Introduction
WWP2 (WW domain-containing protein 2) is a type of ubiquitin protein ligase. Ubiquitination can serve as a signal for degradation, lead to translocation within the cell, and result in altered activity and altered protein-protein interactions. The ubiquitination pathway comprises of ubiquitin-activating (E1), ubiquitin-conjugating (E2) and ubiquitin-ligating (E3) enzymes. WWP2 is a member of the HECT (Homologous to the E6-AP Carboxyl Terminus) E3 Ligase class of enzymes. HECT E3 Ligases accept a ubiquitin molecule from E2 enzymes and transfer the ubiquitin to a Lysine residue in the target signaling molecule or transcription factor [1].
The thioester bond formation between an active site Cystine on HECT E3 Ligases and the ubiquitin ligand differentiates the HECT family of enzymes from the more abundant RING (Really Interesting New Gene) family of ubiquitin ligases which mediate ubiquitin transfer through non-covalent interactions. Within HECT E3 Ligases, WWP2 falls into the NEDD 4 family (named after the instance in which the first member was discovered: a study of developmentally down-regulated proteins in neuronal embryonic mouse cells) which generally target proteins with a PPxY motif. NEDD4 E3 Ligases consist of an amino-terminal C2 domain, between two and four WW domains, and a carboxy-terminal HECT domain. Two conformational states (a ground state inverse T shape and catalytically active L shape) have been observed.
Structure
WWP2 Scheme without C2 Domain
WWP2 Scheme WW2-2,3-linker-HECT
Full-length WWP2 consists of an amino-terminal C2 domain, four WW domains (labeled WW1-WW4), and a carboxy-terminal HECT domain. WW domains are one of the smallest studied protein modules, consisting of less than 40 amino acids, fold into three-stranded beta-sheets. They are characterized by two highly conserved positioned 20-22 amino acids apart and a high affinity for proline-rich motifs. Linkers of varying length and secondary structure connect the C2 domain to WW1, WW1 to WW2, WW2 to WW3, WW3 to WW4, and WW4 to the HECT domain. A chimeric of WWP2 consisting of the , the WW2-WW3 linker (), and the HECT domain is shown on the right.
The HECT domain is divided into two lobes (labeled N and C). The N-lobe serves as a binding site for the E2-ubiquitin complex and includes an exosite for non-covalent ubiquitin binding relevant to autoinhibition while the C-lobe contains an active site with a catalytic Cys residue to which the substrate ubiquitin molecule can covalently attach. The (magenta) connects the N and C lobes of the HECT domain and allows for flexible movement of the lobes as ubiquitin is transferred from the E2-ubiquitin complex docked on the N-lobe to the ubiquitin binding site in the C-lobe. A transthiolation reaction in this active site results in a thioester bond between the ubiquitin and a Cys residue. The HECT domain is in an inverse T shape when inactive (autoinhibited) and takes on an L shape when active. WW2 interaction with HECT is mediated by the six C terminal residues.
The alpha-helical 2,3-linker is subject to tyrosine phosphorylation at either end of the linker at residues . Chen et. al. have shown that the phosphorylation of Tyr369 allows for allosteric activation by ubiquitination at the exosite in the N-lobe, while phosphorylation of Tyr392 residue leads to a destabilization of the T conformation of the HECT domain.
Function
Studies suggest that the C2 domain, with calcium ion and phospholipid binding properties, is required for membrane binding while the WW domains play a critical role in substrate specificity. Genetic deletions of WW domains and linkers have shown increased autoubiquitination activity. The WW domains and linkers play a role in altering protein conformation, with the 2,3-linker playing the greatest role.
This linker plays an autoinhibitory role. The ground state conformation of the protein has the 2,3-linker close to the N-lobe, while WW1 and WW2 domains block the N lobe’s ubiquitin-binding site. Upon phosphorylation the 2,3-linker changes conformation, moving further from the N lobe, allowing the protein to bind ubiquitin in the E2 and N lobe binding sites. This binding will further open the protein up to bind possible substrates, like PTEN.
Interactions between the hinge and the 2,3-linker appear to restrict flexibility of the C-lobe which is necessary for ubiquitin transferase activity. This interaction further locks the protein into the T position. A current model of WWP2 activation by Chen et. al. proposes the following: (1) autoinhibited WWP2 gets phosphorylated at the 2,3-linker, (2) loosening of the 2,3-linker interactions with the hinge and ubiquitin binding exosite allow ubiquitin to bind in the C-lobe, (3) the target substrate protein binds with specifity towards the WW domains, (4) the substrate protein gets ubiquitinated, and (5) the 2,3-linker gets dephosphorylated, allowing WWP2 to return to the autoinhibited ground state conformation. [2]
