Regulation of Itch and Nedd4 E3 Ligase Activity and Degradation by LRAD3.

Itch and Nedd4 are members of the Nedd4 family of E3 ubiquitin ligases that are important in a number of biological processes. Precise regulation of their enzymatic activity is required for normal physiological function. Nedd4-like E3 ligases exist in an inactive form resulting from intramolecular interactions of their catalytic HECT domain with their WW domains. We identified the low-density-lipoprotein receptor class A domain containing 3 (LRAD3), a member of the LDL receptor family, as a potent activator of Itch and Nedd4 as evidenced by their increased auto-ubiquitination when bound to LRAD3. LRAD3 contains two PPxY motifs within its intracellular domain, both of which can bind to the WW domains on Itch and other Nedd4 family members with high affinity. Mutational analysis revealed that binding of Itch to the terminal LRAD3 PPxY motif is required to promote its auto-ubiquitination. We also determined that association of Itch and Nedd4 with LRAD3 leads to increased auto-ubiquitination and subsequent degradation through proteasome-mediated processes. Our findings reveal that LRAD3 is a component of pathways that function effectively to modulate Itch and Nedd4 auto-ubiquitination and levels. The identification of potential ligands for LRAD3 that may modulate LRAD3-induced activation of Itch and Nedd4 is likely to identify additional novel substrates and cellular functions for these important E3 ligases.