Characterization of the DOC1/APC10 subunit of the yeast and the human anaphase-promoting complex.

The anaphase-promoting complex/cyclosome (APC) is a ubiquitin-protein ligase whose activity is essential for progression through mitosis. The vertebrate APC is thought to be composed of 8 subunits, whereas in budding yeast several additional APC-associated proteins have been identified, including a 33-kDa protein called Doc1 or Apc10. Here, we show that Doc1/Apc10 is a subunit of the yeast APC throughout the cell cycle. Mutation of Doc1/Apc10 inactivates the APC without destabilizing the complex. An ortholog of Doc1/Apc10, which we call APC10, is associated with the APC in different vertebrates, including humans and frogs. Biochemical fractionation experiments and mass spectrometric analysis of a component of the purified human APC show that APC10 is a genuine APC subunit whose cellular levels or association with the APC are not cell cycle-regulated. We have further identified an APC10 homology region, which we propose to call the DOC domain, in several protein sequences that also contain either cullin or HECT domains. Cullins are present in several ubiquitination complexes including the APC, whereas HECT domains represent the catalytic core of a different type of ubiquitin-protein ligase. DOC domains may therefore be important for reactions catalyzed by several types of ubiquitin-protein ligases.     

Crystal structure of the APC10/DOC1 subunit of the human anaphase-promoting complex

The anaphase-promoting complex (APC), or cyclosome, is a cell cycle-regulated ubiquitin ligase that controls progression through mitosis and the G1 phase of the cell cycle. The APC is composed of at least 11 subunits; no structure has been determined for any of these subunits. The subunit APC10/DOC1, a one-domain protein consisting of 185 amino acids, has a conserved core (residues 22-161) that is homologous to domains found in several other putative ubiquitin ligases and, therefore, may play a role in ubiquitination reactions. Here we report the crystal structure of human APC10 at 1.6 A resolution. The core of the protein is formed by a beta-sandwich that adopts a jellyroll fold. Unexpectedly, this structure is highly similar to ligand-binding domains of several bacterial and eukaryotic proteins, such as galactose oxidase and coagulation factor Va, raising the possibility that APC10 may function by binding a yet unidentified ligand. We further provide biochemical evidence that the C-terminus of APC10 binds to CDC27/APC3, an APC subunit that contains multiple tetratrico peptide repeats.     

Doc1 mediates the activity of the anaphase-promoting complex by contributing to substrate recognition

The anaphase-promoting complex (APC) is a multisubunit E3 ubiquitin ligase that targets specific cell cycle-related proteins for degradation, regulating progression from metaphase to anaphase and exit from mitosis. The APC is regulated by binding of the coactivator proteins Cdc20p and Cdh1p, and by phosphorylation. We have developed a purification strategy that allowed us to purify the budding yeast APC to near homogeneity and identify two novel APC-associated proteins, Swm1p and Mnd2p. Using an in vitro ubiquitylation system and a native gel binding assay, we have characterized the properties of wild-type and mutant APC. We show that both the D and KEN boxes contribute to substrate recognition and that coactivator is required for substrate binding. APC lacking Apc9p or Doc1p/Apc10 have impaired E3 ligase activities. However, whereas Apc9p is required for structural stability and the incorporation of Cdc27p into the APC complex, Doc1p/Apc10 plays a specific role in substrate recognition by APC-coactivator complexes. These results imply that Doc1p/Apc10 may play a role to regulate the binding of specific substrates, similar to that of the coactivators.     

The Doc1 subunit is a processivity factor for the anaphase-promoting complex

Ubiquitin-mediated proteolysis of securin and mitotic cyclins is essential for exit from mitosis. The final step in ubiquitination of these and other proteins is catalysed by the anaphase-promoting complex (APC), a multi-subunit ubiquitin-protein ligase (E3). Little is known about the molecular reaction resulting in APC-dependent substrate ubiquitination or the role of individual APC subunits in the reaction. Using a well-defined in vitro system, we show that highly purified APC from Saccharomyces cerevisiae ubiquitinates a model cyclin substrate in a processive manner. Analysis of mutant APC lacking the Doc1/Apc10 subunit (APC(doc1 Delta)) indicates that Doc1 is required for processivity. The specific molecular defect in APC(doc1 Delta) is identified by a large increase in apparent K(M) for the cyclin substrate relative to the wild-type enzyme. This suggests that Doc1 stimulates processivity by limiting substrate dissociation. Addition of recombinant Doc1 to APC(doc1 Delta) fully restores enzyme function. Doc1-related domains are found in mechanistically distinct ubiquitin-ligase enzymes and may generally stimulate ubiquitination by contributing to substrate-enzyme affinity.     

