RENAL EPITHELIAL PROTEIN (APX) IS AN ACTIN CYTOSKELETON-REGULATED NA+CHANNEL

Apr, the amphibian protein associated with renal amiloride-sensitive N a+ channel activity and with properties consistent with the pore-formi ng 150-kDa subunit of an epithelial Na+ channel complex initially puri fied by Benos et al. (Benos, D. J., Saccomani, G., and Sariban-Sohraby , S. (1987) J. Biol. Chem. 262, 10613-10618), has previously failed to generate amiloride-sensitive Na+ currents (Staub, O., Verrey, F., Kle yman, T. R., Benos, D. J., Rossier, B. C., and Kraehenbuhl, J.-P. (199 2) J. Cell Biol. 119, 1497-1506). Renal epithelial Na+ channel activit y is tonically inhibited by endogenous actin filaments (Cantiello, H. F., Stow, J., Prat, A. G;., and Ausiello, D. A. (1991) Am. J. Physiol. 261, C882-C888). Thus, Apr was expressed and its function examined in human melanoma cells with a defective actin-based cytoskeleton. Apx-t ransfection was associated with a 60-900% increase in amiloride-sensit ive (K-i = 3 mu M) Na+ currents. Single channel Na+ currents had a sim ilar functional fingerprint to the vasopressin sensitive, and actin-re gulated epithelial Na+ channel of A6 cells, including a 6-7 pS single channel conductance and a perm-selectivity of Na+:K+ of 4:1. Na+ chann el activity was either spontaneous, or induced by addition of actin or protein kinase A plus ATP to the bathing solution of excised inside-o ut patches. Therefore, Apr may be responsible for the ionic conductanc e involved in the vasopressin-activated Na+ reabsorption in the amphib ian kidney.