A closer look of the HECTic ubiquitin ligases

The Lotus japonicus Ubiquitin Ligase SIE3 Interacts With the Transcription Factor SIP1 and Forms a Homodimer

The symbiosis receptor kinase SymRK plays an essential role in symbiotic signal transduction and nodule organogenesis. Several proteins bind to SymRK, but how the symbiosis signals are transduced from SymRK to downstream components remains elusive. We previously demonstrated that both SymRK interacting protein 1 (SIP1, an ARID-type DNA-binding protein) and SymRK interacting E3 ligase [SIE3, a RING (Really Interesting New Gene)-containing E3 ligase] interact with SymRK to regulate downstream cellular responses in Lotus japonicus during the legume-rhizobia symbiosis. Here, we show that SIE3 interacts with SIP1 in both yeast cells and Nicotiana benthamiana. SIE3 associated with itself and formed a homodimer. The cysteine 266 residue was found to be essential for SIE3 dimerization and for promoting nodulation in transgenic hairy roots of L. japonicus. Our findings provide a foundation for further investigating the regulatory mechanisms of the SymRK-mediated signaling pathway, as well as the biological function of E3 ligase dimerization in nodule organogenesis.

NEDD4-1 regulates migration and invasion of glioma cells through CNrasGEF ubiquitination in vitro

Neuronal precursor cell-expressed developmentally down-regulated 4-1 (NEDD4-1) plays a great role in tumor cell growth, but its function and mechanism in cell invasive behavior are totally unknown. Here we report that NEDD4-1 regulates migration and invasion of malignant glioma cells via triggering ubiquitination of cyclic nucleotide Ras guanine nucleotide exchange factor (CNrasGEF) using cultured glioma cells. NEDD4-1 overexpression promoted cell migration and invasion, while its downregulation specifically inhibited them. However, NEDD4-1 did not affect the proliferation and apoptosis of glioma cells. NEDD4-1 physically interacted with CNrasGEF and promoted its poly-ubiquitination and degradation. Contrary to the effect of NEDD4-1, CNrasGEF downregulation promoted cell migration and invasion, while its overexpression inhibited them. Importantly, downregulation of CNrasGEF facilitated the effect of NEDD4-1-induced cell migration and invasion. Interestingly, aberrant up-regulated NEDD4-1 showed reverse correlation with CNrasGEF protein level but not with its mRNA level in glioma tissues. Combined with the in vitro results, the result of glioma tissues indicated post-translationally modification effect of NEDD4-1 on CNrasGEF. Our study suggests that NEDD4-1 regulates cell migration and invasion through ubiquitination of CNrasGEF in vitro.

Strategies for the identification of ubiquitin ligase inhibitors

Dysregulation of the UPS (ubiquitin-proteasome system) has been implicated in a wide range of pathologies including cancer, neurodegeneration and viral infection. Inhibiting the proteasome has been shown to be an effective therapeutic strategy in humans; however, toxicity with this target remains high. E3s (Ub-protein ligases) represent an alternative attractive therapeutic target in the UPS. In this paper, we will discuss current platforms that report on E3 ligase activity and can detect E3 inhibitors, and underline the advantages and disadvantages of each approach.

Crystal structure of SopA, a Salmonella effector protein mimicking a eukaryotic ubiquitin ligase

Bacterial pathogens deliver virulence proteins into host cells to facilitate entry and survival. Salmonella SopA functions as an E3 ligase to manipulate the host proinflammatory response. Here we report the crystal structure of SopA in two conformations. Although it has little sequence similarity to eukaryotic HECT-domain E3s, the C-terminal half of SopA has a bilobal architecture that is reminiscent of the N- and C-lobe arrangement of HECT domains. The SopA structure also contains a putative substrate-binding domain located near the E2-binding site. The two structures of SopA differ in the relative orientations of the C lobe, indicating that SopA possesses the conformational flexibility essential for HECT E3 function. These results suggest that SopA is a unique HECT E3 ligase evolved from the coevolutionary selective pressure at the bacterium-host interface.

Ubiquitin-dependent proteolysis of trihydrophobin 1 (TH1) by the human papilloma virus E6-associated protein (E6-AP)

Human Papilloma virus E6-associated protein (E6-AP), which is known as an E3 ubiquitin ligase, mediates ubiquitination and subsequent degradation of a series of cellular proteins. In this paper, we identify here trihydrophobin 1 (TH1), an integral subunit of the human negative transcription elongation factor (NELF) complex, as a novel E6-AP interaction protein and a target of E6-AP-mediated degradation. Overexpression of E6-AP results in degradation of TH1 in a dose-dependent manner, whereas knock-down of endogenous E6-AP elevates the TH1 protein level. TH1 protein turnover is substantially faster, compared to controls, in cells that overexpressed E6-AP. Wild-type E6-AP promotes the ubiquitination of TH1, while a catalytically inactive point mutant of E6-AP abolishes its ubiquitination. Furthermore, in vitro ubiquitination assay also demonstrates that TH1 can be ubiquitinated by E6-AP. The degradation is blocked by treatment with proteasome inhibitor MG132. Herein, we provide strong evidence that TH1 is a specific substrate that is targeted for degradation through E6-AP-catalyzed polyubiquitination.

Getting into position: the catalytic mechanisms of protein ubiquitylation

The role of protein ubiquitylation in the control of diverse cellular pathways has recently gained widespread attention. Ubiquitylation not only directs the targeted destruction of tagged proteins by the 26 S proteasome, but it also modulates protein activities, protein-protein interactions and subcellular localization. An understanding of the components involved in protein ubiquitylation (E1s, E2s and E3s) is essential to understand how specificity and regulation are conferred upon these pathways. Much of what we know about the catalytic mechanisms of protein ubiquitylation comes from structural studies of the proteins involved in this process. Indeed, structures of ubiquitin-activating enzymes (E1s) and ubiquitin-conjugating enzymes (E2s) have provided insight into their mechanistic details. E3s (ubiquitin ligases) contain most of the substrate specificity and regulatory elements required for protein ubiquitylation. Although several E3 structures are available, the specific mechanistic role of E3s is still unclear. This review will discuss the different types of ubiquitin signals and how they are generated. Recent advances in the field of protein ubiquitylation will be examined, including the mechanisms of E1, E2 and E3. In particular, we discuss the complexity of molecular recognition required to impose selectivity on substrate selection and topology of poly-ubiquitin chains.