The APC11 RING-H2 finger mediates E2-dependent ubiquitination

Polyubiquitination marks proteins for degradation by the 26S proteasome and is carried out by a cascade of enzymes that includes ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s), and ubiquitin ligases (E3s). The anaphase-promoting complex or cyclosome (APC/C) comprises a multisubunit ubiquitin ligase that mediates mitotic progression. Here, we provide evidence that the Saccharomyces cerevisiae RING-H2 finger protein Apc11 defines the minimal ubiquitin ligase activity of the APC. We found that the integrity of the Apc11p RING-H2 finger was essential for budding yeast cell viability, Using purified, recombinant proteins we showed that Apc11p interacted directly with the Ubc4 ubiquitin conjugating enzyme (E2). Furthermore, purified Apc11p was capable of mediating E1- and E2-dependent ubiquitination of protein substrates, including Clb2p, in vitro. The ability of Apc11p to act as an E3 was dependent on the integrity of the RING-H2 finger, but did not require the presence of the cullin-like APC subunit Apc2p. We suggest that Apc11p is responsible for recruiting E2s to the APC and for mediating the subsequent transfer of ubiquitin to APC substrates in vivo.     

APC2 Cullin Protein and APC11 RING Protein Comprise the Minimal Ubiquitin Ligase Module of the Anaphase-promoting Complex

In mitosis, the anaphase-promoting complex (APC) regulates the onset of sister-chromatid separation and exit from mitosis by mediating the ubiquitination and degradation of the securin protein and mitotic cyclins. With the use of a baculoviral expression system, we have reconstituted the ubiquitin ligase activity of human APC. In combination with Ubc4 or UbcH10, a heterodimeric complex of APC2 and APC11 is sufficient to catalyze the ubiquitination of human securin and cyclin B1. However, the minimal APC2/11 ubiquitin ligase module does not possess substrate specificity, because it also ubiquitinates the destruction box deletion mutants of securin and cyclin B1. Both APC11 and UbcH10 bind to the C-terminal cullin homology domain of APC2, whereas Ubc4 interacts with APC11 directly. Zn(2+)-binding and mutagenesis experiments indicate that APC11 binds Zn(2+) at a 1:3 M ratio. Unlike the two Zn(2+) ions of the canonical RING-finger motif, the third Zn(2+) ion of APC11 is not essential for its ligase activity. Surprisingly, with Ubc4 as the E2 enzyme, Zn(2+) ions alone are sufficient to catalyze the ubiquitination of cyclin B1. Therefore, the Zn(2+) ions of the RING finger family of ubiquitin ligases may be directly involved in catalysis.     

Apc10 and Ste9/Srw1, two regulators of the APC-cyclosome, as well as the CDK inhibitor Rum1 are required for G1 cell-cycle arrest in fission yeast.

Many eukaryotic cells arrest the cell cycle at G1 phase upon nutrient deprivation. In fission yeast, during nitrogen starvation, cells divide twice and arrest at G1. We have isolated a novel type of sterile mutant, which undergoes one additional S phase upon starvation and, as a result, arrests at G2. Three loci (apc10, ste9/srw1 and rum1) were identified. The apc10 mutants, previously unidentified, show, in addition to sterility, temperature-sensitive growth with defects in chromosome segregation. apc10(+) is essential for viability, encodes a conserved protein (a homologue of budding yeast Apc10/Doc1) and is required for ubiquitination and degradation of mitotic B-type cyclins. Apc10 does not co-sediment with the 20S APC-cyclosome, a ubiquitin ligase for B-type cyclins, and in the apc10 mutant the 20S complex is intact, suggesting that it is a novel regulator for this complex. A subpopulation of Apc10 does co-immunoprecipitate with the anaphase-promoting complex (APC). A second gene, ste9(+)/srw1(+), encodes a member of the fizzy-related family, also regulators of the APC. Finally, Rum1 is a cyclin-dependent kinase (CDK) inhibitor which exists only in G1. The results suggest that dual downregulation of CDK, one via the APC and the other via the CDK inhibitor, is a universal mechanism that is used to arrest cell cycle progression at G1.     

