14-3-3 proteins play a role in the cell cycle by shielding cdt2 from ubiquitin-mediated degradation

Deubiquitinating enzyme mutagenesis screens identify a USP43-dependent HIF-1 transcriptional response

The ubiquitination and proteasome-mediated degradation of Hypoxia Inducible Factors (HIFs) is central to metazoan oxygen-sensing, but the involvement of deubiquitinating enzymes (DUBs) in HIF signalling is less clear. Here, using a bespoke DUBs sgRNA library we conduct CRISPR/Cas9 mutagenesis screens to determine how DUBs are involved in HIF signalling. Alongside defining DUBs involved in HIF activation or suppression, we identify USP43 as a DUB required for efficient activation of a HIF response. USP43 is hypoxia regulated and selectively associates with the HIF-1α isoform, and while USP43 does not alter HIF-1α stability, it facilitates HIF-1 nuclear accumulation and binding to its target genes. Mechanistically, USP43 associates with 14-3-3 proteins in a hypoxia and phosphorylation dependent manner to increase the nuclear pool of HIF-1. Together, our results highlight the multifunctionality of DUBs, illustrating that they can provide important signalling functions alongside their catalytic roles.

14-3-3ζ suppresses RANKL signaling by destabilizing TRAF6

Macrophages are essential regulators of inflammation and bone loss. Receptor activator of nuclear factor-κβ ligand (RANKL), a pro-inflammatory cytokine, is responsible for macrophage differentiation to osteoclasts and bone loss. We recently showed that 14-3-3ζ-knockout (YwhazKO) rats exhibit increased bone loss in the inflammatory arthritis model. 14-3-3ζ is a cytosolic adaptor protein that actively participates in many signaling transductions. However, the role of 14-3-3ζ in RANKL signaling or bone remodeling is unknown. We investigated how 14-3-3ζ affects osteoclast activity by evaluating its role in RANKL signaling. We utilized 14-3-3ζ-deficient primary bone marrow-derived macrophages obtained from wildtype and YwhazKO animals and RAW264.7 cells generated using CRISPR-Cas9. Our results showed that 14-3-3ζ-deficient macrophages, upon RANKL stimulation, have bigger and stronger tartrate-resistant acid phosphatase-positive multinucleated cells and increased bone resorption activity. The presence of 14-3-3ζ suppressed RANKL-induced MAPK and AKT phosphorylation, transcription factors (NFATC1 and p65) nuclear translocation, and subsequently, gene induction (Rank, Acp5, and Ctsk). Mechanistically, 14-3-3ζ interacts with TRAF6, an essential component of the RANKL receptor complex. Upon RANKL stimulation, 14-3-3ζ-TRAF6 interaction was increased, while RANK-TRAF6 interaction was decreased. Importantly, 14-3-3ζ supported TRAF6 ubiquitination and degradation by the proteasomal pathway, thus dampening the downstream RANKL signaling. Together, we show that 14-3-3ζ regulates TRAF6 levels to suppress inflammatory RANKL signaling and osteoclast activity. To the best of our knowledge, this is the first report on 14-3-3ζ regulation of RANKL signaling and osteoclast activation.

14-3-3 interaction with phosphodiesterase 8A sustains PKA signaling and downregulates the MAPK pathway

The cAMP/PKA and mitogen-activated protein kinase (MAPK) signaling cascade control many cellular processes and are highly regulated for optimal cellular responses upon external stimuli. Phosphodiesterase 8A (PDE8A) is an important regulator that inhibits signaling via cAMP-dependent PKA by hydrolyzing intracellular cAMP pool. Conversely, PDE8A activates the MAPK pathway by protecting CRAF/Raf1 kinase from PKA-mediated inhibitory phosphorylation at Ser259 residue, a binding site of scaffold protein 14-3-3. It still remains enigmatic as to how the cross-talk involving PDE8A regulation influences cAMP/PKA and MAPK signaling pathways. Here, we report that PDE8A interacts with 14-3-3ζ in both yeast and mammalian system, and this interaction is enhanced upon the activation of PKA, which phosphorylates PDE8A's Ser359 residue. Biophysical characterization of phospho-Ser359 peptide with 14-3-3ζ protein further supports their interaction. Strikingly, 14-3-3ζ reduces the catalytic activity of PDE8A, which upregulates the cAMP/PKA pathway while the MAPK pathway is downregulated. Moreover, 14-3-3ζ in complex with PDE8A and cAMP-bound regulatory subunit of PKA, RIα, delays the deactivation of PKA signaling. Our results define 14-3-3ζ as a molecular switch that operates signaling between cAMP/PKA and MAPK by associating with PDE8A.

