HECT E3s and human disease

Physical and functional interaction of the HECT ubiquitin-protein ligases E6AP and HERC2

Deregulation of the ubiquitin-protein ligase E6AP contributes to the development of the Angelman syndrome and to cervical carcinogenesis suggesting that the activity of E6AP needs to be under tight control. However, how E6AP activity is regulated at the post-translational level under non-pathologic conditions is poorly understood. In this study, we report that the giant protein HERC2, which is like E6AP a member of the HECT family of ubiquitin-protein ligases, binds to E6AP. The interaction is mediated by the RCC1-like domain 2 of HERC2 and a region spanning amino acid residues 150-200 of E6AP. Furthermore, we provide evidence that HERC2 stimulates the ubiquitin-protein ligase activity of E6AP in vitro and within cells and that this stimulatory effect does not depend on the ubiquitin-protein ligase activity of HERC2. Thus, the data obtained indicate that HERC2 acts as a regulator of E6AP.

Angelman syndrome protein UBE3A interacts with primary microcephaly protein ASPM, localizes to centrosomes and regulates chromosome segregation

Many proteins associated with the phenotype microcephaly have been localized to the centrosome or linked to it functionally. All the seven autosomal recessive primary microcephaly (MCPH) proteins localize at the centrosome. Microcephalic osteodysplastic primordial dwarfism type II protein PCNT and Seckel syndrome (also characterized by severe microcephaly) protein ATR are also centrosomal proteins. All of the above findings show the importance of centrosomal proteins as the key players in neurogenesis and brain development. However, the exact mechanism as to how the loss-of-function of these proteins leads to microcephaly remains to be elucidated. To gain insight into the function of the most commonly mutated MCPH gene ASPM, we used the yeast two-hybrid technique to screen a human fetal brain cDNA library with an ASPM bait. The analysis identified Angelman syndrome gene product UBE3A as an ASPM interactor. Like ASPM, UBE3A also localizes to the centrosome. The identification of UBE3A as an ASPM interactor is not surprising as more than 80% of Angelman syndrome patients have microcephaly. However, unlike in MCPH, microcephaly is postnatal in Angelman syndrome patients. Our results show that UBE3A is a cell cycle regulated protein and its level peaks in mitosis. The shRNA knockdown of UBE3A in HEK293 cells led to many mitotic abnormalities including chromosome missegregation, abnormal cytokinesis and apoptosis. Thus our study links Angelman syndrome protein UBE3A to ASPM, centrosome and mitosis for the first time. We suggest that a defective chromosome segregation mechanism is responsible for the development of microcephaly in Angelman syndrome.

Cell type-specific filamin complex regulation by a novel class of HECT ubiquitin ligase is required for normal cell motility and patterning

Differential cell motility, which plays a key role in many developmental processes, is perhaps most evident in examples of pattern formation in which the different cell types arise intermingled before sorting out into discrete tissues. This is thought to require heterogeneities in responsiveness to differentiation-inducing signals that result in the activation of cell type-specific genes and 'salt and pepper' patterning. How differential gene expression results in cell sorting is poorly defined. Here we describe a novel gene (hfnA) that provides the first mechanistic link between cell signalling, differential gene expression and cell type-specific sorting in Dictyostelium. HfnA defines a novel group of evolutionarily conserved HECT ubiquitin ligases with an N-terminal filamin domain (HFNs). HfnA expression is induced by the stalk differentiation-inducing factor DIF-1 and is restricted to a subset of prestalk cells (pstO). hfnA(-) pstO cells differentiate but their sorting out is delayed. Genetic interactions suggest that this is due to misregulation of filamin complex activity. Overexpression of filamin complex members phenocopies the hfnA(-) pstO cell sorting defect, whereas disruption of filamin complex function in a wild-type background results in pstO cells sorting more strongly. Filamin disruption in an hfnA(-) background rescues pstO cell localisation. hfnA(-) cells exhibit altered slug phototaxis phenotypes consistent with filamin complex hyperactivity. We propose that HfnA regulates filamin complex activity and cell type-specific motility through the breakdown of filamin complexes. These findings provide a novel mechanism for filamin regulation and demonstrate that filamin is a crucial mechanistic link between responses to differentiation signals and cell movement in patterning based on 'salt and pepper' differentiation and sorting out.

