Mapping Degradation Signals and Pathways in a Eukaryotic N-terminome.

Most eukaryotic proteins are N-terminally acetylated. This modification can be recognized as a signal for selective protein degradation (degron) by the N-end rule pathways. However, the prevalence and specificity of such degrons in the proteome are unclear. Here, by systematically examining how protein turnover is affected by N-terminal sequences, we perform a comprehensive survey of degrons in the yeast N-terminome. We find that approximately 26% of nascent protein N termini encode cryptic degrons. These degrons exhibit high hydrophobicity and are frequently recognized by the E3 ubiquitin ligase Doa10, suggesting a role in protein quality control. In contrast, N-terminal acetylation rarely functions as a degron. Surprisingly, we identify two pathways where N-terminal acetylation has the opposite function and blocks protein degradation through the E3 ubiquitin ligase Ubr1. Our analysis highlights the complexity of N-terminal degrons and argues that hydrophobicity, not N-terminal acetylation, is the predominant feature of N-terminal degrons in nascent proteins.

DNA fragility at topologically associated domain boundaries is promoted by alternative DNA secondary structure and topoisomerase II activity.

CCCTC-binding factor (CTCF) binding sites are hotspots of genome instability. Although many factors have been associated with CTCF binding site fragility, no study has integrated all fragility-related factors to understand the mechanism(s) of how they work together. Using an unbiased, genome-wide approach, we found that DNA double-strand breaks (DSBs) are enriched at strong, but not weak, CTCF binding sites in five human cell types. Energetically favorable alternative DNA secondary structures underlie strong CTCF binding sites. These structures coincided with the location of topoisomerase II (TOP2) cleavage complex, suggesting that DNA secondary structure acts as a recognition sequence for TOP2 binding and cleavage at CTCF binding sites. Furthermore, CTCF knockdown significantly increased DSBs at strong CTCF binding sites and at CTCF sites that are located at topologically associated domain (TAD) boundaries. TAD boundary-associated CTCF sites that lost CTCF upon knockdown displayed increased DSBs when compared to the gained sites, and those lost sites are overrepresented with G-quadruplexes, suggesting that the structures act as boundary insulators in the absence of CTCF, and contribute to increased DSBs. These results model how alternative DNA secondary structures facilitate recruitment of TOP2 to CTCF binding sites, providing mechanistic insight into DNA fragility at CTCF binding sites.

Differential Gene Expression following DHX36/G4R1 Knockout Is Associated with G-Quadruplex Content and Cancer.

G-quadruplexes (G4s) are secondary DNA and RNA structures stabilized by positive cations in a central channel formed by stacked tetrads of Hoogsteen base-paired guanines. G4s form from G-rich sequences across the genome, whose biased distribution in regulatory regions points towards a gene-regulatory role. G4s can themselves be regulated by helicases, such as DHX36 (aliases: G4R1 and RHAU), which possess the necessary activity to resolve these stable structures. G4s have been shown to both positively and negatively regulate gene expression when stabilized by ligands, or through the loss of helicase activity. Using DHX36 knockout Jurkat cell lines, we identified widespread, although often subtle, effects on gene expression that are associated with the presence or number of observed G-quadruplexes in promoters or gene regions. Genes that significantly change their expression, particularly those that show a significant increase in RNA abundance under DHX36 knockout, are associated with a range of cellular functions and processes, including numerous transcription factors and oncogenes, and are linked to several cancers. Our work highlights the direct and indirect role of DHX36 in the transcriptome of T-lymphocyte leukemia cells and the potential for DHX36 dysregulation in cancer.

Discovery of 5-{2-[5-Chloro-2-(5-ethoxyquinoline-8-sulfonamido)phenyl]ethynyl}-4-methoxypyridine-2-carboxylic Acid, a Highly Selective in Vivo Useable Chemical Probe to Dissect MCT4 Biology.

Due to increased lactate production during glucose metabolism, tumor cells heavily rely on efficient lactate transport to avoid intracellular lactate accumulation and acidification. Monocarboxylate transporter 4 (MCT4/SLC16A3) is a lactate transporter that plays a central role in tumor pH modulation. The discovery and optimization of a novel class of MCT4 inhibitors (hit 9a), identified by a cellular screening in MDA-MB-231, is described. Direct target interaction of the optimized compound 18n with the cytosolic domain of MCT4 was shown after solubilization of the GFP-tagged transporter by fluorescence cross-correlation spectroscopy and microscopic studies. In vitro treatment with 18n resulted in lactate efflux inhibition and reduction of cellular viability in MCT4 high expressing cells. Moreover, pharmacokinetic properties of 18n allowed assessment of lactate modulation and antitumor activity in a mouse tumor model. Thus, 18n represents a valuable tool for investigating selective MCT4 inhibition and its effect on tumor biology.

