泛素化和磷酸化协同作用调控蛋白质降解

在真核细胞中,泛素化和磷酸化是2种常见的蛋白质修饰方式。泛素在蛋白酶体降解途径中发挥重要的靶向作用,细胞外信号严格调控着目的蛋白的泛素化。在很多情况下,这种调控依赖于蛋白质的磷酸化。由磷酸化影响的调控步骤可能与E3泛素连接酶对底物的识别有关,也可能与实际的交联反应有关。这种调控是通过对底物或E3连接酶本身的磷酸化实现的。     

ISG15 and immune diseases

ISG15, the product of interferon (IFN)-stimulated gene 15, is the first identified ubiquitin-like protein, consisting of two ubiquitin-like domains. ISG15 is synthesized as a precursor in certain mammals and, therefore, needs to be processed to expose the C-terminal glycine residue before conjugation to target proteins. A set of three-step cascade enzymes, an E1 enzyme (UBE1L), an E2 enzyme (UbcH8), and one of several E3 ligases (e.g., EFP and HERC5), catalyzes ISG15 conjugation (ISGylation) of a specific protein. These enzymes are unique among the cascade enzymes for ubiquitin and other ubiquitin-like proteins in that all of them are induced by type I IFNs or other stimuli, such as exposure to viruses and lipopolysaccharide. Mass spectrometric analysis has led to the identification of several hundreds of candidate proteins that can be conjugated by ISG15. Some of them are type I IFN-induced proteins, such as PKR and RIG-I, and some are the key regulators that are involved in IFN signaling, such as JAK1 and STAT1, implicating the role of ISG15 and its conjugates in type I IFN-mediated innate immune responses. However, relatively little is known about the functional significance of ISG15 induction due to the lack of information on the consequences of its conjugation to target proteins. Here, we describe the recent progress made in exploring the biological function of ISG15 and its reversible modification of target proteins and thus in their implication in immune diseases.     

Role of an N-terminal site of Ubc9 in SUMO-1, -2, and -3 binding and conjugation

Covalent posttranslational modification of target proteins with ubiquitin and ubiquitin-like proteins regulates many important cellular processes. However, the molecular mechanisms by which these proteins are activated and conjugated to substrates has yet to be fully understood. NMR studies have shown that the ubiquitin-like proteins SUMO-1, -2, and -3 interact with the same N-terminal region of the E2 conjugating enzyme Ubc9 with similar affinities. This is correlated to their almost identical utilization by Ubc9 in the SUMO conjugation pathway. To investigate the functional significance of this interaction, site-directed mutagenesis was used to alter residues in the SUMO binding surface of Ubc9, and the effect of the amino acid substitutions on binding and conjugation to SUMO-1 and target protein RanGAP1 was investigated by isothermal titration calorimetry and biochemical analysis. R13A/K14A and R17A/K18A mutations in Ubc9 disrupted the interaction with SUMO-1 but did not completely abolish the interaction with E1. While these Ubc9 mutants displayed a significantly reduced efficiency in the transfer of SUMO-1 from E1 to E2, their ability to recognize substrate and transfer SUMO-1 from E2 to the target protein was unaffected. These results suggest that the noncovalent binding site of SUMO-1 on Ubc9, although distant from the active site, is important for the transfer of SUMO-1 from the E1 to the E2. The conservation of E2 enzymes across the ubiquitin and ubiquitin-like protein pathways indicates that analogous N-terminal sites of E2 enzymes are likely to have similar roles in general.     

NEDD8 overexpression results in neddylation of ubiquitin substrates by the ubiquitin pathway

Ubiquitin and ubiquitin-like proteins use unique E1, E2, and E3 enzymes for conjugation to their substrates. We and others have recently reported that increases in the relative concentration of the ubiquitin-like protein NEDD8 over ubiquitin lead to activation of NEDD8 by the ubiquitin E1 enzyme. We now show that this results in erroneous conjugation of NEDD8 to ubiquitin substrates, such as p53, Caspase 7, and Hif1α, demonstrating that overexpression of NEDD8 is not appropriate for identification of substrates of the NEDD8 pathway.     