E3 ubiquitin ligases in ErbB receptor quantity control

Signaling through ErbB family growth factor receptor tyrosine kinases is necessary for the development and homeostasis of a wide variety of tissue types. However, the intensity of receptor-mediated cellular signaling must fall within a precise range; insufficient signaling can lead to developmental abnormalities or tissue atrophy, while over-signaling can lead to hyperplastic and ultimately neoplastic events. While a plethora of mechanisms have been described that regulate downstream signaling events, it appears that cells also utilize various mechanisms to regulate their ErbB receptor levels. Such mechanisms are collectively termed "ErbB receptor quantity control." Notably, studies over the past few years have highlighted roles for post-transcriptional processes, particularly protein degradation, in ErbB quantity control. Here the involvement of ErbB-directed E3 ubiquitin ligases is discussed, including Nrdp1-mediated ErbB3 degradation, ErbB4 degradation mediated by Nedd4 family E3 ligases, and CHIP-mediated ErbB2 degradation. The hypothesis is forwarded that protein degradation-based ErbB quantity control mechanisms play central roles in suppressing receptor overexpression in normal cells, and that the loss of such mechanisms could facilitate the onset or progression of ErbB-dependent tumors.

Animal HECT ubiquitin ligases: evolution and functional implications

Background

HECT ubiquitin ligases (HECT E3s) are key components of the eukaryotic ubiquitin-proteasome system and are involved in the genesis of several human diseases. In this study, I analyze the patterns of diversification of HECT E3s since animals emerged in order to provide the right framework to understand the functional data available for proteins of this family.

Results

I show that the current classification of HECT E3s into three groups (NEDD4-like E3s, HERCs and single-HECT E3s) is fundamentally incorrect. First, the existence of a "Single-HECT E3s" group is not supported by phylogenetic analyses. Second, the HERC proteins must be divided into two subfamilies (Large HERCs, Small HERCs) that are evolutionarily very distant, their structural similarity being due to convergence and not to a common origin. Sequence and structural analyses show that animal HECT E3s can be naturally classified into 16 subfamilies. Almost all of them appeared either before animals originated or in early animal evolution. More recently, multiple gene losses have occurred independently in some lineages (nematodes, insects, urochordates), the same groups that have also lost genes of another type of E3s (RBR family). Interestingly, the emergence of some animal HECT E3s precedes the origin of key cellular systems that they regulate (TGF-β and EGF signal transduction pathways; p53 family of transcription factors) and it can be deduced that distantly related HECT proteins have been independently co-opted to perform similar roles. This may contribute to explain why distantly related HECT E3s are involved in the genesis of multiple types of cancer.

Conclusions

The complex evolutionary history of HECT ubiquitin ligases in animals has been deciphered. The most appropriate model animals to study them and new theoretical and experimental lines of research are suggested by these results.

Insights into ubiquitin transfer cascades from a structure of a UbcH5B approximately ubiquitin-HECT(NEDD4L) complex

In E1-E2-E3 ubiquitin (Ub) conjugation cascades, the E2 first forms a transient E2 approximately Ub covalent complex and then interacts with an E3 for Ub transfer. For cascades involving E3s in the HECT class, Ub is transferred from an associated E2 to the acceptor cysteine in the HECT domain C lobe. To gain insights into this process, we determined the crystal structure of a complex between the HECT domain of NEDD4L and the E2 UbcH5B bearing a covalently linked Ub at its active site (UbcH5B approximately Ub). Noncovalent interactions between UbcH5B and the HECT N lobe and between Ub and the HECT domain C lobe lead to an overall compact structure, with the Ub C terminus sandwiched between UbcH5B and HECT domain active sites. The structure suggests a model for E2-to-HECT Ub transfer, in which interactions between a donor Ub and an acceptor domain constrain upstream and downstream enzymes for conjugation.

