In vivo formation of a proton-sensitive K+ channel by heteromeric subunit assembly of Kir5.1 with Kir4.1

1. Kir5.1 is an inwardly rectifying K+ channel (Kir) subunit, whose physiol ogical function is unknown. Human embryonic kidney HEK293T cells co-transfe cted with rat Kir5.1 and Kir4.1 cDNA expressed a functional K+ channel, who se properties were significantly different from those of the homomeric Kir4 .1 channel. Formation of a Kir4.1/Kir5.1 assembly in HEK293T was confirmed biochemically. 2. We found that heteromeric Kir4.1/Kir5.1 channel activity was affected by internal pH levels between 6.0 and 8.0, when the homomeric Kir4.1 channel activity was relatively stable. Changing external pH in this range had no e ffect on either Kir channel. 3. Western blot analysis using specific antibodies revealed that Kir4.1 and Kir5.1 proteins were expressed in kidney and brain, but co-immunoprecipita ted only from kidney. 4. These results indicate that the co-assembly of Kir5.1 with Kir4.1 occurs in vivo, at least in kidney. The heteromeric Kir4.1/Kir5.1 channel may the refore sense intracellular pH in renal epithelium and be involved in the re gulation of acid-base homeostasis.