14-3-3 protein augments the protein stability of phosphorylated spastin and promotes the recovery of spinal cord injury through its agonist intervention

Axon regeneration is abortive in the central nervous system following injury. Orchestrating microtubule dynamics has emerged as a promising approach to improve axonal regeneration. The microtubule severing enzyme spastin is essential for axonal development and regeneration through remodeling of microtubule arrangement. To date, however, little is known regarding the mechanisms underlying spastin action in neural regeneration after spinal cord injury. Here, we use glutathione transferase pulldown and immunoprecipitation assays to demonstrate that 14-3-3 interacts with spastin, both in vivo and in vitro, via spastin Ser233 phosphorylation. Moreover, we show that 14-3-3 protects spastin from degradation by inhibiting the ubiquitination pathway and upregulates the spastin-dependent severing ability. Furthermore, the 14-3-3 agonist Fusicoccin (FC-A) promotes neurite outgrowth and regeneration in vitro which needs spastin activation. Western blot and immunofluorescence results revealed that 14-3-3 protein is upregulated in the neuronal compartment after spinal cord injury in vivo. In addition, administration of FC-A not only promotes locomotor recovery, but also nerve regeneration following spinal cord injury in both contusion and lateral hemisection models; however, the application of spastin inhibitor spastazoline successfully reverses these phenomena. Taken together, these results indicate that 14-3-3 is a molecular switch that regulates spastin protein levels, and the small molecule 14-3-3 agonist FC-A effectively mediates the recovery of spinal cord injury in mice which requires spastin participation.

Is 14-3-3 the Combination to Unlock New Pathways to Improve Metabolic Homeostasis and β-Cell Function?

Since their discovery nearly five decades ago, molecular scaffolds belonging to the 14-3-3 protein family have been recognized as pleiotropic regulators of diverse cellular and physiological functions. With their ability to bind to proteins harboring specific serine and threonine phosphorylation motifs, 14-3-3 proteins can interact with and influence the function of docking proteins, enzymes, transcription factors, and transporters that have essential roles in metabolism and glucose homeostasis. Here, we will discuss the regulatory functions of 14-3-3 proteins that will be of great interest to the fields of metabolism, pancreatic β-cell biology, and diabetes. We first describe how 14-3-3 proteins play a central role in glucose and lipid homeostasis by modulating key pathways of glucose uptake, glycolysis, oxidative phosphorylation, and adipogenesis. This is followed by a discussion of the contributions of 14-3-3 proteins to calcium-dependent exocytosis and how this relates to insulin secretion from β-cells. As 14-3-3 proteins are major modulators of apoptosis and cell cycle progression, we will explore if 14-3-3 proteins represent a viable target for promoting β-cell regeneration and discuss the feasibility of targeting 14-3-3 proteins to treat metabolic diseases such as diabetes.

ARTICLE HIGHLIGHTS

14-3-3 proteins are ubiquitously expressed scaffolds with multiple roles in glucose homeostasis and metabolism. 14-3-3ζ regulates adipogenesis via distinct mechanisms and is required for postnatal adiposity and adipocyte function. 14-3-3ζ controls glucose-stimulated insulin secretion from pancreatic β-cells by regulating mitochondrial function and ATP synthesis as well as facilitating cross talk between β-cells and α-cells.

Evidence and Metabolic Implications for a New Non-Canonical Role of Cu-Zn Superoxide Dismutase

Copper-zinc superoxide dismutase 1 (SOD1) has long been recognized as a major redox enzyme in scavenging superoxide radicals. However, there is little information on its non-canonical role and metabolic implications. Using a protein complementation assay (PCA) and pull-down assay, we revealed novel protein-protein interactions (PPIs) between SOD1 and tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein zeta (YWHAZ) or epsilon (YWHAE) in this research. Through site-directed mutagenesis of SOD1, we studied the binding conditions of the two PPIs. Forming the SOD1 and YWHAE or YWHAZ protein complex enhanced enzyme activity of purified SOD1 in vitro by 40% (p < 0.05) and protein stability of over-expressed intracellular YWHAE (18%, p < 0.01) and YWHAZ (14%, p < 0.05). Functionally, these PPIs were associated with lipolysis, cell growth, and cell survival in HEK293T or HepG2 cells. In conclusion, our findings reveal two new PPIs between SOD1 and YWHAE or YWHAZ and their structural dependences, responses to redox status, mutual impacts on the enzyme function and protein degradation, and metabolic implications. Overall, our finding revealed a new unorthodox role of SOD1 and will provide novel perspectives and insights for diagnosing and treating diseases related to the protein.

