NOVEL ISOFORMS OF THE BETA-SUBUNIT AND GAMMA-SUBUNIT OF THE XENOPUS EPITHELIAL NA CHANNEL PROVIDE INFORMATION ABOUT THE AMILORIDE BINDING-SITE AND EXTRACELLULAR-SODIUM SENSING

We have previously identified three homologous subunits alpha, beta, a nd gamma of the highly selective amiloride-sensitive Na channel from t he Xenopus laevis kidney A6 cell line, which forms a tight epithelium in culture. We report here two novel genes, termed beta 2 and gamma 2, which share 90 and 92% sequence identity with the previously characte rized beta and gamma XENaC, respectively. beta 2 and gamma 2 transcrip ts were detected in lung, kidney, and A6 cells grown on porous substra te, The physiological and pharmacological profile of the Na channel ex pressed after alpha beta 2 gamma XENaC cRNA injection in Xenopus oocyt e did not differ from alpha beta gamma XENaC. By contrast, the channel expressed after alpha beta gamma 2 injection showed: (i) a lower maxi mal amiloride-sensitive sodium current, (ii) a higher apparent affinit y for external sodium and inactivation of the sodium current by high s odium concentrations, and (iii) a lower apparent affinity for amilorid e (K-I alpha beta gamma 2; 1.34 mu M versus alpha beta gamma 0.35 mu M ). These data indicate that the gamma (and/or gamma 2) subunit partici pates in amiloride binding and the sensing of the extracellular sodium concentration, The close homology between gamma and gamma 2 will help to define the domains involved in sensing external sodium and in the structure of this important drug receptor.