Structural studies of the nedd4WWDomains and their selectivity for the connexin43 (Cx43) carboxyl trerminus

Neuronal precursor cell-expressed developmentally downregulated 4 (Nedd4) was the first ubiquitin protein ligase identified to interact with connexin43 (Cx43), and its suppressed expression results in accumulation of gap junction plaques at the plasma membrane. Nedd4-mediated ubiquitination of Cx43 is required to recruit Eps15 and target Cx43 to the endocytic pathway. Although the Cx43 residues that undergo ubiquitination are still unknown, in this study we address other unresolved questions pertaining to the molecular mechanisms mediating the direct interaction between Nedd4 (WW1-3 domains) and Cx43 (carboxyl terminus (CT)). All threeWWdomains display a similar three antiparallelβ-strand structure and interact with the same Cx43CT ²⁸³PPXY²⁸⁶ sequence. Although Tyr is essential for the interaction, MAPK phosphorylation of the preceding serine residues (Ser(P)²⁷⁹ and Ser(P)²⁸²) increases the binding affinity by 2-fold for the WW domains (WW2 > WW3 >> WW1). The structure of the WW2 Cx43CT ^276-289(Ser(P)²⁷⁹ Ser(P)²⁸²) complex reveals that coordination of Ser(P)²⁸² with the end of-strand 3 enables Ser(P)²⁷⁹ to interact with the back face of-strand 3 (Tyr286 is on the front face) and loop 2, forming a horseshoe-shaped arrangement. The close sequence identity of WW2 with WW1 and WW3 residues that interact with the Cx43CT PPXY motif and Ser(P)²⁷⁹/Ser(P)²⁸² strongly suggests that the significantly lower binding affinity ofWW1is the result of a more rigid structure. This study presents the first structure illustrating how phosphorylation of the Cx43CT domain helps mediate the interaction with a molecular partner involved in gap junction regulation.