NEDD4-2 (NEDD4L): the ubiquitin ligase for multiple membrane proteins

Abstract

NEDD4-2 (also known as NEDD4L, neural precursor cell expressed developmentally down-regulated 4-like) is a ubiquitin protein ligase of the Nedd4 family which is known to bind and regulate a number of membrane proteins to aid in their internalization and turnover. Several of the NEDD4-2 substrates include ion channels, such as the epithelial and voltage-gated sodium channels. Given the critical function of NEDD4-2 in regulating membrane proteins, this ligase is essential for the maintenance of cellular homeostasis. In this article we review the biology and function of this important ubiquitin-protein ligase and discuss its pathophysiological significance.

Keywords: Epithelial sodium channel (ENaC); Hypertension; NEDD4 family; Ubiquitin; Ubiquitin protein ligases; Voltage-gated sodium channels (Na(v)s).

Figure 1.. (A) The primary structure of NEDD4–2 protein.

NEDD4–2 protein comprises an amino-terminal C2 domain, 4 WW domains and a carboxy terminal HECT domain. C2 mediates lipid membrane binding, as well as acting as a protein-protein interaction motif. The WW domains are the main mediators of NEDD4–2 interaction with its substrates and adaptors. The HECT is the catalytic domain responsible for binding E2 and for transferring the ubiquitin to the substrate. (B) The phylogenetic relationship between NEDD4–2 proteins from various species. The neighbour joining phylogenetic tree construction was carried out using NCBI phylogenetic tree view (http://www.ncbi.nlm.nih.gov/blast/treeview/treeView.cgi). b: Cattle (BosTaurus); d: Dog (Canis lupus familaris); ma: Monkey (Macaca mulatta); p: Chimpanzee (Pan troglodytes); g: Chicken (Gallus gallus); z: Zebrafish (Danio rerio); h: Human (Homo sapiens); m: Mouse (Mus musculus); r: Rat (Rattus novergicus); and x: Frog (Xenopus tropicalis).

Figure 2.. NEDD4–2 is a critical regulator of ENaC.

Through the WW domains 3 and 4 NEDD4–2 directly interacts with the PY motifs found in the carboxyl termini of the three ENaC subunits. Under conditions where ENaC mediated Na⁺ uptake is not required (such as when intracellular Na⁺ concentration is high) the ubiquitination of ENaC by NEDD4–2 leads to channel endocytosis and degradation. A disruption of this control mechanism results in increased cell surface ENaC and high ENaC activity, as seen in Liddle’s syndrome and the NEDD4–2 KO mice. Under conditions where Na⁺ intake through ENaC is active NEDD4–2 acts (at least in part) as a converging point for hormones, such as aldosterone and insulin, which stimulate ENaC activity (directly or indirectly through inactivating NEDD4–2 function). Under this scenario, signaling kinases Sgk1 (serum glucocorticoid-inducible kinase) and Akt (PKB), phosphorylate NEDD4–2. This promotes binding of NEDD4–2 to 14-3-3, thus inhibiting the ability of NEDD4–2 to bind ENaC subunits.