the equivalent position (GIRK1-D173) and, consequently, it should be less sensitive to Cs+. Cs+ caused voltage-dependent block of GIRK1/GIRK4 current (measured with the two-microelectrode voltage-clamp technique). The apparent fraction of the electrical field through which Cs+ moves in order to reach its site of block (delta approximately equals 1.66) is comparable to that in IRK1, suggesting that Cs+ binds to a similar site in the two channels. GIRK1/GIRK4 was less sensitive than IRK1 to Cs+ -the Kd was 3.0-8.5 times greater and at potentials more negative than approximately or = to 130 mV there was voltage-dependent relief of block of GIRK1/GIRK4 (not the case with IRK1). However, the mutations GIRK1-D173A and GIRK1-D173Q increased the sensitivity of the channel to Cs+, while adding a negatively charged aspartate residue to GIRK4 at the equivalent position (GIRK4-N 79D) decreased Cs+ sensitivity. GIRK1-D173 cannot be the site of Cs+ block of GIRK1/GIRK4.
Dibb
K M
KM
School of Biomedical Sciences, University of Leeds, UK.
Leach
R
R
Lancaster
M K
MK
Findlay
J B
JB
Boyett
M R
MR
D000595:Amino Acid Sequence / Q000235:genetics; D000818:Animals; D001224:Aspartic Acid; D002586:Cesium / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004553:Electric Conductivity; D005260:Female; D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D006801:Humans; D016297:Mutagenesis, Site-Directed; D009154:Mutation / Q000502:physiology; D009206:Myocardium / Q000378:metabolism*; D009865:Oocytes; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000235:genetics* / Q000502:physiology; D024661:Potassium Channels, Inwardly Rectifying; D014982:Xenopus laevis
D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D024661:Potassium Channels, Inwardly Rectifying; D002586:Cesium; D001224:Aspartic Acid