ESCRT proteins restrict constitutive NF-κB signaling by trafficking cytokine receptors.

Because signaling mediated by the transcription factor nuclear factor κB (NF-κB) is initiated by ligands and receptors that can undergo internalization, we investigated how endocytic trafficking regulated this key physiological pathway. We depleted all of the ESCRT (endosomal sorting complexes required for transport) subunits, which mediate receptor trafficking and degradation, and found that the components Tsg101, Vps28, UBAP1, and CHMP4B were essential to restrict constitutive NF-κB signaling in human embryonic kidney 293 cells. In the absence of exogenous cytokines, depletion of these proteins led to the activation of both canonical and noncanonical NF-κB signaling, as well as the induction of NF-κB-dependent transcriptional responses in cultured human cells, zebrafish embryos, and fat bodies in flies. These effects depended on cytokine receptors, such as the lymphotoxin ß receptor (LTßR) and tumor necrosis factor receptor 1 (TNFR1). Upon depletion of ESCRT subunits, both receptors became concentrated on and signaled from endosomes. Endosomal accumulation of LTßR induced its ligand-independent oligomerization and signaling through the adaptors TNFR-associated factor 2 (TRAF2) and TRAF3. These data suggest that ESCRTs constitutively control the distribution of cytokine receptors in their ligand-free state to restrict their signaling, which may represent a general mechanism to prevent spurious activation of NF-κB.

Endocytic Adaptor Protein Tollip Inhibits Canonical Wnt Signaling.

Many adaptor proteins involved in endocytic cargo transport exhibit additional functions in other cellular processes which may be either related to or independent from their trafficking roles. The endosomal adaptor protein Tollip is an example of such a multitasking regulator, as it participates in trafficking and endosomal sorting of receptors, but also in interleukin/Toll/NF-κB signaling, bacterial entry, autophagic clearance of protein aggregates and regulation of sumoylation. Here we describe another role of Tollip in intracellular signaling. By performing a targeted RNAi screen of soluble endocytic proteins for their additional functions in canonical Wnt signaling, we identified Tollip as a potential negative regulator of this pathway in human cells. Depletion of Tollip potentiates the activity of ß-catenin/TCF-dependent transcriptional reporter, while its overproduction inhibits the reporter activity and expression of Wnt target genes. These effects are independent of dynamin-mediated endocytosis, but require the ubiquitin-binding CUE domain of Tollip. In Wnt-stimulated cells, Tollip counteracts the activation of ß-catenin and its nuclear accumulation, without affecting its total levels. Additionally, under conditions of ligand-independent signaling, Tollip inhibits the pathway after the stage of ß-catenin stabilization, as observed in human cancer cell lines, characterized by constitutive ß-catenin activity. Finally, the regulation of Wnt signaling by Tollip occurs also during early embryonic development of zebrafish. In summary, our data identify a novel function of Tollip in regulating the canonical Wnt pathway which is evolutionarily conserved between fish and humans. Tollip-mediated inhibition of Wnt signaling may contribute not only to embryonic development, but also to carcinogenesis. Mechanistically, Tollip can potentially coordinate multiple cellular pathways of trafficking and signaling, possibly by exploiting its ability to interact with ubiquitin and the sumoylation machinery.

Tandem fluorescent protein timers for in vivo analysis of protein dynamics.

The functional state of a cell is largely determined by the spatiotemporal organization of its proteome. Technologies exist for measuring particular aspects of protein turnover and localization, but comprehensive analysis of protein dynamics across different scales is possible only by combining several methods. Here we describe tandem fluorescent protein timers (tFTs), fusions of two single-color fluorescent proteins that mature with different kinetics, which we use to analyze protein turnover and mobility in living cells. We fuse tFTs to proteins in yeast to study the longevity, segregation and inheritance of cellular components and the mobility of proteins between subcellular compartments; to measure protein degradation kinetics without the need for time-course measurements; and to conduct high-throughput screens for regulators of protein turnover. Our experiments reveal the stable nature and asymmetric inheritance of nuclear pore complexes and identify regulators of N-end rule­mediated protein degradation.