Expression of a functional Kir4 family inward rectifier K+ channel from a gene cloned from mouse liver

1. A low stringency polymerase chain reaction (PCR) homology screening proc edure was used to probe a mouse liver cDNA Library to identify novel inward rectifier K+ channel genes. A single gene (mLV1) was identified that exhib ited extensive sequence homology with previously cloned inward rectifier K channel genes. The mLV1. gene showed greatest sequence identity with genes belonging to the Kir4 subfamily. The amino acid sequence of mLV1 was 96% i dentical to a Kir channel cloned from human kidney (hKir4.2), and similar t o 60% identical to the Kir4.1. channel cloned from human and rat, so that m LV1 was classified as mKir4.2. 2. Xenopus oocytes injected with cRNA encoding mKir4.2 displayed a large in wardly rectifying K+ current, while control oocytes injected with H2O displ ayed no similar K+ current. The current was blocked by Ba2+ and Cs+ in a vo ltage-dependent fashion and displayed inward rectification that was interme diate between that of the strong inward rectifier Kir2.1 and the weak inwar d rectifier Kh1.1. The current was weakly blocked by TEA in a voltage-indep endent fashion. 3. mKir4.2 current was subject to modulation by several distinct mechanisms . Intracellular acidification decreased mKir4.2 current in a reversible fas hion, while activation of protein kinase C decreased mKir4.2 current ill a. manner that was not rapidly reversible. Incubation of oocytes in elevated [K+] produced a slowly developing enhancement of current. 4. Oocytes co-injected with cRNA for mKir4.2 and Kir5.1, a protein that doe s not form functional homomeric channels, displayed membrane currents with properties distinct from those expressing mKir4.2 alone. Go-injected oocyte s displayed larger currents than mKir4.2, with novel kinetic properties and an increased sensitivity to Ba2+ block at negative potentials, suggesting that mKir4.2 forms functional heteromultimeric channels with Kir5.1, as has been shown for Kir4.1 5. These results demonstrate for the first time that a Kir4.2 channel gene product forms functional channels in Xenopus oocytes, that these Kir channe ls display novel properties, and that Kir4.2 subunits may be responsible fo r physiological modulation of functional Kir channels.