RACTK1: a putative inward rectifier potassium channel of the distal nephron. Investigation in Xenopus laevis oocytes

To date, cloning and examination of the functional properties of RACTK1, a K⁺ channel present in the distal nephron, have been performed in Chinese hamster ovary cells, but not using the oocyte expression system. We examined the expression and cytological localization of RACTK1 protein in Xenopus oocytes. In vitro transcribed cRNA of ROMK1, RACTK1, streptavidin‐tagged constructs of RACTK1 and ROMK1, alone or in concert, were injected into oocytes. Protein expression was evaluated by two‐electrode voltage clamp, followed by immunocytochemistry. ROMK1 and co‐injected oocytes (tagged and untagged) exhibited barium‐sensitive K⁺ conductances significantly greater than both RACTK1 (tagged and untagged) and water‐injected oocytes. Water‐injected oocytes failed to demonstrate protein expression. Expression of membrane protein was evident in ROMK1‐injected and RACTK1/ROMK1 co‐injected oocytes, but not in RACTK1‐injected oocytes. RACTK1 does not produce an extracellular membrane protein in oocytes. Therefore, this system is not suitable for further study or verification of the properties of RACTK1.