Identification of a substrate recognition site on Ubc9

Human Ubc9 is homologous to ubiquitin-conjugating enzymes. However, instead of conjugating ubiquitin, it conjugates a ubiquitin homologue, small ubiquitin-like modifier 1 (SUMO-1), also known as UBL1, GMP1, SMTP3, PIC1, and sentrin. The SUMO-1 conjugation pathway is very similar to that of ubiquitin with regard to the primary sequences of the ubiquitin-activating enzymes (E1), the three-dimensional structures of the ubiquitin-conjugating enzymes (E2), and the chemistry of the overall conjugation pathway. The interaction of substrates with Ubc9 has been studied using NMR spectroscopy. Peptides with sequences that correspond to those of the SUMO-1 conjugation sites from p53 and c-Jun both bind to a surface adjacent to the active site Cys93 of human Ubc9, which has been previously shown to include residues that demonstrate the most significant dynamics on the microsecond to millisecond time scale. Mutations in this region, Q126A, Q130A, A131D, E132A, Y134A, and T135A, were constructed to evaluate the role of these residues in SUMO-1 conjugation. These alterations have significant effects on the conjugation of SUMO-1 with the target proteins p53, E1B, and promyelocytic leukemia protein and define a substrate binding site on Ubc9. Furthermore, the SUMO-1 conjugation site of p53 does not form any defined secondary structure when either free or bound to Ubc9. This suggests that a defined secondary structure at SUMO-1 conjugation sites in target proteins is not necessary for recognition and conjugation by the SUMO-1 pathway.     

Ubiquitin and Ubiquitin-like Modifiers in Plants

Posttranslational modifications of proteins by small polypeptides including ubiquitination, neddylation (related to ubiquitin (RUB) conjugation), and sumoylation are implicated in plant growth and development, and they regulate protein degradation, location, and interaction with other proteins. Ubiquitination mediates the selective degradation of proteins by the ubiquitin (Ub)/proteasome pathway. The ubiquitin-like protein RUB is conjugated to cullins, which are part of a ubiquitin E3 ligase complex that is involved in auxin hormonal signaling. Sumoylation, by contrast, is known for its involvement in guiding protein interactions related to abiotic and biotic stresses and in the regulation of flowering time. ATG8/ATG12-mediated autophagy influences degradation and recycling of cellular components. Other ubiquitin-like modifiers (ULPs) such as homology to Ub-1, ubiquitin-fold modifier 1, and membrane-anchored Ub-fold are also found in Arabidopsis. ULPs share similar three-dimensional structures and a conjugation system, including E1 activating enzymes, E2 conjugation enzymes, and E3 ligases, as well as proteases for deconjugation and recycling of the tags. However, each of the ULP posttranslational modifications possesses its own specific enzymes and modifies its specific targets selectively. This review discusses recent findings on ubiquitination and ubiquitin-like modifier processes and their roles in the posttranslational modification of proteins in Arabidopsis.     

Immunity by ubiquitylation: a reversible process of modification

The conjugation of ubiquitin, a 76-amino-acid peptide, to a protein substrate provides a tag that either marks the labelled protein for degradation or modulates its function. The process of protein ubiquitylation--which is catalysed by coordinated enzymatic reactions that are mediated by enzymes known as E1, E2 and E3--has an important role in the modulation of immune responses. Importantly, protein ubiquitylation is a reversible process, and removal of ubiquitin molecules is mediated by de-ubiquitylating enzymes: for example, A20, which has been implicated in the regulation of immune responses. In addition, the conjugation of ubiquitin-like molecules, such as ISG15 (interferon-stimulated protein of 15 kDa), to proteins is also involved in immune regulation. This Review covers recent progress in our understanding of protein ubiquitylation in the immune system.     

Protein-protein interactions regulate Ubl conjugation

The ubiquitin-like proteins (Ubls) can be covalently linked to target proteins to provide a critical signal in diverse cellular processes. Members of the Ubl family include ubiquitin itself and a growing number of homologs such as SUMO, Nedd8, ISG15 and Atg8. The enzymatic mechanism of Ubl conjugation involves an E1, E2, E3 cascade of enzymes that is well conserved between the Ubls. In the past two years, novel structural details of Ubl conjugation were uncovered through analysis of protein-protein complexes. This has given insight in activation of E1, the role of the target lysine in E2-dependent catalysis, the role of noncovalent Ubl binding in Ubl chain formation and the importance of dimerization of Ring-type E3 ligases.     

ISG15, not just another ubiquitin-like protein

ISG15 is a ubiquitin-like protein containing two ubiquitin homology domains and becomes conjugated to a variety of proteins when cells are treated with type I interferon or lipopolysaccharide. Although ISG15 shares several common properties with those of other ubiquitin-like molecules, it is a unique member, whose expression and conjugation to target proteins are tightly regulated by specific signaling pathways, indicating it may be associated with specialized functions in innate immune system. Loss of UBP43 (USP18), a protease that specifically removes ISG15 from ISG15-modified proteins, in mice leads to decreased life span, brain cell injury, and hypersensitivity to interferon stimulation. In UBP43 deficient cells, interferon induces a prolonged Stat1 tyrosine phosphorylation and DNA binding, which result in a prolonged and enhanced activation of interferon-stimulated genes.     

