Ubiquitin ligase

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Ubiquitin—protein ligase
4a4c.png
E3 ubiquitin ligase Cbl (blue) in complex with E2 (cyan) and substrate peptide (green). PDB entry 4a4c [1]
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EC no. 2.3.2.27
CAS no. 74812-49-0
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Ubiquitin ligase
Identifiers
SymbolUbiquitin ligase
OPM superfamily 471
OPM protein 4v6p
Membranome 240

A ubiquitin ligase (also called an E3 ubiquitin ligase) is a protein that recruits an E2 ubiquitin-conjugating enzyme that has been loaded with ubiquitin, recognizes a protein substrate, and assists or directly catalyzes the transfer of ubiquitin from the E2 to the protein substrate. In simple and more general terms, the ligase enables movement of ubiquitin from a ubiquitin carrier to another thing (the substrate) by some mechanism. The ubiquitin, once it reaches its destination, ends up being attached by an isopeptide bond to a lysine residue, which is part of the target protein. [2] E3 ligases interact with both the target protein and the E2 enzyme, and so impart substrate specificity to the E2. Commonly, E3s polyubiquitinate their substrate with Lys48-linked chains of ubiquitin, targeting the substrate for destruction by the proteasome. However, many other types of linkages are possible and alter a protein's activity, interactions, or localization. Ubiquitination by E3 ligases regulates diverse areas such as cell trafficking, DNA repair, and signaling and is of profound importance in cell biology. E3 ligases are also key players in cell cycle control, mediating the degradation of cyclins, as well as cyclin dependent kinase inhibitor proteins. [3] The human genome encodes over 600 putative E3 ligases, allowing for tremendous diversity in substrates. [4]

Contents

Ubiquitination system

Schematic diagram of the ubiquitylation system. Ubiquitylation.png
Schematic diagram of the ubiquitylation system.

The ubiquitin ligase is referred to as an E3, and operates in conjunction with an E1 ubiquitin-activating enzyme and an E2 ubiquitin-conjugating enzyme. There is one major E1 enzyme, shared by all ubiquitin ligases, that uses ATP to activate ubiquitin for conjugation and transfers it to an E2 enzyme. The E2 enzyme interacts with a specific E3 partner and transfers the ubiquitin to the target protein. The E3, which may be a multi-protein complex, is, in general, responsible for targeting ubiquitination to specific substrate proteins.[ citation needed ]

The ubiquitylation reaction proceeds in three or four steps depending on the mechanism of action of the E3 ubiquitin ligase. In the conserved first step, an E1 cysteine residue attacks the ATP-activated C-terminal glycine on ubiquitin, resulting in a thioester Ub-S-E1 complex. The energy from ATP and diphosphate hydrolysis drives the formation of this reactive thioester, and subsequent steps are thermoneutral. Next, a transthiolation reaction occurs, in which an E2 cysteine residue attacks and replaces the E1. HECT domain type E3 ligases will have one more transthiolation reaction to transfer the ubiquitin molecule onto the E3, whereas the much more common RING finger domain type ligases transfer ubiquitin directly from E2 to the substrate. [5] The final step in the first ubiquitylation event is an attack from the target protein lysine amine group, which will remove the cysteine, and form a stable isopeptide bond. [6] One notable exception to this is p21 protein, which appears to be ubiquitylated using its N-terminal amine, thus forming a peptide bond with ubiquitin. [7]

Ubiquitin ligase families

Humans have an estimated 500-1000 E3 ligases, which impart substrate specificity onto the E1 and E2. [8] The E3 ligases are classified into four families: HECT, RING-finger, U-box, and PHD-finger. [8] The RING-finger E3 ligases are the largest family and contain ligases such as the anaphase-promoting complex (APC) and the SCF complex (Skp1-Cullin-F-box protein complex). SCF complexes consist of four proteins: Rbx1, Cul1, Skp1, which are invariant among SCF complexes, and an F-box protein, which varies. Around 70 human F-box proteins have been identified. [9] F-box proteins contain an F-box, which binds the rest of the SCF complex, and a substrate binding domain, which gives the E3 its substrate specificity. [8]

Mono- and poly-ubiquitylation

Ubiquitin with lysine residues (red), N-terminal methionine (blue), and C-terminal glycine (yellow). Ubiquitin Lysines.png
Ubiquitin with lysine residues (red), N-terminal methionine (blue), and C-terminal glycine (yellow).