14-3-3 proteins regulate cullin 7-mediated Eag1 degradation

BACKGROUND

Mutations in the human gene encoding the neuron-specific Eag1 (K_V10.1; KCNH1) potassium channel are linked to congenital neurodevelopmental diseases. Disease-causing mutant Eag1 channels manifest aberrant gating function and defective protein homeostasis. Both the E3 ubiquitin ligase cullin 7 (Cul7) and the small acid protein 14-3-3 serve as binding partners of Eag1. Cul7 mediates proteasomal and lysosomal degradation of Eag1 protein, whereas over-expression of 14-3-3 notably reduces Eag1 channel activity. It remains unclear whether 14-3-3 may also contribute to Eag1 protein homeostasis.

RESULTS

In human cell line and native rat neurons, disruptions of endogenous 14-3-3 function with the peptide inhibitor difopein or specific RNA interference up-regulated Eag1 protein level in a transcription-independent manner. Difopein hindered Eag1 protein ubiquitination at the endoplasmic reticulum and the plasma membrane, effectively promoting the stability of both immature and mature Eag1 proteins. Suppression of endogenous 14-3-3 function also reduced excitotoxicity-associated Eag1 degradation in neurons. Difopein diminished Cul7-mediated Eag1 degradation, and Cul7 knock-down abolished the effect of difopein on Eag1. Inhibition of endogenous 14-3-3 function substantially perturbed the interaction of Eag1 with Cul7. Further structural analyses suggested that the intracellular Per-Arnt-Sim (PAS) domain and cyclic nucleotide-binding homology domain (CNBHD) of Eag1 are essential for the regulatory effect of 14-3-3 proteins. Significantly, suppression of endogenous 14-3-3 function reduced Cul7-mediated degradation of disease-associated Eag1 mutant proteins.

CONCLUSION

Overall these results highlight a chaperone-like role of endogenous 14-3-3 proteins in regulating Eag1 protein homeostasis, as well as a therapeutic potential of 14-3-3 modulators in correcting defective protein expression of disease-causing Eag1 mutants.

PIM kinases phosphorylate lactate dehydrogenase A at serine 161 and suppress its nuclear ubiquitination

Lactate dehydrogenase A (LDHA) is a glycolytic enzyme catalysing the reversible conversion of pyruvate to lactate. It has been implicated as a substrate for PIM kinases, yet the relevant target sites and functional consequences of phosphorylation have remained unknown. Here, we show that all three PIM family members can phosphorylate LDHA at serine 161. When we investigated the physiological consequences of this phosphorylation in PC3 prostate cancer and MCF7 breast cancer cells, we noticed that it suppressed ubiquitin-mediated degradation of nuclear LDHA and promoted interactions between LDHA and 14-3-3 proteins. By contrast, in CRISPR/Cas9-edited knock-out cells lacking all three PIM family members, ubiquitination of nuclear LDHA was dramatically increased followed by its decreased expression. Our data suggest that PIM kinases support nuclear LDHA expression and activities by promoting phosphorylation-dependent interactions of LDHA with 14-3-3ε, which shields nuclear LDHA from ubiquitin-mediated degradation.

Serine 13 of the human cytomegalovirus viral cyclin-dependent kinase UL97 is required for regulatory protein 14-3-3 binding and UL97 stability

