PERMEATION PROPERTIES AND DIFFERENTIAL EXPRESSION ACROSS THE AUDITORYRECEPTOR EPITHELIUM OF AN INWARD RECTIFIER K-EAR( CHANNEL CLONED FROMTHE CHICK INNER)

The auditory receptor epithelium is an excellent model system for stud ying the differential expression of ion channel genes, An inward recti fier potassium cur rent is among those which have been measured in onl y subsets of chick cochlear hair cells. We have cloned and characteriz ed an inward rectifier potassium channel (cIRK1) from the chick cochle ar sensory epithelium, cIRK1 functional properties are similar to thos e of the native channel, and the transcript encoding cIRK1 is limited to the low frequency half of the epithelium. This localization is in a greement with the distribution of the native hair cell current, sugges ting that the differential current expression is transcriptionally reg ulated. The primary structure of cIRK1 is highly homologous to the mou se inward rectifier IRK1. However, we found that cIRK1 exhibited reduc ed single-channel conductance (17 picosiemens) and lower sensitivity t o Ba2+ block (K-1/2 = 12 mu M). We identified Gln-125 near the putativ e pore region as being responsible for these differences. Site-directe d mutagenesis was used to change Gln-125 to Glu (the residue in IRK1), resulting in a channel with a single channel conductance of 28 picosi emens and a Ba2+ block of K-1/2 = 2 mu M. We propose that Gln-125 may form part of the external vestibule of the pore.