Ubiquitin signaling relies on the diversity of ubiquitin tags for the specificity of its message. A protein can be tagged with a single ubiquitin molecule (monoubiquitylation), or variety of different chains of ubiquitin molecules (polyubiquitylation). [11] E3 ubiquitin ligases catalyze polyubiquitination events much in the same way as the single ubiquitylation mechanism, using instead a lysine residue from a ubiquitin molecule currently attached to substrate protein to attack the C-terminus of a new ubiquitin molecule. [6] [11] For example, a common 4-ubiquitin tag, linked through the lysine at position 48 (K48) recruits the tagged protein to the proteasome, and subsequent degradation. [11] However, all seven of the ubiquitin lysine residues (K6, K11, K27, K29, K33, K48, and K63), as well as the N-terminal methionine are used in chains in vivo. [11]

Monoubiquitination has been linked to membrane protein endocytosis pathways. For example, phosphorylation of the Tyrosine at position 1045 in the Epidermal Growth Factor Receptor (EGFR) can recruit the RING type E3 ligase c-Cbl, via an SH2 domain. C-Cbl monoubiquitylates EGFR, signaling for its internalization and trafficking to the lysosome. [12]

Monoubiquitination also can regulate cytosolic protein localization. For example, the E3 ligase MDM2 ubiquitylates p53 either for degradation (K48 polyubiquitin chain), or for nuclear export (monoubiquitylation). These events occur in a concentration dependent fashion, suggesting that modulating E3 ligase concentration is a cellular regulatory strategy for controlling protein homeostasis and localization. [13]

Substrate recognition

Ubiquitin ligases are the final, and potentially the most important determinant of substrate specificity in ubiquitination of proteins. [14] The ligases must simultaneously distinguish their protein substrate from thousands of other proteins in the cell, and from other (ubiquitination-inactive) forms of the same protein. This can be achieved by different mechanisms, most of which involve recognition of degrons: specific short amino acid sequences or chemical motifs on the substrate. [15]

N-degrons

Proteolytic cleavage can lead to exposure of residues at the N-terminus of a protein. According to the N-end rule, different N-terminal amino acids (or N-degrons) are recognized to a different extent by their appropriate ubiquitin ligase (N-recognin), influencing the half-life of the protein. [16] For instance, positively charged (Arg, Lys, His) and bulky hydrophobic amino acids (Phe, Trp, Tyr, Leu, Ile) are recognized preferentially and thus considered destabilizing degrons since they allow faster degradation of their proteins. [17]

Phosphodegrons

A phosphorylated degron (green) is stabilized by hydrogen bonding (yellow) between oxygen atoms of its phosphate (red) and side chains of the SCF ubiquitin ligase (blue). The relevant part of the ubiquitin ligase is shown in gray. PDB entry 2ovr Phosphodegron binding by ubiquitin ligase.png
A phosphorylated degron (green) is stabilized by hydrogen bonding (yellow) between oxygen atoms of its phosphate (red) and side chains of the SCF ubiquitin ligase (blue). The relevant part of the ubiquitin ligase is shown in gray. PDB entry 2ovr

A degron can be converted into its active form by a post-translational modification [19] such as phosphorylation of a tyrosine, serine or threonine residue. [20] In this case, the ubiquitin ligase exclusively recognizes the phosphorylated version of the substrate due to stabilization within the binding site. For example, FBW7, the F-box substrate recognition unit of an SCF FBW7ubiquitin ligase, stabilizes a phosphorylated substrate by hydrogen binding its arginine residues to the phosphate, as shown in the figure to the right. In absence of the phosphate, residues of FBW7 repel the substrate. [18]

Oxygen and small molecule dependent degrons

Presence of oxygen or other small molecules can influence degron recognition. [18] The von Hippel-Lindau (VHL) protein (substrate recognition part of a specific E3 ligase), for instance, recognizes the hypoxia-inducible factor alpha (HIF-α) only under normal oxygen conditions, when its proline is hydroxylated. Under hypoxia, on the other hand, HIF-a is not hydroxylated, evades ubiquitination and thus operates in the cell at higher concentrations which can initiate transcriptional response to hypoxia. [21] Another example of small molecule control of protein degradation is phytohormone auxin in plants. [22] Auxin binds to TIR1 (the substrate recognition domain of SCF TIR1ubiquitin ligase) increasing the affinity of TIR1 for its substrates (transcriptional repressors: Aux/IAA), and promoting their degradation.