Structure of the HHARI Catalytic Domain Shows Glimpses of a HECT E3 Ligase

The ubiquitin-signaling pathway utilizes E1 activating, E2 conjugating, and E3 ligase enzymes to sequentially transfer the small modifier protein ubiquitin to a substrate protein. During the last step of this cascade different types of E3 ligases either act as scaffolds to recruit an E2 enzyme and substrate (RING), or form an ubiquitin-thioester intermediate prior to transferring ubiquitin to a substrate (HECT). The RING-inBetweenRING-RING (RBR) proteins constitute a unique group of E3 ubiquitin ligases that includes the Human Homologue of Drosophila Ariadne (HHARI). These E3 ligases are proposed to use a hybrid RING/HECT mechanism whereby the enzyme uses facets of both the RING and HECT enzymes to transfer ubiquitin to a substrate. We now present the solution structure of the HHARI RING2 domain, the key portion of this E3 ligase required for the RING/HECT hybrid mechanism. The structure shows the domain possesses two Zn²⁺-binding sites and a single exposed cysteine used for ubiquitin catalysis. A structural comparison of the RING2 domain with the HECT E3 ligase NEDD4 reveals a near mirror image of the cysteine and histidine residues in the catalytic site. Further, a tandem pair of aromatic residues exists near the C-terminus of the HHARI RING2 domain that is conserved in other RBR E3 ligases. One of these aromatic residues is remotely located from the catalytic site that is reminiscent of the location found in HECT E3 enzymes where it is used for ubiquitin catalysis. These observations provide an initial structural rationale for the RING/HECT hybrid mechanism for ubiquitination used by the RBR E3 ligases.     

Role of the RING-CH Domain of Viral Ligase mK3 in Ubiquitination of Non-Lysine and Lysine MHC I Residues

A plethora of ubiquitin ligases determine the intracellular location and fate of numerous proteins in a substrate-specific manner. However, the mechanisms for these functions are incompletely understood. Most ligases have structurally related RING domains that are critical for ligase activity including the recruitment of ubiquitin conjugating enzymes. Here we probe the function of the RING-CH domain of murine γ-herpesvirus-68 ligase mK3 that functions as an immune evasin by targeting major histocompatibility complex (MHC) class I heavy chains for endoplasmic reticulum-associated degradation (ERAD). Interestingly, mK3 mediates ubiquitin conjugation via ester bonds to S or T residues in addition to conventional isopeptide linkages to K residues. To determine the mechanism of non-K ubiquitination of substrates, we introduced into an mK3 background the RING-CH domains of related viral and cellular MARCH ( m embrane a ssociated R ING- CH ) ligases. We found that although a conserved W present in all viral RING-CH domains is critical for mK3 function, sequences outside the RING-CH domain determine whether and which non-lysine substrate residues can be ubiquitinated by mK3. Our findings support the model that viral ligases have evolved a highly effective strategy to optimally orient their RING domain with substrate allowing them to ubiquitinate non-K residues.     

Regulation of Nedd4-2 self-ubiquitination and stability by a PY motif located within its HECT-domain

Ubiquitin ligases play a pivotal role in substrate recognition and ubiquitin transfer, yet little is known about the regulation of their catalytic activity. Nedd4 (neural-precursor-cell-expressed, developmentally down-regulated 4)-2 is an E3 ubiquitin ligase composed of a C2 domain, four WW domains (protein-protein interaction domains containing two conserved tryptophan residues) that bind PY motifs (L/PPXY) and a ubiquitin ligase HECT (homologous with E6-associated protein C-terminus) domain. In the present paper we show that the WW domains of Nedd4-2 bind (weakly) to a PY motif (LPXY) located within its own HECT domain and inhibit auto-ubiquitination. Pulse-chase experiments demonstrated that mutation of the HECT PY-motif decreases the stability of Nedd4-2, suggesting that it is involved in stabilization of this E3 ligase. Interestingly, the HECT PY-motif mutation does not affect ubiquitination or down-regulation of a known Nedd4-2 substrate, ENaC (epithelial sodium channel). ENaC ubiquitination, in turn, appears to promote Nedd4-2 self-ubiquitination. These results support a model in which the inter- or intra-molecular WW-domain-HECT PY-motif interaction stabilizes Nedd4-2 by preventing self-ubiquitination. Substrate binding disrupts this interaction, allowing self-ubiquitination of Nedd4-2 and subsequent degradation, resulting in down-regulation of Nedd4-2 once it has ubiquitinated its target. These findings also point to a novel mechanism employed by a ubiquitin ligase to regulate itself differentially compared with substrate ubiquitination and stability.     