ISG15 modification of Ubc13 suppresses its ubiquitin-conjugating activity

ISG15 is one of the interferon-stimulated genes and is classified as a ubiquitin-like protein. Upon interferon stimuli, ISG15 is upregulated and becomes conjugated to various cellular proteins (ISGylation). Several target proteins for ISGylation have recently been identified, but the biological consequence of protein ISGylation remains unclear. In the course of our study to identify components of the ISGylation system, we found that Ubc13, an E2 enzyme for ubiquitin conjugation, is covalently modified with ISG15. To determine the meaning of ISGylation of Ubc13, we isolated ISG15-modified Ubc13 protein and compared its ubiquitin-conjugating activity with that of an unmodified one. We found that ISGylation of Ubc13 suppresses its ability to form a thioester intermediate with ubiquitin.     

Detection of modification by ubiquitin-like proteins

Ubiquitin and ubiquitin-like proteins (Ubls) are conjugated to many target proteins either as monomeric units or as polymeric chains. There are at least 12 members of the ubiquitin family in the human genome and their conjugation dramatically alters the properties of the modified protein. The presence of highly active proteases that specifically deconjugate Ubls often means that, in the cell, the steady state level of modified protein is low. Detection of protein species modified by Ubls can therefore represent a significant challenge. Here, we describe methods that have been developed to allow detection of Ubl modified proteins both in vivo and in vitro.     

Changes in the ratio of free NEDD8 to ubiquitin triggers NEDDylation by ubiquitin enzymes

Ubiquitin and UBL (ubiquitin-like) modifiers are small proteins that covalently modify other proteins to alter their properties or behaviours. Ubiquitin modification (ubiquitylation) targets many substrates, often leading to their proteasomal degradation. NEDD8 (neural-precursor-cell-expressed developmentally down-regulated 8) is the UBL most closely related to ubiquitin, and its best-studied role is the activation of CRLs (cullin-RING ubiquitin ligases) by its conjugation to a conserved C-terminal lysine residue on cullin proteins. The attachment of UBLs requires three UBL-specific enzymes, termed E1, E2 and E3, which are usually well insulated from parallel UBL pathways. In the present study, we report a new mode of NEDD8 conjugation (NEDDylation) whereby the UBL NEDD8 is linked to proteins by ubiquitin enzymes in vivo. We found that this atypical NEDDylation is independent of classical NEDD8 enzymes, conserved from yeast to mammals, and triggered by an increase in the NEDD8 to ubiquitin ratio. In cells, NEDD8 overexpression leads to this type of NEDDylation by increasing the concentration of NEDD8, whereas proteasome inhibition has the same effect by depleting free ubiquitin. We show that bortezomib, a proteasome inhibitor used in cancer therapy, triggers atypical NEDDylation in tissue culture, which suggests that a similar process may occur in patients receiving this treatment.     

Link between the ubiquitin conjugation system and the ISG15 conjugation system: ISG15 conjugation to the UbcH6 ubiquitin E2 enzyme

ISG15 is a ubiquitin-like protein that is upregulated on treatment with interferon. ISG15 is considered to be covalently conjugated to cellular proteins through a sequential reaction similar to that of the ubiquitin conjugation system consisting of E1/E2/E3 enzymes: UBE1L and UbcH8 have been reported to function as E1 and E2 enzymes, respectively, for ISG15 conjugation. Several cellular proteins have been identified as targets for ISG15 conjugation, but the roles of ISG15 conjugation remain unclear. In this study, we found that UbcH6 and UbcH8, E2 enzymes for ubiquitin conjugation, are covalently modified by ISG15. We also found that UbcH6 is capable of forming a thioester intermediate with ISG15 through Cys131. We determined that the Lys136 residue near the catalytic site Cys131 is the ISG15 conjugation site in UbcH6. We isolated ISG15-modified and unmodified UbcH6 proteins, and analyzed their abilities to form thioester intermediates with ubiquitin. A ubiquitin thioester intermediate was detected in the case of unmodified UbcH6, but not in that of ISG15-modified UbcH6, strongly suggesting that ISG15 conjugation to UbcH6 suppresses its ubiquitin E2 enzyme activity. Thus, we provide evidence for a link between the ubiquitin conjugation system and the ISG15 conjugation system.     

A novel protein modification pathway related to the ubiquitin system.

