Identification of a novel bacterial K(+) channel.

In an attempt to explore unknown K(+) channels in mammalian cells, especially ATP-sensitive K(+) (K(ATP)) channels, we compared the sequence homology of Kir6.1 and Kir6.2, two pore-forming subunits of mammalian K(ATP) channel genes, with bacterial genes that code for selective proteins with confirmed or putative ion transport properties. BLAST analysis revealed that a prokaryotic gene (ydfJ) expressed in Escherichia coli K12 strain shared 8.6% homology with Kir6.1 and 8.3% with Kir6.2 genes. Subsequently, we cloned and sequenced ydfJ gene from E. coli K12 and heterologously expressed it in mammalian HEK-293 cells. The whole-cell patch-clamp technique was used to record ion channel currents generated by ydfJ-encoded protein. Heterologous expression of ydfJ gene in HEK-293 cells yielded a novel K(+) channel current that was inwardly rectified and had a reversal potential close to K(+) equilibrium potential. The expressed ydfJ channel was blocked reversibly by low concentration of barium in a dose-dependent fashion. Specific K(ATP) channel openers or blockers did not alter the K(+) current generated by ydfJ expression alone or ydfJ coexpressed with rvSUR1 or rvSUR2B subunits of K(ATP) channel complex. Furthermore, this coexpressed ydfJ/rvSUR1 channels were not inhibited by ATP dialysis. On the other hand, ydfJ K(+) currents were inhibited by protopine (a nonspecific K(+) channel blocker) but not by dofetilide (a HERG channel blocker). In summary, heterologously expressed prokaryotic ydfJ gene formed a novel functional K(+) channel in mammalian cells.