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Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina, Chapel Hill, NC 27599

Correspondence to Agustín García-Caballero: acaballe{at}med.unc.edu

Epithelial sodium channels (ENaCs) perform diverse physiological roles by mediating Na⁺ absorption across epithelial surfaces throughout the body. Excessive Na⁺ absorption in kidney and colon elevates blood pressure and in the airways disrupts mucociliary clearance. Potential therapies for disorders of Na⁺ absorption require better understanding of ENaC regulation. Recent work has established partial and selective proteolysis of ENaCs as an important means of channel activation. In particular, channel-activating transmembrane serine proteases (CAPs) and cognate inhibitors may be important in tissue-specific regulation of ENaCs. Although CAP2 (TMPRSS4) requires catalytic activity to activate ENaCs, there is not yet evidence of ENaC fragments produced by this serine protease and/or identification of the site(s) where CAP2 cleaves ENaCs. Here, we report that CAP2 cleaves at multiple sites in all three ENaC subunits, including cleavage at a conserved basic residue located in the vicinity of the degenerin site (-K561, β-R503, and -R515). Sites in -ENaC at K149/R164/K169/R177 and furin-consensus sites in -ENaC (R205/R231) and -ENaC (R138) are responsible for ENaC fragments observed in oocytes coexpressing CAP2. However, the only one of these demonstrated cleavage events that is relevant for the channel activation by CAP2 takes place in -ENaC at position R138, the previously identified furin-consensus cleavage site. Replacement of arginine by alanine or glutamine (,β,R138A/Q) completely abolished both the Na⁺ current (I_Na) and a 75-kD -ENaC fragment at the cell surface stimulated by CAP2. Replacement of -ENaC R138 with a conserved basic residue, lysine, preserved both the CAP2-induced I_Na and the 75-kD -ENaC fragment. These data strongly support a model where CAP2 activates ENaCs by cleaving at R138 in -ENaC.

© 2008 García-Caballero et al. This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://www.jgp.org/misc/terms.shtml). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 3.0 Unported license, as described at http://creativecommons.org/licenses/by-nc-sa/3.0/).

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