A structural element within the HUWE1 HECT domain modulates self-ubiquitination and substrate ubiquitination activities

E3 ubiquitin ligases catalyze the final step of ubiquitin conjugation and regulate numerous cellular processes. The HECT class of E3 ubiquitin (Ub) ligases directly transfers Ub from bound E2 enzyme to a myriad of substrates. The catalytic domain of HECT Ub ligases has a bilobal architecture that separates the E2 binding region and catalytic site. An important question regarding HECT domain function is the control of ligase activity and specificity. Here we present a functional analysis of the HECT domain of the E3 ligase HUWE1 based on crystal structures and show that a single N-terminal helix significantly stabilizes the HECT domain. We observe that this element modulates HECT domain activity, as measured by self-ubiquitination induced in the absence of this helix, as distinct from its effects on Ub conjugation of substrate Mcl-1. Such subtle changes to the protein may be at the heart of the vast spectrum of substrate specificities displayed by HECT domain E3 ligases.

Viral avoidance and exploitation of the ubiquitin system

The versatility of ubiquitin in regulating protein function and cell behaviour through post-translational protein modification makes it a particularly attractive target for viruses. Here we review how viruses manipulate the ubiquitin system to favour their propagation by redirecting cellular ubiquitin enzymes or encoding their own ubiquitin components to enable replication, egress and immune evasion. These studies not only reveal the many cellular processes requiring ubiquitin but also illustrate how viruses usurp their host cells.

Physiological functions of the HECT family of ubiquitin ligases

The ubiquitylation of proteins is carried out by E1, E2 and E3 (ubiquitin ligase) enzymes, and targets them for degradation or for other cellular fates. The HECT enzymes, including Nedd4 family members, are a major group of E3 enzymes that dictate the specificity of ubiquitylation. In addition to ubiquitylating proteins for degradation by the 26S proteasome, HECT E3 enzymes regulate the trafficking of many receptors, channels, transporters and viral proteins. The physiological functions of the yeast HECT E3 ligase Rsp5 are the best known, but the functions of HECT E3 enyzmes in metazoans are now becoming clearer from in vivo studies.

Polyubiquitination by HECT E3s and the determinants of chain type specificity

Polyubiquitination can mediate several different biochemical functions, determined in part by which lysine of ubiquitin is used to link the polyubiquitin chain. Among the HECT domain ubiquitin ligases, some, such as human E6AP, preferentially catalyze the formation of K48-linked polyubiquitin chains, while others, including Saccharomyces cerevisiae Rsp5 and human Itch, preferentially catalyze the formation of K63-linked chains. The features of HECT E3s that determine their chain type specificities have not been identified. We show here that chain type specificity is a function solely of the Rsp5 HECT domain, that the identity of the cooperating E2 protein does not influence the chain type specificity, that single chains produced by Rsp5 contain between 12 and 30 ubiquitin moieties, and that the determinants of chain type specificity are located within the last 60 amino acids of the C lobe of the HECT domain. Our results are also consistent with a simple sequential-addition mechanism for polyubiquitination by Rsp5, rather than a mechanism involving the formation of either E2- or E3-linked polyubiquitin chain transfers.

Two proteolytic pathways regulate DNA repair by cotargeting the Mgt1 alkylguanine transferase

O(6)-methylguanine (O(6)meG) and related modifications of guanine in double-stranded DNA are functionally severe lesions that can be produced by many alkylating agents, including N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), a potent carcinogen. O(6)meG is repaired through its demethylation by the O(6)-alkylguanine-DNA alkyltransferase (AGT). This protein is called Mgmt (or MGMT) in mammals and Mgt1 in the yeast Saccharomyces cerevisiae. AGT proteins remove methyl and other alkyl groups from an alkylated O(6) in guanine by transferring the adduct to an active-site cysteine residue. The resulting S-alkyl-Cys of AGT is not restored back to Cys, so repair proteins of this kind can act only once. We report here that S. cerevisiae Mgt1 is cotargeted for degradation, through a degron near its N terminus, by 2 ubiquitin-mediated proteolytic systems, the Ubr1/Rad6-dependent N-end rule pathway and the Ufd4/Ubc4-dependent ubiquitin fusion degradation (UFD) pathway. The cotargeting of Mgt1 by these pathways is synergistic, in that it increases not only the yield of polyubiquitylated Mgt1, but also the processivity of polyubiquitylation. The N-end rule and UFD pathways comediate both the constitutive and MNNG-accelerated degradation of Mgt1. Yeast cells lacking the Ubr1 and Ufd4 ubiquitin ligases were hyperresistant to MNNG but hypersensitive to the toxicity of overexpressed Mgt1. We consider ramifications of this discovery for the control of DNA repair and mechanisms of substrate targeting by the ubiquitin system.

