Arl15 upregulates the TGFß family signaling by promoting the assembly of the Smad-complex.

The hallmark event of the canonical transforming growth factor ß (TGFß) family signaling is the assembly of the Smad-complex, consisting of the common Smad, Smad4, and phosphorylated receptor-regulated Smads. How the Smad-complex is assembled and regulated is still unclear. Here, we report that active Arl15, an Arf-like small G protein, specifically binds to the MH2 domain of Smad4 and colocalizes with Smad4 at the endolysosome. The binding relieves the autoinhibition of Smad4, which is imposed by the intramolecular interaction between its MH1 and MH2 domains. Activated Smad4 subsequently interacts with phosphorylated receptor-regulated Smads, forming the Smad-complex. Our observations suggest that Smad4 functions as an effector and a GTPase activating protein (GAP) of Arl15. Assembly of the Smad-complex enhances the GAP activity of Smad4 toward Arl15, therefore dissociating Arl15 before the nuclear translocation of the Smad-complex. Our data further demonstrate that Arl15 positively regulates the TGFß family signaling.

Amino acids stimulate the endosome-to-Golgi trafficking through Ragulator and small GTPase Arl5.

The endosome-to-Golgi or endocytic retrograde trafficking pathway is an important post-Golgi recycling route. Here we show that amino acids (AAs) can stimulate the retrograde trafficking and regulate the cell surface localization of certain Golgi membrane proteins. By testing components of the AA-stimulated mTORC1 signaling pathway, we demonstrate that SLC38A9, v-ATPase and Ragulator, but not Rag GTPases and mTORC1, are essential for the AA-stimulated trafficking. Arl5, an ARF-like family small GTPase, interacts with Ragulator in an AA-regulated manner and both Arl5 and its effector, the Golgi-associated retrograde protein complex (GARP), are required for the AA-stimulated trafficking. We have therefore identified a mechanistic connection between the nutrient signaling and the retrograde trafficking pathway, whereby SLC38A9 and v-ATPase sense AA-sufficiency and Ragulator might function as a guanine nucleotide exchange factor to activate Arl5, which, together with GARP, a tethering factor, probably facilitates the endosome-to-Golgi trafficking.

Regulation of the NRF2 transcription factor by andrographolide and organic extracts from plant endophytes.

The transcription factor NF-E2 Related Factor-2 (NRF2) is an important drug target. Activation of NRF2 has chemopreventive effects in cancer and exerts beneficial effects in a number of diseases, including neurodegenerative diseases, inflammatory diseases, hepatosteatosis, obesity and insulin resistance. Hence, there have been great efforts to discover and characterize novel NRF2 activators. One reported NRF2 activator is the labdane diterpenoid andrographolide. In this study, we identified the mechanism through which andrographolide activates NRF2. We showed that andrographolide inhibits the function of KEAP1, a protein that together with CUL3 and RBX1 forms an E3 ubiquitin ligase that polyubiquitinates NRF2. Andrographolide partially inhibits the interaction of KEAP1 with CUL3 in a manner dependent on Cys151 in KEAP1. This suggests that andrographolide forms Michael acceptor dependent adducts with Cys151 in KEAP1 in vivo, leading to inhibition of NRF2 ubiquitination and consequently accumulation of the transcription factor. Interestingly, we also showed that at higher concentrations andrographolide increases NRF2 protein expression in a Cys151 independent, but likely KEAP1 dependent manner, possibly through modification of other Cys residues in KEAP1. In this study we also screened secondary metabolites produced by endophytes isolated from non-flowering plants for NRF2-inducing properties. One of the extracts, ORX 41, increased both NRF2 protein expression and transcriptional activity markedly. These results suggest that endophytes isolated from non-flowering or other plants may be a good source of novel NRF2 inducing compounds.

Mammalian Mon2/Ysl2 regulates endosome-to-Golgi trafficking but possesses no guanine nucleotide exchange activity toward Arl1 GTPase.

Arl1 is a member of Arf family small GTPases that is essential for the organization and function of Golgi complex. Mon2/Ysl2, which shares significant homology with Sec7 family Arf guanine nucleotide exchange factors, was poorly characterized in mammalian cells. Here, we report the first in depth characterization of mammalian Mon2. We found that Mon2 localized to trans-Golgi network which was dependent on both its N and C termini. The depletion of Mon2 did not affect the Golgi localized or cellular active form of Arl1. Furthermore, our in vitro assay demonstrated that recombinant Mon2 did not promote guanine nucleotide exchange of Arl1. Therefore, our results suggest that Mon2 could be neither necessary nor sufficient for the guanine nucleotide exchange of Arl1. We demonstrated that Mon2 was involved in endosome-to-Golgi trafficking as its depletion accelerated the delivery of furin and CI-M6PR to Golgi after endocytosis.

Characterization of the role of COP9 signalosome in regulating cullin E3 ubiquitin ligase activity.

Cullin RING ligases (CRLs) are the largest family of cellular E3 ubiquitin ligases and mediate polyubiquitination of a number of cellular substrates. CRLs are activated via the covalent modification of the cullin protein with the ubiquitin-like protein Nedd8. This results in a conformational change in the cullin carboxy terminus that facilitates the ubiquitin transfer onto the substrate. COP9 signalosome (CSN)-mediated cullin deneddylation is essential for CRL activity in vivo. However, the mechanism through which CSN promotes CRL activity in vivo is currently unclear. In this paper, we provide evidence that cullin deneddylation is not intrinsically coupled to substrate polyubiquitination as part of the CRL activation cycle. Furthermore, inhibiting substrate-receptor autoubiquitination is unlikely to account for the major mechanism through which CSN regulates CRL activity. CSN also did not affect recruitment of the substrate-receptor SPOP to Cul3, suggesting it may not function to facilitate the exchange of Cul3 substrate receptors. Our results indicate that CSN binds preferentially to CRLs in the neddylation-induced, active conformation. Binding of the CSN complex to active CRLs may recruit CSN-associated proteins important for CRL regulation. The deneddylating activity of CSN would subsequently promote its own dissociation to allow progression through the CRL activation cycle.

