Stoichiometry and modular retrieval of ENaC

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The epithelial Na channel (ENaC) plays a fundamental role in establishing blood pressure. ENaC activity is set, in part, by its level in the plasma membrane. Membrane levels of ENaC reflect constitutive delivery and regulated retrieval. The mechanisms and domains within ENaC involved in retrieval are not completely understood. We are interested in two conserved, overlying domains (S/TPPPxYxS/TL) found within all ENaC subunits: the PY (xPPxY) and tyrosine-based endocytic (YxxL) motifs. The prior motif targets the channel for ubiquitinylation via Nedd4 ubiquitin ligases; and the latter motif is known to interact with the mu2 subunit of the AP-2 complex, which targets proteins for clathrin coated-pit endocytosis mediated by dynamin. We will test whether these domains are modular and thus, separable or whether they act in concert. Physiological signaling cascades (e.g. MAPK) decrease ENaC activity by promoting channel degradation. Thus, we also test the hypothesis that MAPK signaling decreases channel activity via these modular retrieval domains. Moreover, we will determine whether absolutely conserved S/T just preceding the PY and within the YxxL motifs, which fit the consensus sequence for MAPK, are targets for MAPK impacting channel activity and membrane level. ENaC is a heteromeric channel comprised of 3 distinct subunits with channels containing two or fewer types of subunits having decreased activity. Thus, one possible outcome of subunit retrieval is production of homomeric channels or channels containing only two types of subunits. Since, subunit stoichiometry of membrane ENaC may not be fixed, we also ask whether MAPK signaling via the PY and YxxL domains changes ENaC subunit stoichiometry and/or composition to modulate channel activity. Here, we address four specific aims: 1) Determine subunit stoichiometry of membrane ENaC; 2) Determine whether membrane ENaC levels are controlled by modular PY and YxxL endocytic domains and assign significance to each domain; 3) Determine whether physiological cell signaling cascades modulate channel retrieval via the PY and YxxL motifs and whether differential phosphorylation of conserved S/T modulate this retrieval; and 4) Determine if retrieval of ENaC subunits from the plasma membrane is coordinated. This research will provide critical insight about the molecualr architecture of ENaC and regulation of this important channel.