The human cytomegalovirus (HCMV) UL97 protein is a conserved herpesvirus protein kinase (CHPK) and a viral cyclin-dependent kinase (v-CDK). However, mechanisms regulating its activity in the context of infection are unknown. Here, we identified several cellular regulatory 14-3-3 proteins as UL97-interacting partners that promote UL97 stability. Humans are known to encode seven isoforms of 14-3-3 proteins (β, ε, η, γ, σ, θ, and ζ) that bind phosphoserines or phosphothreonines to impact protein structure, stability, activity, and localization. Our proteomic analysis of UL97 identified 49 interacting partners, including 14-3-3 isoforms β, η, and γ. Furthermore, coimmunoprecipitation with Western blotting assays demonstrated that UL97 interaction with 14-3-3 isoforms β, ε, η, γ, and θ occurs in a kinase activity-dependent manner. Using mutational analysis, we determined the serine residue at amino acid 13 of UL97 is crucial for 14-3-3 interaction. We demonstrate UL97 S13A (serine to alanine substitution at residue 13) retains kinase activity but the mutant protein accumulated at lower levels than WT UL97. Finally, we show both laboratory (AD169) and clinical (TB40/E) strains of HCMV encoding UL97 S13A replicated with WT kinetics in fibroblasts but showed decreased UL97 accumulation. Taken together, we conclude that 14-3-3 proteins interact with and stabilize UL97 during HCMV infection.

miR-34a negatively regulates cell cycle factor Cdt2/DTL in HPV infected cervical cancer cells

MicroRNAs have emerged as an important regulator of cell cycle and various other cellular processes. Aberration in microRNAs has been linked with development of several cancers and other diseases but still very little is known about the mechanism by which they regulate these cellular events. High risk human papilloma virus (HR HPV) is the causative agent of 99% of cervical cancer cases which attenuates multiple tumor suppressors and checkpoint factors of the host cell. The viral proteins also stabilize many oncogenic factors, including an essential cell cycle regulator Cdt2/DTL which in turn promotes cell transformation and proliferation. In this study, we report that a micro-RNA, miR-34a by suppressing HPV E6 protein, destabilizes Cdt2/DTL protein level in HPV infected cervical cancer cell lines. Destabilization of Cdt2 stabilizes pro-apoptotic and onco-suppressor proteins like p21 and Set8 and suppresses cell proliferation, invasion and migration capabilities of the HPV positive cervical cancer cells. Overexpression of either HPV E6 or Cdt2 genes along with miR-34a restored back the suppressed proliferation rate. This study is the first-ever report to show that miR-34a regulates cell cycle factor Cdt2 by suppressing viral E6 protein level, thus opening up the possibility of exploring miR-34a as a specific therapy for cervical cancer treatment.

Unraveling the links between neurodegeneration and Epstein-Barr virus-mediated cell cycle dysregulation

The Epstein-Barr virus is a well-known cell cycle modulator. To establish successful infection in the host, EBV alters the cell cycle at multiple steps via antigens such as EBNAs, LMPs, and certain other EBV-encoded transcripts. Interestingly, several recent studies have indicated the possibility of EBV's neurotrophic potential. However, the effects and outcomes of EBV infection in the CNS are under-explored. Additionally, more and more epidemiological evidence implicates the cell-cycle dysregulation in neurodegeneration. Numerous hypotheses which describe the triggers that force post-mitotic neurons to re-enter the cell cycle are prevalent. Apart from the known genetic and epigenetic factors responsible, several reports have shown the association of microbial infections with neurodegenerative pathology. Although, studies implicating the herpesvirus family members in neurodegeneration exist, the involvement of Epstein-Barr virus (EBV), in particular, is under-evaluated. Interestingly, a few clinical studies have reported patients of AD or PD to be seropositive for EBV. Based on the findings mentioned above, in this review, we propose that EBV infection in neurons could drive it towards neurodegeneration through dysregulation of cell-cycle events and induction of apoptosis.

Transcriptome Analysis of Near-Isogenic Lines Provides Novel Insights into Genes Associated with Seed Low-Temperature Germination Ability in Maize (Zea mays L.)

