Number of subunits comprising the epithelial sodium channel

Abstract

The human epithelial sodium channel (hENaC) is a hetero-oligomeric complex composed of three subunits, α, β, and γ. Understanding the structure and function of this channel and its abnormal behavior in disease requires knowledge of the number of subunits that comprise the channel complex. We used freeze-fracture electron microscopy and electrophysiological methods to evaluate the number of subunits in the ENaC complex expressed in Xenopus laevis oocytes. In oocytes expressing wild-type hENaC (α, β, and γ subunits), clusters of particles appeared in the protoplasmic face of the plasma membrane. The total number of particles in the clusters was consistent with the whole-cell amiloride-sensitive current measured in the same cells. The size frequency histogram for the particles in the clusters suggested the presence of an integral membrane protein complex composed of 17 ± 2 transmembrane α-helices. Because each ENaC subunit has two putative transmembrane helices, these data suggest that in the oocyte plasma membrane, the ENaC complex is composed of eight or nine subunits. At high magnification, individual ENaC particles exhibited a near-square geometry. Functional studies using wild-type αβ-hENaC coexpressed with γ-hENaC mutants, which rendered the functional channel differentially sensitive to methanethiosulfonate reagents and cadmium, suggested that the functional channel complex contains more than one γ subunit. These data suggest that functional ENaC consists of eight or nine subunits of which a minimum of two are γ subunits.