Functional analysis of the RING-type ubiquitin ligase family of Arabidopsis

Approximately 5% of the Arabidopsis (Arabidopsis thaliana) proteome is predicted to be involved in the ubiquitination/26S proteasome pathway. The majority of these predicted proteins have identity to conserved domains found in E3 ligases, of which there are multiple types. The RING-type E3 is characterized by the presence of a cysteine-rich domain that coordinates two zinc atoms. Database searches followed by extensive manual curation identified 469 predicted Arabidopsis RING domain-containing proteins. In addition to the two canonical RING types (C3H2C3 or C3HC4), additional types of modified RING domains, named RING-v, RING-D, RING-S/T, RING-G, and RING-C2, were identified. The modified RINGs differ in either the spacing between metal ligands or have substitutions at one or more of the metal ligand positions. The majority of the canonical and modified RING domain-containing proteins analyzed were active in in vitro ubiquitination assays, catalyzing polyubiquitination with the E2 AtUBC8. To help identity regions of the proteins that may interact with substrates, domain analyses of the amino acids outside the RING domain classified RING proteins into 30 different groups. Several characterized protein-protein interaction domains were identified, as well as additional conserved domains not described previously. The two largest classes of RING proteins contain either no identifiable domain or a transmembrane domain. The presence of such a large and diverse number of RING domain-containing proteins that function as ubiquitin E3 ligases suggests that target-specific proteolysis by these E3 ligases is a complex and important part of cellular regulation in Arabidopsis.     

UHRF2, a Ubiquitin E3 Ligase,Acts as a Small Ubiquitin-like Modifier E3 Ligase for Zinc Finger Protein 131

Small ubiquitin-like modifier (SUMO), a member of the ubiquitin-related protein family, is covalently conjugated to lysine residues of its substrates in a process referred to as SUMOylation. SUMOylation occurs through a series of enzymatic reactions analogous to that of the ubiquitination pathway, resulting in modification of the biochemical and functional properties of substrates. To date, four mammalian SUMO isoforms, a single heterodimeric SUMO-activating E1 enzyme SAE1/SAE2, a single SUMO-conjugating E2 enzyme ubiquitin-conjugating enzyme E2I (UBC9), and a few subgroups of SUMO E3 ligases have been identified. Several SUMO E3 ligases such as topoisomerase I binding, arginine/serine-rich (TOPORS), TNF receptor-associated factor 7 (TRAF7), and tripartite motif containing 27 (TRIM27) have dual functions as ubiquitin E3 ligases. Here, we demonstrate that the ubiquitin E3 ligase UHRF2 also acts as a SUMO E3 ligase. UHRF2 effectively enhances zinc finger protein 131 (ZNF131) SUMOylation but does not enhance ZNF131 ubiquitination. In addition, the SUMO E3 activity of UHRF2 on ZNF131 depends on the presence of SET and RING finger-associated and nuclear localization signal-containing region domains, whereas the critical ubiquitin E3 activity RING domain is dispensable. Our findings suggest that UHRF2 has independent functional domains and regulatory mechanisms for these two distinct enzymatic activities.     