Maize originated from tropical regions and is extremely sensitive to low temperature during germination. Our previous work identified a major QTL, qp1ER1-1, for low temperature germination ability (LTGA) of maize. Here, we introgressed qp1ER1-1 from the tolerant line L220 into the sensitive line PH4CV to generate two near isogenic lines NIL^220-3 and NIL^220-25. When germinated under cold temperature for 25 days (Cold-25), the NILs showed similar seedling root length and shoot length to L220, but significantly higher than PH4CV. However, when germinated under cold temperature for 15 days (Cold-15) or under normal temperature (25 °C) for 3 days (CK-3), all lines showed similar seedling performance, indicating that introgression of qp1ER1-1 from L220 into PH4CV could improve LTGA of NIL^220-3 and NIL^220-25. The whole seedlings, including root and shoot, of Cold-15 and CK-3 were harvested for transcriptome analysis, when both stayed at a similar developmental stage. Dry seed embryo was sequenced as a non-germination control (CK-0). Compared with PH4CV, the tolerant line (L220, NIL^220-3, and NIL^220-25) specifically expressed genes (different expressed genes, DEGs) were identified for CK-0, Cold-15, and CK-3. Then, DEGs identified from Cold-15, but not from CK-0 or CK-3, were defined as tolerant line specifically expressed LTGA genes. Finally, 1786, 174, and 133 DEGs were identified as L220, NIL^220-3, and NIL^220-25 specifically expressed LTGA genes, respectively. Of them, 27 were common LTGA genes that could be identified from all three tolerant lines, with two (Zm00001d031209 and Zm00001d031292) locating in the confidence interval of qp1ER1-1. In addition, GO analysis revealed that L220 specifically expressed LTGA genes were majorly enriched in the cell division process and plasma membrane related categories. Taken together, these results provided new insight into the molecular mechanism of maize seed LTGA and facilitated the cloning of the qp1ER1-1 gene.

Exploiting Interdata Relationships in Prostate Cancer Proteomes: Clinical Significance of HO-1 Interactors

Prostate cancer (PCa) cells display abnormal expression of proteins resulting in an augmented capacity to resist chemotherapy and colonize distant organs. We have previously shown the anti-tumoral role of heme oxygenase 1 (HO-1) in this disease. In this work, we undertook a mass spectrometry-based proteomics study to identify HO-1 molecular interactors that might collaborate with its modulatory function in PCa. Among the HO-1 interactors, we identified proteins with nuclear localization. Correlation analyses, using the PCa GSE70770 dataset, showed a significant and positive correlation between HMOX1 and 6 of those genes. Alternatively, HMOX1 and YWHAZ showed a negative correlation. Univariable analyses evidenced that high expression of HNRNPA2B1, HSPB1, NPM1, DDB1, HMGA1, ZC3HAV1, and HMOX1 was associated with increased relapse-free survival (RFS) in PCa patients. Further, PCa patients with high HSPB1/HMOX1, DDB1/HMOX1, and YWHAZ/HMOX1 showed a worse RFS compared with patients with lower ratios. Moreover, a decrease in RFS for patients with higher scores of this signature was observed using a prognostic risk score model. However, the only factor significantly associated with a higher risk of relapse was high YWHAZ. Multivariable analyses confirmed HSPB1, DDB1, and YWHAZ independence from PCa clinic-pathological parameters. In parallel, co-immunoprecipitation analysis in PCa cells ascertained HO-1/14-3-3ζ/δ (protein encoded by YWHAZ) interaction. Herein, we describe a novel protein interaction between HO-1 and 14-3-3ζ/δ in PCa and highlight these factors as potential therapeutic targets.

Transcriptomic Analyses of the Adenoma-Carcinoma Sequence Identify Hallmarks Associated With the Onset of Colorectal Cancer

The concept of the adenoma-carcinoma sequence in colorectal cancer (CRC) is widely accepted. However, the relationship between the characteristics of the transcriptome and the adenoma-carcinoma sequence in CRC remains unclear. Here, the transcriptome profiles of 15 tissue samples from five CRC patients were generated by RNAseq. Six specific dynamic expression patterns of differentially expressed genes (DEGs) were generated by mFuzz. Weighted correlation network analysis showed that DEGs in cluster 4 were associated with carcinoma tissues, and those in cluster 6 were associated with non-normal tissues. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses identified metabolic dysregulation as a consistent finding throughout the transition process, whereas downregulation of the immune response occurred during normal to adenoma transition, and the upregulation of canonical pathways was associated with adenoma to carcinoma transition. Overall survival analysis of patients in cluster 6 identified TPD52L1 as a marker of poor prognosis, and cell proliferation, colony formation, wound healing, and Transwell invasion assays showed that high expression levels of TPD52L1 promoted malignant behaviors. In total, 70 proteins were identified as potential partners of hD53 by mass spectrometry. CRC formation was associated with three cancer hallmarks: dysregulation of metabolism, inactivation of the immune response, and activation of canonical cancer pathways. The TPD52L1 gene was identified as a potential marker to track tumor formation in CRC and as an indicator of poor patient prognosis.

