Redefining the catalytic HECT domain boundaries for the HECT E3 ubiquitin ligase family.

There are 28 unique human members of the homologous to E6AP C-terminus (HECT) E3 ubiquitin ligase family. Each member of the HECT E3 ubiquitin ligases contains a conserved bilobal HECT domain of approximately 350 residues found near their C-termini that is responsible for their respective ubiquitylation activities. Recent studies have begun to elucidate specific roles that each HECT E3 ubiquitin ligase has in various cancers, age-induced neurodegeneration, and neurological disorders. New structural models have been recently released for some of the HECT E3 ubiquitin ligases, but many HECT domain structures have yet to be examined due to chronic insolubility and/or protein folding issues. Building on these recently published structural studies coupled with our in-house experiments discussed in the present study, we suggest that the addition of ∼50 conserved residues preceding the N-terminal to the current UniProt defined boundaries of the HECT domain are required for isolating soluble, stable, and active HECT domains. We show using in silico bioinformatic analyses coupled with secondary structural prediction software that this predicted N-terminal α-helix found in all 28 human HECT E3 ubiquitin ligases forms an obligate amphipathic α-helix that binds to a hydrophobic pocket found within the HECT N-terminal lobe. The present study brings forth the proposal to redefine the residue boundaries of the HECT domain to include this N-terminal extension that will likely be critical for future biochemical, structural, and therapeutic studies on the HECT E3 ubiquitin ligase family.

An Angelman syndrome substitution in the HECT E3 ubiquitin ligase C-terminal Lobe of E6AP affects protein stability and activity.

Angelman syndrome (AS) is a rare neurodevelopmental disorder characterized by speech impairment, intellectual disability, ataxia, and epilepsy. AS is caused by mutations in the maternal copy of UBE3A located on chromosome 15q11-13. UBE3A codes for E6AP (E6 Associated Protein), a prominent member of the HECT (Homologous to E6AP C-Terminus) E3 ubiquitin ligase family. E6AP catalyzes the posttranslational attachment of ubiquitin via its HECT domain onto various intracellular target proteins to regulate DNA repair and cell cycle progression. The HECT domain consists of an N-lobe, required for E2~ubiquitin recruitment, while the C-lobe contains the conserved catalytic cysteine required for ubiquitin transfer. Previous genetic studies of AS patients have identified point mutations in UBE3A that result in amino acid substitutions or premature termination during translation. An AS transversion mutation (codon change from ATA to AAA) within the region of the gene that codes for the catalytic HECT domain of E6AP has been annotated (I827K), but the molecular basis for this loss of function substitution remained elusive. Here, we demonstrate that the I827K substitution destabilizes the 3D fold causing protein aggregation of the C-terminal lobe of E6AP using a combination of spectropolarimetry and nuclear magnetic resonance (NMR) spectroscopy. Our fluorescent ubiquitin activity assays with E6AP-I827K show decreased ubiquitin thiolester formation and ubiquitin discharge. Using 3D models in combination with our biochemical and biophysical results, we rationalize why the I827K disrupts E6AP-dependent ubiquitylation. This work provides new insight into the E6AP mechanism and how its malfunction can be linked to the AS phenotype.

1H, 13C, and 15N resonance assignments of the C-terminal lobe of the human HECT E3 ubiquitin ligase ITCH.

ITCH (aka Atrophin-1-interacting protein 4) is a prominent member of the NEDD4 HECT (Homologous to E6AP C-Terminus) E3 ubiquitin ligase family that regulates numerous cellular functions including inflammatory responses through T-cell activation, cell differentiation, and apoptosis. Known intracellular targets of ITCH-dependent ubiquitylation include receptor proteins, signaling molecules, and transcription factors. The HECT C-terminal lobe of ITCH contains the conserved catalytic cysteine required for the covalent attachment of ubiquitin onto a substrate and polyubiquitin chain assembly. We report here the complete experimentally determined 1H, 13C, and 15N backbone and sidechain resonance assignments for the HECT C-terminal lobe of ITCH (residues 784-903) using heteronuclear, multidimensional NMR spectroscopy. These resonance assignments will be used in future NMR-based studies to examine the role of dynamics and conformational flexibility in HECT-dependent ubiquitylation as well as deciphering the structural and biochemical basis for polyubiquitin chain synthesis and specificity by ITCH.

Solution structure of the E3 ligase HOIL-1 Ubl domain.