Ubiquitin conjugation is known to target protein substrates primarily to degradation by the proteasome or via the endocytic route. Here we describe a novel protein modification pathway in yeast which mediates the conjugation of RUB1, a ubiquitin-like protein displaying 53% amino acid identity to ubiquitin. We show that RUB1 conjugation requires at least three proteins in vivo. ULA1 and UBA3 are related to the N- and C-terminal domains of the E1 ubiquitin-activating enzyme, respectively, and together fulfil E1-like functions for RUB1 activation. RUB1 conjugation also requires UBC12, a protein related to E2 ubiquitin-conjugating enzymes, which functions analogously to E2 enzymes in RUB1-protein conjugate formation. Conjugation of RUB1 is not essential for normal cell growth and appears to be selective for a small set of substrates. Remarkably, CDC53/cullin, a common subunit of the multifunctional SCF ubiquitin ligase, was found to be a major substrate for RUB1 conjugation. This suggests that the RUB1 conjugation pathway is functionally affiliated to the ubiquitin-proteasome system and may play a regulatory role.     

The ubiquitin E1 enzyme Ube1 mediates NEDD8 activation under diverse stress conditions

Modification of proteins with ubiquitin and ubiquitin-like molecules is involved in the regulation of almost every biological process. Historically, each conjugation pathway has its unique set of E1, E2 and E3 enzymes that lead to activation and conjugation of their cognate molecules. Here, we present the unexpected finding that under stress conditions, the ubiquitin E1 enzyme Ube1 mediates conjugation of the ubiquitin-like molecule NEDD8. Inhibition of the 26S proteasome, heat shock and oxidative stress cause a global increase in NEDDylation. Surprisingly, this does not depend on the NEDD8 E1-activating enzyme, but rather on Ube1. A common event in the tested stress conditions is the depletion of “free” ubiquitin. A decrease in “free” ubiquitin levels in the absence of additional stress is sufficient to stimulate NEDDylation through Ube1. Further analysis on the NEDD8 proteome shows that the modified NEDDylated proteins are simultaneously ubiquitinated. Mass spectrometry on the complex proteome under stress reveals the existence of mixed chains between NEDD8 and ubiquitin. We further show that NEDDylation of the p53 tumor suppressor upon stress is mediated mainly through Ube1. Our studies reveal an unprecedented interplay between NEDD8 and ubiquitin pathways operating in diverse cellular stress conditions.     

Structural basis for E2-mediated SUMO conjugation revealed by a complex between ubiquitin-conjugating enzyme Ubc9 and RanGAP1

E2 enzymes catalyze attachment of ubiquitin and ubiquitin-like proteins to lysine residues directly or through E3-mediated reactions. The small ubiquitin-like modifier SUMO regulates nuclear transport, stress response, and signal transduction in eukaryotes and is essential for cell-cycle progression in yeast. In contrast to most ubiquitin conjugation, the SUMO E2 enzyme Ubc9 is sufficient for substrate recognition and lysine modification of known SUMO targets. Crystallographic analysis of a complex between mammalian Ubc9 and a C-terminal domain of RanGAP1 at 2.5 A reveals structural determinants for recognition of consensus SUMO modification sequences found within SUMO-conjugated proteins. Structure-based mutagenesis and biochemical analysis of Ubc9 and RanGAP1 reveal distinct motifs required for substrate binding and SUMO modification of p53, IkappaBalpha, and RanGAP1.     

Role of two residues proximal to the active site of Ubc9 in substrate recognition by the Ubc9.SUMO-1 thiolester complex

The small ubiquitin-like modifier SUMO-1 is covalently attached to lysine residues on target proteins by a specific conjugation pathway involving the E1 enzyme SAE1/SAE2 and the E2 enzyme Ubc9. In an ATP-dependent manner, the C-terminus of SUMO-1 forms consecutive thiolester bonds with cysteine residues in the SAE2 subunit and Ubc9, before the Ubc9.SUMO-1 thiolester complex catalyzes the formation of an isopeptide bond between SUMO-1 and the epsilon-amino group of the target lysine residue on the protein substrate. The SUMO-1 conjugation pathway bears many similarities with that of ubiquitin and other ubiquitin-like protein modifiers (Ubls), and because of its production of a singly conjugated substrate and the lack of absolute requirement in vitro for E3 enzymes, the SUMO-1/Ubc9 system is a good model for the analysis of protein conjugation pathways that share this basic chemistry. Here we describe methods of both steady-state and half-reaction kinetic analysis of Ubc9, and use these techniques to determine the role of two residues, Asp(100) and Lys(101) of Ubc9 which are not found in E2 enzymes from other protein conjugation pathways. These residues are found close to the active site Cys in the tertiary structure of Ubc9, and although they are shown to inhibit the transesterification reaction from SAE1/SAE2, they are important for substrate recognition in the context of the thiolester complex with SUMO-1.