FBXO11-mediated proteolysis of BAHD1 relieves PRC2-dependent transcriptional repression in erythropoiesis

The histone mark H3K27me3 and its reader/writer polycomb repressive complex 2 (PRC2) mediate widespread transcriptional repression in stem and progenitor cells. Mechanisms that regulate this activity are critical for hematopoietic development but are poorly understood. Here we show that the E3 ubiquitin ligase F-box only protein 11 (FBXO11) relieves PRC2-mediated repression during erythroid maturation by targeting its newly identified substrate bromo adjacent homology domain-containing 1 (BAHD1), an H3K27me3 reader that recruits transcriptional corepressors. Erythroblasts lacking FBXO11 are developmentally delayed, with reduced expression of maturation-associated genes, most of which harbor bivalent histone marks at their promoters. In FBXO11-/- erythroblasts, these gene promoters bind BAHD1 and fail to recruit the erythroid transcription factor GATA1. The BAHD1 complex interacts physically with PRC2, and depletion of either component restores FBXO11-deficient erythroid gene expression. Our studies identify BAHD1 as a novel effector of PRC2-mediated repression and reveal how a single E3 ubiquitin ligase eliminates PRC2 repression at many developmentally poised bivalent genes during erythropoiesis.

A biophysical and structural analysis of the interaction of BLNK with 14-3-3 proteins

B-cell linker protein (BLNK) is an adaptor protein that orchestrates signalling downstream of B-cell receptors. It has been reported to undergo proteasomal degradation upon binding to 14-3-3 proteins. Here, we report the first biophysical and structural study of this protein-protein interaction (PPI). Specifically, we investigated the binding of mono- and di- phosphorylated BLNK peptides to 14-3-3 using fluorescent polarization (FP) and isothermal titration calorimetry assays (ITC). Our results suggest that BLNK interacts with 14-3-3 according to the gatekeeper model, where HPK1 mediated phosphorylation of Thr152 (pT152) allows BLNK anchoring to 14-3-3, and an additional phosphorylation of Ser285 (pS285) by AKT, then further improves the affinity. Finally, we have also solved a crystal structure of the BLNKpT152 peptide bound to 14-3-3σ. These findings could serve as important tool for compound discovery programs aiming to modulate this interaction with 14-3-3.

The 14-3-3/SLP76 protein-protein interaction in T-cell receptor signalling: a structural and biophysical characterization

The SH2 domain-containing protein of 76 kDa, SLP76, is an important adaptor protein that coordinates a complex protein network downstream of T-cell receptors (TCR), ultimately regulating the immune response. Upon phosphorylation on Ser376, SLP76 interacts with 14-3-3 adaptor proteins, which leads to its proteolytic degradation. This provides a negative feedback mechanism by which TCR signalling can be controlled. To gain insight into the 14-3-3/SLP76 protein-protein interaction (PPI), we have determined a high-resolution crystal structure of a SLP76 synthetic peptide containing Ser376 with 14-3-3σ. We then characterized its binding to 14-3-3 proteins biophysically by means of fluorescence polarization and isothermal titration calorimetry. Furthermore, we generated two recombinant SLP76 protein constructs and characterized their binding to 14-3-3. Our work lays the foundation for drug design efforts aimed at targeting the 14-3-3/SLP76 interaction and, thereby, TCR signalling.

Loss of the conserved PKA sites of SIK1 and SIK2 increases sleep need

Although sleep is one of the most conserved behaviors, the intracellular mechanism regulating sleep/wakefulness remains unknown. We recently identified a protein kinase, SIK3, as a sleep-regulating molecule. Mice that lack a well-conserved protein kinase A (PKA) phosphorylation site, S551, showed longer non-rapid eye movement (NREM) sleep and increased NREMS delta density. S551 of SIK3 is conserved in other members of the SIK family, such as SIK1 (S577) and SIK2 (S587). Here, we examined whether the PKA phosphorylation sites of SIK1 and SIK2 are involved in sleep regulation by generating Sik1^S577A and Sik2^S587A mice. The homozygous Sik1^S577A mice showed a shorter wake time, longer NREMS time, and higher NREMS delta density than the wild-type mice. The heterozygous and homozygous Sik2^S587A mice showed increased NREMS delta density. Both the Sik1^S577A and Sik2^S587A mice exhibited proper homeostatic regulation of sleep need after sleep deprivation. Despite abundant expression of Sik1 in the suprachiasmatic nucleus, the Sik1^S577A mice showed normal circadian behavior. Although Sik2 is highly expressed in brown adipose tissue, the male and female Sik2^S587A mice that were fed either a chow or high-fat diet showed similar weight gain as the wild-type littermates. These results suggest that PKA-SIK signaling is involved in the regulation of sleep need.

