Biosynthesis and processing of epithelial sodium channels in Xenopus oocytes.

The epithelial sodium channel (ENaC) provides the rate-limiting step in the reabsorption of sodium by many epithelia. The number of channels at the cell surface is tightly regulated; most cells express only a few channels. We have examined the biosynthesis and cell surface expression of ENaC in Xenopus oocytes. The subunits of ENaC are readily synthesized in the endoplasmic reticulum, but most of them remain as immature proteins in pre-Golgi compartments, where they are degraded by the proteasomal pathway without apparent ubiquitination. Even when the three subunits, alpha, beta, and gamma, are expressed in the same cell, only a very small fraction of the total channel population leave the endoplasmic reticulum, acquire complex oligosaccharides, and reach the plasma membrane. Overexpression of subunits does not increase the number of channels in the plasma membrane but results in the appearance of cytoplasmic subunits in a form not membrane bound. The data indicate that maturation and assembly of the subunits are slow and inefficient processes, and constitute limiting steps for the expression of functional ENaC channels in the plasma membrane.