Targeting of protein ubiquitination by BTB-Cullin 3-Roc1 ubiquitin ligases

The concentrations and functions of many cellular proteins are regulated by the ubiquitin pathway. Cullin family proteins bind with the RING-finger protein Roc1 to recruit the ubiquitin-conjugating enzyme (E2) to the ubiquitin ligase complex (E3). Cul1 and Cul7, but not other cullins, bind to an adaptor protein, Skp1. Cul1 associates with one of many F-box proteins through Skp1 to assemble various SCF-Roc1 E3 ligases that each selectively ubiquitinate one or more specific substrates. Here, we show that Cul3, but not other cullins, binds directly to multiple BTB domains through a conserved amino-terminal domain. In vitro, Cul3 promoted ubiquitination of Caenorhabditis elegans MEI-1, a katanin-like protein whose degradation requires the function of both Cul3 and BTB protein MEL-26. We suggest that in vivo there exists a potentially large number of BCR3 (BTB-Cul3-Roc1) E3 ubiquitin ligases.     

Crystal Structure of the N-terminal Domain of Anaphase-promoting Complex Subunit 7

Anaphase-promoting complex or cyclosome (APC/C) is an unusual E3 ubiquitin ligase and an essential protein that controls mitotic progression. APC/C includes at least 13 subunits, but no structure has been determined for any tetratricopeptide repeat (TPR)-containing subunit (Apc3 and -6-8) in the TPR subcomplex of APC/C. Apc7 is a TPR-containing subunit that exists only in vertebrate APC/C. Here we report the crystal structure of quad mutant of nApc7 (N-terminal fragment, residues 1-147) of human Apc7 at a resolution of 2.5 Å. The structure of nApc7 adopts a TPR-like motif and has a unique dimerization interface, although the protein does not contain the conserved TPR sequence. Based on the structure of nApc7, in addition to previous experimental findings, we proposed a putative homodimeric structure for full-length Apc7. This model suggests that TPR-containing subunits self-associate and bind to adaptors and substrates via an IR peptide in TPR-containing subunits of APC/C.Anaphase-promoting complex/cyclosome (APC/C)² is an E3 ubiquitin ligase that controls mitotic progression (1). APC/C is an ∼1.7-MDa protein complex that is composed of at least 13 subunits, and it contains a cullin homolog (Apc2), a ring-H2 finger domain (Apc11), and a tetratricopeptide repeat (TPR)-containing subunit (TPR subunit; Apc3 and -6-8) (2). Most TPR subunits are essential and evolutionarily conserved in eukaryotes (3).APC/C requires two adaptors that contain a C-terminal WD40 domain, Cdc20 and Cdh1, to recruit and select various substrates at different stages of the cell cycle. Moreover, both adaptors and specific APC/C subunits contribute to substrate recognition (4).APC/C specifically ubiquitinates cell cycle regulatory proteins that contain destruction (D) or KEN box motifs (5-7), which target them for destruction by the 26 S proteosome (8). During the cell cycle, APC/C mediates the metaphase-anaphase transition by ubiquitinating and degrading securin, a separase inhibitor, which participates in the degradation of chromatic cohesion complexes and ubiquitinates B-type cyclin, thereby accelerating transition from the late mitotic phase to G₁ (9). In addition to its primary role in cell cycle regulation, APC/C participates in postmitotic processes, such as regulation of synaptic size and axon growth (10, 11).To assess the mechanism that underlies cell cycle regulation by APC/C and the various roles of its subunits, we need to understand how APC/C is organized into higher order structures and the manner in which the subunits assemble. Although little is known regarding the crystal structures of APC/C components, three-dimensional models of APC/C have recently been obtained by cryo-negative staining EM in human, Xenopus laevis, Saccharomyces cerevisiae, and Schizosaccharomyces pombe (12-15). Several studies have indicated that APC/C assumes an asymmetric triangular shape that is composed of an outer shell and a cavity that extends through its center (12, 14). Furthermore, APC/C includes a catalytic subcomplex (Doc1/Apc10, Apc11, and Apc2), a structural complex (Apc1, Apc4, and Apc5), and a TPR subcomplex (TPR-containing subunits and nonessential subunits) (16).A TPR unit consists of a 34-residue repeat motif that adopts a helix-turn-helix conformation, which is associated with protein-protein interactions (17). Multiple copies of TPR-containing subunits are organized into the TPR subcomplex within APC/C, and this subcomplex is functionally important for the recruitment of adaptors and substrates (18). In fact, adaptors (Cdc20 and Cdh1) and Doc1/Apc10 bind to the C-terminal domain of the TPR-containing subunits Apc3 and Apc7 via the IR peptide tail sequence (7, 16, 19). It is unknown, however, how TPR-containing subunits form homo- and heterosubunit complexes, although studies have demonstrated that TPR-containing subunits self-associate in vivo and in vitro (15) and that they interact with other TPR-containing subunits (20).Apc7 is found only in vertebrate APC/C and is estimated to contain 9-15 TPR motifs, similar to other TPR-containing subunits (9). Apc7 is considered to be a molecular descendant of the same ancestral protein that gave rise to Apc3. Furthermore, the N-terminal domain of Apc7 has been reported to contain cell cycle-regulated phosphorylation sites (21), and the C-terminal TPR domain of Apc7 interacts with Cdh1 and Cdc20 (19). In Drosophila APC/C, the homolog of vertebrate Apc7 participates in synergistic genetic interactions with other TPR-containing subunits (22).The function of Apc7 within vertebrate APC/C, however, is poorly understood. Moreover, although the C-terminal regions of Apc3 and Apc7 include a tandem of nine TPR motifs, the N-terminal domains of human Apc3 and Apc7 share little homology with the canonical TPR sequence. Therefore, the N-terminal domain of human Apc7 is expected to have a significant function in vertebrate APC/C.In this study, we determined the crystal structure of the N-terminal fragment of human Apc7 (residues 1-147, denoted nApc7), and the homodimeric self-association of nApc7 structure led us to insights into mechanisms of vertebrate APC/C.     