Inhibition of cullin RING ligases by cycle inhibiting factor: evidence for interference with Nedd8-induced conformational control.

Cycle inhibiting factor (Cif) is produced by pathogenic intracellular bacteria and injected into the host cells via a type III secretion system. Cif is known to interfere with the eukaryotic cell cycle by inhibiting the function of cullin RING E3 ubiquitin ligases (CRLs). Cullin proteins form the scaffold protein of CRLs and are modified with the ubiquitin-like protein Nedd8, which exerts important conformational control required for CRL activity. Cif has recently been shown to catalyze the deamidation of Gln40 in Nedd8 to Glu. Here, we addressed how Nedd8 deamidation inhibits CRL activity. Our results indicate that Burkholderia pseudomallei Cif (also known as CHBP) inhibits the deconjugation of Nedd8 in vivo by inhibiting binding of the deneddylating COP9 signalosome (CSN) complex. We provide evidence that the reduced binding of CSN and the inhibition of CRL activity by Cif are due to interference with Nedd8-induced conformational control, which is dependent on the interaction between the Nedd8 hydrophobic patch and the cullin winged-helix B subdomain. Of note, mutation of Gln40 to Glu in ubiquitin, an additional target of Cif, inhibits the interaction between the hydrophobic surface of ubiquitin and the ubiquitin-binding protein p62/SQSTM1, showing conceptually that Cif activity can impair ubiquitin/ubiquitin-like protein non-covalent interactions. Our results also suggest that Cif may exert additional cellular effects by interfering with the association between ubiquitin and ubiquitin-binding proteins.

Neddylation-induced conformational control regulates cullin RING ligase activity in vivo.

Cullin RING ligases (CRLs) constitute the largest family of ubiquitin ligases with diverse cellular functions. Conjugation of the ubiquitin-like molecule Nedd8 to a conserved lysine residue on the cullin scaffold is essential for the activity of CRLs. Using structural studies and in vitro assays, it has been demonstrated that neddylation stimulates CRL activity through conformational rearrangement of the cullin C-terminal winged-helix B domain and Rbx1 RING subdomain from a closed architecture to an open and dynamic structure, thus promoting ubiquitin transfer onto the substrate. Here, we tested whether the proposed mechanism operates in vivo in intact cells and applies to other CRL family members. To inhibit cellular neddylation, we used a cell line with tetracycline-inducible expression of a dominant-negative form of the Nedd8 E2 enzyme or treatment of cells with the Nedd8 E1 inhibitor MLN4924. Using these cellular systems, we show that different mutants of Cul2 and Cul3 and of Rbx1 that confer increased Rbx1 flexibility mimic neddylation and rescue CRL activity in intact cells. Our findings indicate that in vivo neddylation functions by inducing conformational changes in the C-terminal domain of Cul2 and Cul3 that free the RING domain of Rbx1 and bridge the gap for ubiquitin transfer onto the substrate.

Regulation of cullin RING E3 ubiquitin ligases by CAND1 in vivo.

Cullin RING ligases are multi-subunit complexes consisting of a cullin protein which forms a scaffold onto which the RING protein Rbx1/2 and substrate receptor subunits assemble. CAND1, which binds to cullins that are not conjugated with Nedd8 and not associated with substrate receptors, has been shown to function as a positive regulator of Cullin ligases in vivo. Two models have been proposed to explain this requirement: (i) CAND1 sequesters cullin proteins and thus prevents autoubiquitination of substrate receptors, and (ii) CAND1 is required to promote the exchange of bound substrate receptors. Using mammalian cells, we show that CAND1 is predominantly cytoplasmically localized and that cullins are the major CAND1 interacting proteins. However, only small amounts of CAND1 bind to Cul1 in cells, despite low basal levels of Cul1 neddylation and approximately equal cytoplasmic endogenous protein concentrations of CAND1 and Cul1. Compared to F-box protein substrate receptors, binding of CAND1 to Cul1 in vivo is weak. Furthermore, preventing binding of F-box substrate receptors to Cul1 does not increase CAND1 binding. In conclusion, our study suggests that CAND1 does not function by sequestering cullins in vivo to prevent substrate receptor autoubiquitination and is likely to regulate cullin RING ligase activity via alternative mechanisms.

Characterization of erythrosine B binding to bovine serum albumin and bilirubin displacement.

The interaction of crythrosine B (ErB), a commonly used dye for coloring foods and drinks, with bovine scrum albumin (BSA) was investigated both in the absence and presence of bilirubin (BR) using absorption and absorption difference spectroscopy. ErB binding to BSA was reflected from a significant red shift of 11 nm in the absorption maximum of ErB (527 nm) with the change in absorbance at lamdamax. Analysis of absorption difference spectroscopic titration results of BSA with increasing concentrations of ErB3 using Benesi-Hildebrand equation gave the association constant, K as 6.9 x 10(4) M(-1). BR displacing action of ErB was revealed by a significant blue shift in the absorption maximum, accompanied by a decrease in absorbance difference at lamdamax in the difference spectrum of BR-BSA complex upon addition of increasing concentrations of ErB. This was further substantiated by fluorescence spectroscopy, as addition of increasing concentrations of ErB to BR-BSA complex caused a significant decrease in fluoresccnce at 510 nm. The results suggest that ErB binds to a site in the vicinity of BR binding site on BSA. Therefore, intake of ErB may increase the risk of hyperbilirubinemia in the healthy subjects.