Misfolded and sugar degrons

In addition to recognizing amino acids, ubiquitin ligases can also detect unusual features on substrates that serve as signals for their destruction. [14] For example, San1 (Sir antagonist 1), a nuclear protein quality control in yeast, has a disordered substrate binding domain, which allows it to bind to hydrophobic domains of misfolded proteins. [14] Misfolded or excess unassembled glycoproteins of the ERAD pathway, on the other hand, are recognized by Fbs1 and Fbs2, mammalian F-box proteins of E3 ligases SCF Fbs1and SCFFbs2. [23] These recognition domains have small hydrophobic pockets allowing them to bind high-mannose containing glycans.

Structural motifs

In addition to linear degrons, the E3 ligase can in some cases also recognize structural motifs on the substrate. [14] In this case, the 3D motif can allow the substrate to directly relate its biochemical function to ubiquitination. This relation can be demonstrated with TRF1 protein (regulator of human telomere length), which is recognized by its corresponding E3 ligase (FBXO4) via an intermolecular beta sheet interaction. TRF1 cannot be ubiquinated while telomere bound, likely because the same TRF1 domain that binds to its E3 ligase also binds to telomeres. [14]

Disease relevance

E3 ubiquitin ligases regulate homeostasis, cell cycle, and DNA repair pathways, and as a result, a number of these proteins are involved in a variety of cancers, including famously MDM2, BRCA1, and Von Hippel-Lindau tumor suppressor. [24] For example, a mutation of MDM2 has been found in stomach cancer, [25] renal cell carcinoma, [26] and liver cancer [27] (amongst others) to deregulate MDM2 concentrations by increasing its promoter’s affinity for the Sp1 transcription factor, causing increased transcription of MDM2 mRNA. [25] Several proteomics-based experimental techniques are available for identifying E3 ubiquitin ligase-substrate pairs, [28] such as proximity-dependent biotin identification (BioID), ubiquitin ligase-substrate trapping, and tandem ubiquitin-binding entities (TUBEs).

Examples

Individual E3 ubiquitin ligases

See also

Related Research Articles

<span class="mw-page-title-main">Proteasome</span> Protein complexes which degrade unnecessary or damaged proteins by proteolysis

Proteasomes are protein complexes which degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks peptide bonds. Enzymes that help such reactions are called proteases.

<span class="mw-page-title-main">Ubiquitin</span> Regulatory protein found in most eukaryotic tissues

Ubiquitin is a small regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ubiquitously. It was discovered in 1975 by Gideon Goldstein and further characterized throughout the late 1970s and 1980s. Four genes in the human genome code for ubiquitin: UBB, UBC, UBA52 and RPS27A.

<span class="mw-page-title-main">Anaphase-promoting complex</span> Cell-cycle regulatory complex

Anaphase-promoting complex is an E3 ubiquitin ligase that marks target cell cycle proteins for degradation by the 26S proteasome. The APC/C is a large complex of 11–13 subunit proteins, including a cullin (Apc2) and RING (Apc11) subunit much like SCF. Other parts of the APC/C have unknown functions but are highly conserved.

<span class="mw-page-title-main">Deubiquitinating enzyme</span>

Deubiquitinating enzymes (DUBs), also known as deubiquitinating peptidases, deubiquitinating isopeptidases, deubiquitinases, ubiquitin proteases, ubiquitin hydrolases, ubiquitin isopeptidases, are a large group of proteases that cleave ubiquitin from proteins. Ubiquitin is attached to proteins in order to regulate the degradation of proteins via the proteasome and lysosome; coordinate the cellular localisation of proteins; activate and inactivate proteins; and modulate protein-protein interactions. DUBs can reverse these effects by cleaving the peptide or isopeptide bond between ubiquitin and its substrate protein. In humans there are nearly 100 DUB genes, which can be classified into two main classes: cysteine proteases and metalloproteases. The cysteine proteases comprise ubiquitin-specific proteases (USPs), ubiquitin C-terminal hydrolases (UCHs), Machado-Josephin domain proteases (MJDs) and ovarian tumour proteases (OTU). The metalloprotease group contains only the Jab1/Mov34/Mpr1 Pad1 N-terminal+ (MPN+) (JAMM) domain proteases.

