Permeation properties and differential expression across the auditory receptor epithelium of an inward rectifier K+ channel cloned from the chick inner ear.

The auditory receptor epithelium is an excellent model system for studying the differential expression of ion channel genes. An inward rectifier potassium current is among those which have been measured in only subsets of chick cochlear hair cells. We have cloned and characterized an inward rectifier potassium channel (cIRK1) from the chick cochlear sensory epithelium. cIRK1 functional properties are similar to those of the native channel, and the transcript encoding cIRK1 is limited to the low frequency half of the epithelium. This localization is in agreement with the distribution of the native hair cell current, suggesting that the differential current expression is transcriptionally regulated. The primary structure of cIRK1 is highly homologous to the mouse inward rectifier IRK1. However, we found that cIRK1 exhibited reduced single-channel conductance (17 picosiemens) and lower sensitivity to Ba2+ block (K1/2 = 12 microM). We identified Gln-125 near the putative pore region as being responsible for these differences. Site-directed mutagenesis was used to change Gln-125 to Glu (the residue in IRK1), resulting in a channel with a single-channel conductance of 28 picosiemens and a Ba2+ block of K1/2 = 2 microM. We propose that Gln-125 may form part of the external vestibule of the pore.