The Molecular Chaperones, ERdj3 and ERdj4, Two Hsp40s, Select ENaC for ER Associated Degradation (ERAD)

Abstract

The epithelial Na+ channel (ENaC) is a vital channel and mutations in ENaC may contribute to hyper‐ and hypotension and cystic fibrosis. ENaC consists of 3 homologous subunits (α, β, γ) that assemble in the ER and traffic to the plasma membrane. ENaC assembly is inefficient with <20% reaching the plasma membrane, and a large fraction of ENaC subunits are targeted for ER‐associated degradation (ERAD). Commonly, selection of ERAD substrates occurs via the chaperones Hsp70s and Hsp40s; however, little is known about the chaperones involved in the ERAD of ENaC. We therefore employed a yeast expression system and identified two Hsp40s, Jem1 and Scj1, that function as ENaC chaperones. The mammalian homologs of these Hsp40s are ERdj3 and ERdj4, and our hypothesis was that ERdj3 and ERdj4 over‐expression would increase ENaC degradation and cause a decrease in ENaC function‐expression. Using a Xenopus oocyte expression system and two‐electrode voltage clamp (TEV), we discovered that ENaC current was reduced >30% by co‐expression of ERdj3 or ERdj4. This reduction in current was proteosome‐mediated and correlated with a decrease in ENaC surface expression. However, these Hsp40s neither affected the rate of ENaC endocytosis nor exocytosis. These data support the hypothesis that Hsp40s are important in the ERAD of ENaC, and that data obtained in our yeast system can be recapitulated in a vertebrate‐expression system. Funded by NIH: GM75061, DK65161, and DK79307