KH And NYN Domain Containing Protein (KHNYN)

KHNYN belongs to the N4BP1 family. N4BP1 has been identified as a protein interactor and a substrate of the homologous to E6AP C terminus (HECT) domain-containing E3 ubiquitin–protein ligase (E3), Nedd4. N4BP1 association with the second WW domain (WW2) of Itch interferes with E3 binding to its substrates. N4BP1 and p73a, a target of Itch-mediated ubiquitin/proteasome proteolysis, share the same binding site. By competing with p73a for binding to the WW2 domain, N4BP1 reduces the ability of Itch to recruit and ubiquitylate p73a and inhibits Itch autoubiquitylation activity both in in vitro and in vivo ubiquitylation assays. Similarly, both c-Jun and p63 polyubiquitylation by Itch are inhibited by N4BP1. As a consequence, genetic and RNAi knockdown of N4BP1 diminish the steady-state protein levels and significantly impair the transcriptional activity of Itch substrates.