The Mechanism of Ubiquitination in the Cullin-RING E3 Ligase Machinery: Conformational Control of Substrate Orientation

In cullin-RING E3 ubiquitin ligases, substrate binding proteins, such as VHL-box, SOCS-box or the F-box proteins, recruit substrates for ubiquitination, accurately positioning and orienting the substrates for ubiquitin transfer. Yet, how the E3 machinery precisely positions the substrate is unknown. Here, we simulated nine substrate binding proteins: Skp2, Fbw7, β-TrCP1, Cdc4, Fbs1, TIR1, pVHL, SOCS2, and SOCS4, in the unbound form and bound to Skp1, ASK1 or Elongin C. All nine proteins have two domains: one binds to the substrate; the other to E3 ligase modules Skp1/ASK1/Elongin C. We discovered that in all cases the flexible inter-domain linker serves as a hinge, rotating the substrate binding domain, optimally and accurately positioning it for ubiquitin transfer. We observed a conserved proline in the linker of all nine proteins. In all cases, the prolines pucker substantially and the pucker is associated with the backbone rotation toward the E2/ubiquitin. We further observed that the linker flexibility could be regulated allosterically by binding events associated with either domain. We conclude that the flexible linker in the substrate binding proteins orients the substrate for the ubiquitin transfer. Our findings provide a mechanism for ubiquitination and polyubiquitination, illustrating that these processes are under conformational control.     

The lore of the RINGs: substrate recognition and catalysis by ubiquitin ligases

Recently, many new examples of E3 ubiquitin ligases or E3 enzymes have been found to regulate a host of cellular processes. These E3 enzymes direct the formation of multiubiquitin chains on specific protein substrates, and - typically - the subsequent destruction of those proteins. We discuss how the modular architecture of E3 enzymes connects one of two distinct classes of catalytic domains to a wide range of substrate-binding domains. In one catalytic class, a HECT domain transfers ubiquitin directly to substrate bound to a non-catalytic domain. Members of the other catalytic class, found in the SCF, VBC and APC complexes, use a RING finger domain to facilitate ubiquitylation. The separable substrate-recognition domains of E3 enzymes provides a flexible means of linking a conserved ubiquitylation function to potentially thousands of ubiquitylated substrates in eukaryotic cells.     