<span class="mw-page-title-main">UBE3A</span> Protein-coding gene in Homo sapiens

Ubiquitin-protein ligase E3A (UBE3A) also known as E6AP ubiquitin-protein ligase (E6AP) is an enzyme that in humans is encoded by the UBE3A gene. This enzyme is involved in targeting proteins for degradation within cells.

<span class="mw-page-title-main">Mdm2</span> Protein-coding gene in the species Homo sapiens

Mouse double minute 2 homolog (MDM2) also known as E3 ubiquitin-protein ligase Mdm2 is a protein that in humans is encoded by the MDM2 gene. Mdm2 is an important negative regulator of the p53 tumor suppressor. Mdm2 protein functions both as an E3 ubiquitin ligase that recognizes the N-terminal trans-activation domain (TAD) of the p53 tumor suppressor and as an inhibitor of p53 transcriptional activation.

<span class="mw-page-title-main">Endoplasmic-reticulum-associated protein degradation</span>

Endoplasmic-reticulum-associated protein degradation (ERAD) designates a cellular pathway which targets misfolded proteins of the endoplasmic reticulum for ubiquitination and subsequent degradation by a protein-degrading complex, called the proteasome.

<span class="mw-page-title-main">SCF complex</span>

Skp, Cullin, F-box containing complex is a multi-protein E3 ubiquitin ligase complex that catalyzes the ubiquitination of proteins destined for 26S proteasomal degradation. Along with the anaphase-promoting complex, SCF has important roles in the ubiquitination of proteins involved in the cell cycle. The SCF complex also marks various other cellular proteins for destruction.

<span class="mw-page-title-main">Ubiquitin-activating enzyme</span> Class of enzymes

Ubiquitin-activating enzymes, also known as E1 enzymes, catalyze the first step in the ubiquitination reaction, which can target a protein for degradation via a proteasome. This covalent bond of ubiquitin or ubiquitin-like proteins to targeted proteins is a major mechanism for regulating protein function in eukaryotic organisms. Many processes such as cell division, immune responses and embryonic development are also regulated by post-translational modification by ubiquitin and ubiquitin-like proteins.

Ubiquitin-conjugating enzymes, also known as E2 enzymes and more rarely as ubiquitin-carrier enzymes, perform the second step in the ubiquitination reaction that targets a protein for degradation via the proteasome. The ubiquitination process covalently attaches ubiquitin, a short protein of 76 amino acids, to a lysine residue on the target protein. Once a protein has been tagged with one ubiquitin molecule, additional rounds of ubiquitination form a polyubiquitin chain that is recognized by the proteasome's 19S regulatory particle, triggering the ATP-dependent unfolding of the target protein that allows passage into the proteasome's 20S core particle, where proteases degrade the target into short peptide fragments for recycling by the cell.

<span class="mw-page-title-main">CUL4A</span> Protein-coding gene in the species Homo sapiens

Cullin-4A is a protein that in humans is encoded by the CUL4A gene. CUL4A belongs to the cullin family of ubiquitin ligase proteins and is highly homologous to the CUL4B protein. CUL4A regulates numerous key processes such as DNA repair, chromatin remodeling, spermatogenesis, haematopoiesis and the mitotic cell cycle. As a result, CUL4A has been implicated in several cancers and the pathogenesis of certain viruses including HIV. A component of a CUL4A complex, Cereblon, was discovered to be a major target of the teratogenic agent thalidomide.

