Epithelial Na* channels (ENaCs) are expressed at the apical membrane of renal collecting duct principal cells and have a key role in transepithelial Na + absorption and in the regulation of extracellular fluid volume and blood pressure. Na + channels are composed of three structurally related subunits that assemble in the endoplasmic reticulum into a tetrameric structure. Channel assembly appears to be an inefficient process, and quality control mechanisms within the ER likely have an important role in preventing exit from the ER of individual subunits while promoting the exit of properly assembled oligomeric channels for delivery to the cell surface. Channel subunits undergo post-translational processing that includes cleavage by proteases and processing of N-linked glycans. Proposed studies in Aim 1 will identify ER chaperones that interact with ENaC subunits during Na + channel assembly, examine the role of ubiquitination and proteasomal degradation in the disposal of unassembled or improperly folded channel subunits, determine whether ER retention or ER exit signals are present within the cytoplasmic domains of the channel subunits, and examine the role of a lectin-based quality control system as a late checkpoint that discriminates channels for either ER export or degradation. Proposed studies in Aim 2 will identify proteases that process ENaC in the biosynthetic pathway and determine whether proteolytic processing of channel subunits affects functional properties of ENaC, channel surface expression or turnover. These studies should generate new information regarding the regulation of ENaC assembly, ER export and post-translational processing that provide additional levels of control of the cellular and surface pool ofNa + channels and of channel gating.
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