To ubiquitinate or to deubiquitinate: it all depends on the partners

Ub (ubiquitin) and Ubls (Ub-like molecules) are peptide modifiers that change the fate and function of their substrates. A plethora of enzyme activities and protein cofactors are required for either the conjugation (mainly E3 ligases) or deconjugation of Ub and Ubls. Most of the data have been gathered on describing individual enzymes and their partners, but an increasing number of reports point to the formation of multisubunit complexes regulated by cross-talk between Ub and Ubl systems and which contain opposing conjugation/deconjugation activities. This minireview focuses on these latest reports and proposes that these complexes, which are able to recruit transient partners, shift cofactors and integrate different signalling stimuli, are a common strategy to regulate highly dynamic processes, in a switch-on/switch-off type of mechanism, thus responding promptly to cellular requirements.

The ubiquitin system, disease, and drug discovery

The ubiquitin system of protein modification has emerged as a crucial mechanism involved in the regulation of a wide array of cellular processes. As our knowledge of the pathways in this system has grown, so have the ties between the protein ubiquitin and human disease. The power of the ubiquitin system for therapeutic benefit blossomed with the approval of the proteasome inhibitor Velcade in 2003 by the FDA. Current drug discovery activities in the ubiquitin system seek to (i) expand the development of new proteasome inhibitors with distinct mechanisms of action and improved bioavailability, and (ii) validate new targets. This review summarizes our current understanding of the role of the ubiquitin system in various human diseases ranging from cancer, viral infection and neurodegenerative disorders to muscle wasting, diabetes and inflammation. I provide an introduction to the ubiquitin system, highlight some emerging relationships between the ubiquitin system and disease, and discuss current and future efforts to harness aspects of this potentially powerful system for improving human health.

Republished from Current BioData's Targeted Proteins database (TPdb; ).

gamma2-Adaptin, a ubiquitin-interacting adaptor, is a substrate to coupled ubiquitination by the ubiquitin ligase Nedd4 and functions in the endosomal pathway

gamma2-Adaptin is a putative member of the clathrin adaptor protein family with unknown physiological function. We previously reported that gamma2-adaptin acts as a ubiquitin receptor by virtue of its ubiquitin-interacting motif. Here we demonstrate that this motif mediates a specific physical interaction with the ubiquitin ligase Nedd4 and promotes ubiquitination of gamma2-adaptin. By mapping regions of Nedd4 involved in binding to gamma2-adaptin, we identified its C2 domain to be essential, whereas the WW and HECT domains are dispensable. Consistent with this, we uncovered that the C2 domain of Nedd4 is ubiquitinated itself and as such is recruited by the ubiquitin-interacting motif of gamma2-adaptin for subsequent ubiquitin conjugation. Unlike known coupled ubiquitination reactions, this novel type of interaction leads to mono- and multi/polyubiquitinated gamma2-adaptin. In addition, we show that gamma2-adaptin functions in the endosomal/multivesicular body (MVB) pathway. Depletion of gamma2-adaptin impairs the degradation of internalized epidermal growth factor and results in defective MVB morphology characterized by significantly enlarged vesicles. These defects cannot be rescued by gamma1-adaptin, a closely related homolog of gamma2-adaptin, which is unable to bind ubiquitin. Together, these results indicate that gamma2-adaptin may operate within the MVB sorting system in a manner different from that of classic adaptins.

The HECT family of E3 ubiquitin ligases: multiple players in cancer development

The involvement of the homologous to E6-AP carboxyl terminus (HECT)-type E3s in crucial signaling pathways implicated in tumorigenesis is presently an area of intense research and extensive scientific interest. This review highlights recent discoveries on the ubiquitin-mediated degradation of crucial tumor suppressor molecules catalyzed by the HECT-type E3s. By providing a portrait of their protein targets, we intend to link the substrate specificity of HECT-type E3s with their contribution to tumorigenesis. Moreover, we discuss the relevance of targeting the HECT E3s, through the development of small-molecule inhibitors, as an anticancer therapeutic strategy.