The E3 Ubiquitin Ligase Atrophin Interacting Protein 4 Binds Directly To The Chemokine Receptor CXCR4 Via a Novel WW Domain-mediated Interaction

The E3 ubiquitin ligase atrophin interacting protein 4 (AIP4) mediates ubiquitination and down-regulation of the chemokine receptor CXCR4. AIP4 belongs to the Nedd4-like homologous to E6-AP carboxy terminus domain family of E3 ubiquitin ligases, which typically bind proline-rich motifs within target proteins via the WW domains. The intracellular domains of CXCR4 lack canonical WW domain binding motifs; thus, whether AIP4 is targeted to CXCR4 directly or indirectly via an adaptor protein remains unknown. Here, we show that AIP4 can interact directly with CXCR4 via a novel noncanonical WW domain-mediated interaction involving serine residues 324 and 325 within the carboxy-terminal tail of CXCR4. These serine residues are critical for mediating agonist-promoted binding of AIP4 and subsequent ubiquitination and degradation of CXCR4. These residues are phosphorylated upon agonist activation and phosphomimetic mutants show enhanced binding to AIP4, suggesting a mechanism whereby phosphorylation mediates the interaction between CXCR4 and AIP4. Our data reveal a novel noncanonical WW domain-mediated interaction involving phosphorylated serine residues in the absence of any proline residues and suggest a novel mechanism whereby an E3 ubiquitin ligase is targeted directly to an activated G protein-coupled receptor.     

Mechanisms of mono- and poly-ubiquitination: Ubiquitination specificity depends on compatibility between the E2 catalytic core and amino acid residues proximal to the lysine

Ubiquitination involves the attachment of ubiquitin to lysine residues on substrate proteins or itself, which can result in protein monoubiquitination or polyubiquitination. Ubiquitin attachment to different lysine residues can generate diverse substrate-ubiquitin structures, targeting proteins to different fates. The mechanisms of lysine selection are not well understood. Ubiquitination by the largest group of E3 ligases, the RING-family E3 s, is catalyzed through co-operation between the non-catalytic ubiquitin-ligase (E3) and the ubiquitin-conjugating enzyme (E2), where the RING E3 binds the substrate and the E2 catalyzes ubiquitin transfer. Previous studies suggest that ubiquitination sites are selected by E3-mediated positioning of the lysine toward the E2 active site. Ultimately, at a catalytic level, ubiquitination of lysine residues within the substrate or ubiquitin occurs by nucleophilic attack of the lysine residue on the thioester bond linking the E2 catalytic cysteine to ubiquitin. One of the best studied RING E3/E2 complexes is the Skp1/Cul1/F box protein complex, SCF^Cdc4, and its cognate E2, Cdc34, which target the CDK inhibitor Sic1 for K48-linked polyubiquitination, leading to its proteasomal degradation. Our recent studies of this model system demonstrated that residues surrounding Sic1 lysines or lysine 48 in ubiquitin are critical for ubiquitination. This sequence-dependence is linked to evolutionarily conserved key residues in the catalytic region of Cdc34 and can determine if Sic1 is mono- or poly-ubiquitinated. Our studies indicate that amino acid determinants in the Cdc34 catalytic region and their compatibility to those surrounding acceptor lysine residues play important roles in lysine selection. This may represent a general mechanism in directing the mode of ubiquitination in E2 s.     

Distinct functional domains contribute to degradation of the low density lipoprotein receptor (LDLR) by the E3 ubiquitin ligase inducible Degrader of the LDLR (IDOL)

We recently identified the liver X receptor-regulated E3 ubiquitin ligase inducible degrader of the LDL receptor (IDOL) as a modulator of lipoprotein metabolism. Acting as an E3 ubiquitin ligase, IDOL triggers ubiquitination and subsequent degradation of the low density lipoprotein receptor (LDLR). We demonstrate here that this outcome requires the conserved FERM and RING domains present in IDOL. The RING domain promotes ubiquitination in vitro and Lys-63-specific ubiquitination of the LDLR in vivo in response to IDOL or liver X receptor activation. We further identify RING residues that differentially influence ubiquitination of the LDLR or stability of IDOL. The FERM domain interacts with the LDLR and in living cells co-localizes with the receptor at the plasma membrane. Homology modeling revealed a phosphotyrosine-binding element embedded in the FERM domain. Mutating residues within this region or residues in the LDLR preceding the NPVY endocytosis motif abrogate LDLR degradation by IDOL. Collectively, our results indicate that both the FERM and RING domains are required for promoting lysosomal degradation of the LDLR by IDOL. Our findings may facilitate development of structure-based IDOL inhibitors aimed at increasing LDLR abundance in therapeutic strategies to treat cardiovascular disease.