<span class="mw-page-title-main">UBE2L3</span> Protein-coding gene in humans

Ubiquitin-conjugating enzyme E2 L3 (UBE2L3), also called UBCH7, is a protein that in humans is encoded by the UBE2L3 gene. As an E2 enzyme, UBE2L3 participates in ubiquitination to target proteins for degradation. The role of UBE2L3 in the ubiquitination of the NF-κB precursor implicated it in various major autoimmune diseases, including rheumatoid arthritis (RA), celiac disease, Crohn's disease (CD), and systemic lupus erythematosus.

<span class="mw-page-title-main">CDC34</span> Protein-coding gene in the species Homo sapiens

CDC34 is a gene that in humans encodes the protein Ubiquitin-conjugating enzyme E2 R1. This protein is a member of the ubiquitin-conjugating enzyme family, which catalyzes the covalent attachment of ubiquitin to other proteins.

<span class="mw-page-title-main">Ubiquitin D</span> Protein-coding gene in the species Homo sapiens

Ubiquitin D is a protein that in humans is encoded by the UBD gene, also known as FAT10. UBD acts like ubiquitin, by covalently modifying proteins and tagging them for destruction in the proteasome.

<span class="mw-page-title-main">UBE2D1</span> Protein-coding gene in the species Homo sapiens

Ubiquitin-conjugating enzyme E2 D1 is a protein that in humans is encoded by the UBE2D1 gene.

<span class="mw-page-title-main">CUL3</span> Protein encoded by the CUL3 gene in humans

Cullin 3 is a protein that in humans is encoded by the CUL3 gene.

<span class="mw-page-title-main">Cullin</span> Hydrophobic scaffold protein

Cullins are a family of hydrophobic scaffold proteins which provide support for ubiquitin ligases (E3). All eukaryotes appear to have cullins. They combine with RING proteins to form Cullin-RING ubiquitin ligases (CRLs) that are highly diverse and play a role in myriad cellular processes, most notably protein degradation by ubiquitination.

<span class="mw-page-title-main">Cell division control protein 4</span>

Cdc4 is a substrate recognition component of the SCF ubiquitin ligase complex, which acts as a mediator of ubiquitin transfer to target proteins, leading to their subsequent degradation via the ubiquitin-proteasome pathway. Cdc4 targets primarily cell cycle regulators for proteolysis. It serves the function of an adaptor that brings target molecules to the core SCF complex. Cdc4 was originally identified in the model organism Saccharomyces cerevisiae. CDC4 gene function is required at G1/S and G2/M transitions during mitosis and at various stages during meiosis.

<span class="mw-page-title-main">HECT domain</span>

In molecular biology, the HECT domain is a protein domain found in ubiquitin-protein ligases. The name HECT comes from 'Homologous to the E6-AP Carboxyl Terminus'. Proteins containing this domain at the C terminus include ubiquitin-protein ligase, which regulates ubiquitination of CDC25. Ubiquitin-protein ligase accepts ubiquitin from an E2 ubiquitin-conjugating enzyme in the form of a thioester, and then directly transfers the ubiquitin to targeted substrates. A cysteine residue is required for ubiquitin-thiolester formation. Human thyroid receptor interacting protein 12 (TRIP12), which also contains this domain, is a component of an ATP-dependent multisubunit protein that interacts with the ligand binding domain of the thyroid hormone receptor. It could be an E3 ubiquitin-protein ligase. Human E6AP ubiquitin-protein ligase interacts with the E6 protein of the cancer-associated Human papillomavirus type 16 and Human papillomavirus type 18. The E6/E6-AP complex binds to and targets the p53 tumour-suppressor protein for ubiquitin-mediated proteolysis.

A proteolysis targeting chimera (PROTAC) is a heterobifunctional molecule composed of two active domains and a linker, capable of removing specific unwanted proteins. Rather than acting as a conventional enzyme inhibitor, a PROTAC works by inducing selective intracellular proteolysis. PROTACs consist of two covalently linked protein-binding molecules: one capable of engaging an E3 ubiquitin ligase, and another that binds to a target protein meant for degradation. Recruitment of the E3 ligase to the target protein results in ubiquitination and subsequent degradation of the target protein via the proteasome. Because PROTACs need only to bind their targets with high selectivity, there are currently many efforts to retool previously ineffective inhibitor molecules as PROTACs for next-generation drugs.

References

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