Skp1 Independent Function of Cdc53/Cul1 in F-box Protein Homeostasis

Cadmium binding by the F-box domain induces p97-mediated SCF complex disassembly to activate stress response programs

The F-box domain is a highly conserved structural motif that defines the largest class of ubiquitin ligases, Skp1/Cullin1/F-box protein (SCF) complexes. The only known function of the F-box motif is to form the protein interaction surface with Skp1. Here we show that the F-box domain can function as an environmental sensor. We demonstrate that the F-box domain of Met30 is a cadmium sensor that blocks the activity of the SCFMet30 ubiquitin ligase during cadmium stress. Several highly conserved cysteine residues within the Met30 F-box contribute to binding of cadmium with a KD of 8 µM. Binding induces a conformational change that allows for Met30 autoubiquitylation, which in turn leads to recruitment of the segregase Cdc48/p97/VCP followed by active SCFMet30 disassembly. The resulting inactivation of SCFMet30 protects cells from cadmium stress. Our results show that F-box domains participate in regulation of SCF ligases beyond formation of the Skp1 binding interface.

Active-site cysteine 215 sulfonation targets protein tyrosine phosphatase PTP1B for Cullin1 E3 ligase-mediated degradation

Reactive oxygen species (ROS), released as byproducts of mitochondrial metabolism or as products of NADPH oxidases and other processes, can directly oxidize the active-site cysteine (Cys) residue of protein tyrosine phosphatases (PTPs) in a mammalian cell. Robust degradation of irreversibly oxidized PTPs is essential for preventing accumulation of these permanently inactive enzymes. However, the mechanism underlying the degradation of these proteins was unknown. In this study, we found that the active-site Cys215 of endogenous PTP1B is sulfonated in H9c2 cardiomyocytes under physiological conditions. The sulfonation of Cys215 led PTP1B to exhibit a conformational change, and drive the subsequent ubiquitination and degradation of this protein. We then discovered that Cullin1, an E3 ligase, interacts with the Cys215-sulfonated PTP1B. The functional impairment of Cullin1 prevented PTP1B from oxidation-dependent ubiquitination and degradation in H9c2 cells. Moreover, delivery of the terminally oxidized PTP1B resulted in proteotoxicity-caused injury in the affected cells. In conclusion, we elucidate how sulfonation of the active-site Cys215 can direct turnover of endogenous PTP1B through the engagement of ubiquitin-proteasome system. These data highlight a novel mechanism that maintains PTP homeostasis in cardiomyocytes with constitutive ROS production.

F-box receptor mediated control of substrate stability and subcellular location organizes cellular development of Aspergillus nidulans

Fungal growth and development are coordinated with specific secondary metabolism. This coordination requires 8 of 74 F-box proteins of the filamentous fungus Aspergillus nidulans. F-box proteins recognize primed substrates for ubiquitination by Skp1-Cul1-Fbx (SCF) E3 ubiquitin RING ligases and degradation by the 26S proteasome. 24 F-box proteins are found in the nuclear fraction as part of SCFs during vegetative growth. 43 F-box proteins interact with SCF proteins during growth, development or stress. 45 F-box proteins are associated with more than 700 proteins that have mainly regulatory roles. This corroborates that accurate surveillance of protein stability is prerequisite for organizing multicellular fungal development. Fbx23 combines subcellular location and protein stability control, illustrating the complexity of F-box mediated regulation during fungal development. Fbx23 interacts with epigenetic methyltransferase VipC which interacts with fungal NF-κB-like velvet domain regulator VeA that coordinates fungal development with secondary metabolism. Fbx23 prevents nuclear accumulation of methyltransferase VipC during early development. These results suggest that in addition to their role in protein degradation, F-box proteins also control subcellular accumulations of key regulatory proteins for fungal development.

Removal of sulfamethazine and Cu2+ by Sakaguchia cladiensis A5: Performance and transcriptome analysis

