Primary structure, developmental expression and functional properties of an inward rectifier K+ channel of the tunicate

A cDNA encoding for a tunicate inward rectifier K+ channel (TuIRK) was isol ated. TuIRK exhibited the highest similarity (approximately 50%) with mamma lian Kir2 (IRK) subfamily. Maternal RNA of TuIRK was detected by RT-PCR in unfertilized eggs. By in situ hybridization, the transcript was observed at the 32-cell stage, restricted at the 64-cell stage in anterior epidermal c ells of a4-2 blastomere lineage, and disappeared at the late gastrula stage . Therefore, TuIRK was identified to be the inward rectifier whose expressi on was previously reported to change dramatically upon the neural/epidermal cell fate selection. In Xenopus oocytes, TuIRK expressed a strongly inward rectifying K+ current. The basic electrophysiological properties of TuIRK were similar to those of the mouse IRK1 (mIRK1), except that the sensitivit y to the block by extracellular Mg2+ was much lower than that of mIRK1. To identify the structural determinant, we made mutants of the pore region, an d then of the extracellular loop (N226 of TuIRK, and E125 of mIRK1). In E12 5N mutant of mIRK1, the sensitivity to the Mg2+ block was decreased signifi cantly, whereas N226E of TuIRK1 did not acquire the sensitivity. These resu lts demonstrate the contribution of this site to the Mg2+ block and the pre sence of additional determinant(s).