The E3 ligases HOIL-1 and parkin are each comprised of an N-terminal ubiquitin-like (Ubl) domain followed by a zinc-binding region and C-terminal RING-In-between-RING-RING domains. These two proteins, involved in the ubiquitin-mediated degradation pathway, are the only two known E3 ligases to share this type of multidomain architecture. Further, the Ubl domain of both HOIL-1 and parkin has been shown to interact with the S5a subunit of the 26S proteasome. The solution structure of the HOIL-1 Ubl domain was solved using NMR spectroscopy to compare it with that of parkin to determine the structural elements responsible for S5a intermolecular interactions. The final ensemble of 20 structures had a ß-grasp Ubl-fold with an overall backbone RMSD of 0.59 ± 0.10 Å in the structured regions between I55 and L131. HOIL-1 had a unique extension of both ß1 and ß2 sheets compared to parkin and other Ubl domains, a result of a four-residue insertion in this region. A similar 15-residue hydrophobic core in the HOIL-1 Ubl domain resulted in a comparable stability to the parkin Ubl, but significantly lower than that observed for ubiquitin. A comparison with parkin and other Ubl domains indicates that HOIL-1 likely uses a conserved hydrophobic patch (W58, V102, Y127, Y129) found on the ß1 face, the ß3-ß4 loop and ß5, as well as a C-terminal basic residue (R134) to recruit the S5a subunit as part of the ubiquitin-mediated proteolysis pathway.

Identification of a novel Zn2+-binding domain in the autosomal recessive juvenile Parkinson-related E3 ligase parkin.

Missense mutations in park2, encoding the parkin protein, account for approximately 50% of autosomal recessive juvenile Parkinson disease (ARJP) cases. Parkin belongs to the family of RBR (RING-between-RING) E3 ligases involved in the ubiquitin-mediated degradation and trafficking of proteins such as Pael-R and synphillin-1. The proposed architecture of parkin, based largely on sequence similarity studies, consists of N-terminal ubiquitin-like and C-terminal RBR domains. These domains are separated by a approximately 160-residue unique parkin sequence having no recognizable domain structure. We used limited proteolysis experiments on bacterially expressed and purified parkin to identify a new domain (RING0) within the unique parkin domain sequence. RING0 comprises two distinct, conserved cysteine-rich clusters between Cys(150)-Cys(169) and Cys(196)-His(215) consisting of CX(2)-(3)CX(11)CX(2)C and CX(4-6)CX(10-16)-CX(2)(H/C) motifs. The positions of the cysteine/histidine residues in this region bear similarity to parkin RING1 and RING2 domains, as well as other E3 ligase RING domains. However, in parkin a 26-residue linker region separates the motifs, which is not typical of other RING domain structures. Further, the RING0 domain includes all but one of the known ARJP mutation sites between the ubiquitin-like and RBR regions of parkin. Using electrospray ionization mass spectrometry and inductively coupled plasma-atomic emission spectrometry analysis, we determined that the RING0, RING1, IBR, and RING2 domains each bind two Zn(2+) ions, the first observation of an E3 ligase with the ability to bind eight metal ions. Removal of the zinc from parkin causes near complete unfolding of the protein, an observation that rationalizes cysteine-based ARJP mutations found throughout parkin, including RING0 (C212Y) that form cellular inclusions and/or are defective for ubiquitination likely because of poor zinc binding and misfolding. The identification of the RING0 domain in parkin provides a new overall domain structure for the protein that will be important in assessing the roles of ARJP mutations and designing experiments aimed at understanding the disease.

Structure of the Parkin in-between-ring domain provides insights for E3-ligase dysfunction in autosomal recessive Parkinson's disease.

Mutations in Parkin are one of the predominant hereditary factors found in patients suffering from autosomal recessive juvenile Parkinsonism. Parkin is a member of the E3 ubiquitin ligase family that is defined by a tripartite RING1-in-between-ring (IBR)-RING2 motif. In Parkin, the IBR domain has been shown to augment binding of the E2 proteins UbcH7 and UbcH8, and the subsequent ubiquitination of the proteins synphilin-1, Sept5, and SIM2. To facilitate our understanding of Parkin function, the solution structure of the Parkin IBR domain was solved by using NMR spectroscopy. Folding of the IBR domain (residues M327-S378) was found to be zinc dependent, and the structure reveals the domain forms a unique pair scissor-like and GAG knuckle-like zinc-binding sites, different from other zinc-binding motifs such as the RING, LIM, PHD, or B-box motifs. The N terminus of the IBR domain, residues E307-E322, is unstructured. The disease causing mutation T351P causes global unfolding, whereas the mutation R334C causes some structural rearrangement of the domain. In contrast, the protein containing the mutation G328E appears to be properly folded. The structure of the Parkin IBR domain, in combination with mutational data, allows a model to be proposed where the IBR domain facilitates a close arrangement of the adjacent RING1 and RING2 domains to facilitate protein interactions and subsequent ubiquitination.