Cell-Surface Expression of the Channel Activating Protease xCAP-1 Is Required for Activation of ENaC in the Xenopus Oocyte

Abstract

ABSTRACT. Sodium balance, extracellular fluid volume, and ultimately BP are maintained by precise regulation of the activity of the epithelial sodium channel (ENaC). Using a functional complementation assay in the Xenopuslaevis oocyte expression system, a channel-activating protease (CAP-1) that increases ENaC activity two to threefold in the Xenopus oocyte expression system is here identified. External application of trypsin mimics the effect of Xenopus CAP-1 (xCAP-1) on ENaC activity, which can be blocked by aprotinin, a serine protease inhibitor, suggesting the existence of a novel extracellular pathway for controlling ENaC activity. Sequence analysis predicts that CAP-1 is a secreted and/or glycosyl-phosphatidyl-inositol (GPI)-anchored protein. The aim of the present study was to determine whether cell-surface expression of xCAP-1 is required for ENaC activation. By site-directed mutagenesis of xCAP-1, the importance of the catalytic site, n-glycosylation, and the GPI anchor of xCAP-1 on ENaC activity were analyzed. Glycosylation or catalytic activity is not required for cell-surface expression of xCAP-1, whereas the deletion of the GPI anchor consensus motif at the C-terminus of xCAP-1 (G305Stop) abolishes cell-surface expression and ENaC activation. G305Stop-mutated xCAP-1 is recovered as a secreted protein in the external medium. A catalytic mutant of xCAP-1 significantly decreased ENaC activation but did not fully abolish the effect of xCAP-1. The data indicate the critical role of the GPI anchor in ENaC activation and suggest that catalytic and noncatalytic mechanisms are involved.