To reduce the potential risks of contamination of antibiotics and heavy metals to ecological environment and human safety, biological removal of these composite pollutants is the focus of much study. One previously identified isolate, Sakaguchia cladiensis A5, was used to decompose sulfamethazine (SMZ) and adsorb Cu²⁺. The ability of A5 to remove SMZ was enhanced by pre-induced culture, which reached 49.8% on day 9. The removal of SMZ could be also increased to 37.6% on day 3 in the presence of Cu²⁺, but only to 12.2% in the system without Cu²⁺. The biosorption of Cu²⁺ mainly occurred on the cell walls, while the biodegradation of SMZ was inside the cells. By comparative transcriptome analysis for A5, 1270 and 2220 differentially expressed genes (DEGs) were identified after treating single SMZ and SMZ/Cu²⁺, respectively. The Gene expression pattern analysis suggested a suppression of transcriptional changes in A5 responding to SMZ/Cu²⁺ as compared to under the sole stress of SMZ. The DEGs functional enrichment analysis suggested that the antioxidant and sulfate assimilation pathways played a key role on SMZ biodegradation and Cu²⁺ biosorption. The DEGs of proteins CAT, PRDX5, SAT, and CYSC were up-regulated to facilitate the resistance of A5 against oxidative toxicity of Cu²⁺. Moreover, the protein MET30 activated by Cu²⁺ was also overexpressed to promote the transmembrane transport of SMZ, such that A5 could decompose SMZ more effectively in SMZ/Cu²⁺ system. The results of this study would provide new insights into the mechanism of biodegradation and biosorption of SMZ/Cu²⁺.

Cdc48 cofactor Shp1 regulates signal-induced SCFMet30 disassembly

Organisms can adapt to a broad spectrum of sudden and dramatic changes in their environment. These abrupt changes are often perceived as stress and trigger responses that facilitate survival and eventual adaptation. The ubiquitin-proteasome system (UPS) is involved in most cellular processes. Unsurprisingly, components of the UPS also play crucial roles during various stress response programs. The budding yeast SCFMet30 complex is an essential cullin-RING ubiquitin ligase that connects metabolic and heavy metal stress to cell cycle regulation. Cadmium exposure results in the active dissociation of the F-box protein Met30 from the core ligase, leading to SCFMet30 inactivation. Consequently, SCFMet30 substrate ubiquitylation is blocked and triggers a downstream cascade to activate a specific transcriptional stress response program. Signal-induced dissociation is initiated by autoubiquitylation of Met30 and serves as a recruitment signal for the AAA-ATPase Cdc48/p97, which actively disassembles the complex. Here we show that the UBX cofactor Shp1/p47 is an additional key element for SCFMet30 disassembly during heavy metal stress. Although the cofactor can directly interact with the ATPase, Cdc48 and Shp1 are recruited independently to SCFMet30 during cadmium stress. An intact UBX domain is crucial for effective SCFMet30 disassembly, and a concentration threshold of Shp1 recruited to SCFMet30 needs to be exceeded to initiate Met30 dissociation. The latter is likely related to Shp1-mediated control of Cdc48 ATPase activity. This study identifies Shp1 as the crucial Cdc48 cofactor for signal-induced selective disassembly of a multisubunit protein complex to modulate activity.

The Biology of F-box Proteins: The SCF Family of E3 Ubiquitin Ligases

F-box proteins function as substrate adaptors for the S-phase kinase-associated protein 1 (SKP1)-cullin 1 (CUL1)-F-box protein (SCF) ubiquitin ligase complexes, which mediate the proteasomal degradation of a diverse range of regulatory proteins. 20 years since the F-box protein family has been discovered, our understanding of substrate-recognition regulation and the roles F-box proteins play in cellular processes has continued to expand. Here, we provide an introduction to the discovery and classification of F-box proteins, the overall structural assembly of SCF complexes, the varied mechanisms by which F-box proteins recognize their substrates, and the role F-box proteins play in diseases and their potentials in targeted therapies.

CENP-W inhibits CDC25A degradation by destabilizing the SCFβ-TrCP-1 complex at G2/M

Skp, Cullin, F-box (SCF)β-TrCP-1 ubiquitin ligases play a central role in cell cycle regulation and tumorigenesis via proteolytic cleavage of many essential cell cycle regulators. In this study, we propose that centromere protein (CENP)-W, a newly identified kinetochore component, is a novel negative regulator of the SCFβ-TrCP-1 complex. CENP-W interacts with Cullin (CUL)-1 and β-Transducin repeat-containing protein (β-TrCP)-1 through highly overlapped binding sites with S-phase kinase-associated protein (SKP)-1. CENP-W is incorporated into the SCFβ-TrCP-1 complex to promote complex disassembly. Unlike other known regulators that increase SCFβ-TrCP-1 ubiquitin ligase activity by promoting complex reassociation, CENP-W-mediated complex disorganization induced β-TrCP1 degradation and consequently decreased its activity. The association between CENP-W and the SCFβ-TrCP-1 complex was prominent during the G2/M transition in the nucleus. Especially, CENP-W knockdown decreased the cell division cycle-25A protein level, leading to a delay in mitotic progression. We propose that CENP-W participates in cell cycle regulation by modulating SCFβ-TrCP-1 ubiquitin ligase activity.-Cheon, Y., Lee, S. CENP-W inhibits CDC25A degradation by destabilizing the SCFβ-TrCP-1 complex at G2/M.