14-3-3 scaffold proteins mediate the inactivation of trim25 and inhibition of the type I interferon response by herpesvirus deconjugases

The 14-3-3 molecular scaffolds promote type I interferon (IFN) responses by stabilizing the interaction of RIG-I with the TRIM25 ligase. Viruses have evolved unique strategies to halt this cellular response to support their replication and spread. Here, we report that the ubiquitin deconjugase (DUB) encoded in the N-terminus of the Epstein-Barr virus (EBV) large tegument protein BPLF1 harnesses 14-3-3 molecules to promote TRIM25 autoubiquitination and sequestration of the ligase into inactive protein aggregates. Catalytically inactive BPLF1 induced K48-linked autoubiquitination and degradation of TRIM25 while the ligase was mono- or di-ubiquitinated in the presence of the active viral enzyme and formed cytosolic aggregates decorated by the autophagy receptor p62/SQSTM1. Aggregate formation and the inhibition of IFN response were abolished by mutations of solvent exposed residues in helix-2 of BPLF1 that prevented binding to 14-3-3 while preserving both catalytic activity and binding to TRIM25. 14-3-3 interacted with the Coiled-Coil (CC) domain of TRIM25 in in vitro pulldown, while BPLF1 interacted with both the CC and B-box domains, suggesting that 14-3-3 positions BPLF1 at the ends of the CC dimer, close to known autoubiquitination sites. Our findings provide a molecular understanding of the mechanism by which a viral deubiquitinase inhibits the IFN response and emphasize the role of 14-3-3 proteins in modulating antiviral defenses.

14-3-3ζ inhibits heme oxygenase-1 (HO-1) degradation and promotes hepatocellular carcinoma proliferation: involvement of STAT3 signaling

Background

Heme oxygenase 1 (HO-1) has been reported to be very important in the pathogenesis or progression of multiple types of cancer. Identification of novel hmox1 binding proteins may reveal undefined oncogenes, tumor suppressors, signaling pathways, and possible treatment targets.

Methods

Immunoprecipitation and mass spectrometry analyses were used to identify novel regulators of HO-1. The association of the 14–3-3ζ protein with HO-1 and modulation of the stability of HO-1 were investigated by co-immunoprecipitation, immunofluorescence, western blotting, and quantitative RT-PCR. Degradation and in vivo ubiquitination assays were utilized to examine whether 14–3-3ζ stabilizes the HO-1 protein by inhibiting its ubiquitination. The effect of 14–3-3ζ on proliferation was investigated by function assays conducted in vitro using the CCK-8 and colony formation assays and in vivo in a xenograft mouse model. The biological functions of the 14–3-3ζ/HO-1 axis were demonstrated by western blotting and rescue experiments. Using gain-of-function and loss-of-function strategies, we further clarified the impact of 14–3-3ζ/HO-1 complex on the signal transducers and activators of transcription 3 (STAT3) signaling pathway in cancer cells.

Results

We identified 14–3-3ζ as a novel HO-1 binding protein. The binding inhibited the ubiquitination and proteasome-mediated degradation of HO-1, thus facilitating its stabilization. Enforced expression of 14–3-3ζ significantly promoted cell proliferation in vitro, as well as tumorigenesis in vivo, while 14–3-3ζ knockdown had opposite effects. The data indicated that 14–3-3ζ can stabilize HO-1 expression and thus influence cancer cell proliferation. We further demonstrated the involvement of the STAT3 pathway in 14–3-3ζ/HO-1 regulation of hepatocellular carcinoma cell proliferation.

Conclusions

Collectively, these data show that 14–3-3ζ regulates the stability of HO-1 to promote cancer cell proliferation and STAT3 signaling activation. The data establish the 14–3-3ζ-HO-1-STAT3 axis as an important regulatory mechanism of cancer cell growth and implicate HO-1 and 14–3-3ζ as potential therapeutic targets in hepatocellular carcinoma.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-1007-9) contains supplementary material, which is available to authorized users.

The Deubiquitinase USP46 Is Essential for Proliferation and Tumor Growth of HPV-Transformed Cancers

High-risk human papilloma viruses (HPVs) cause cervical, anal, and oropharyngeal cancers, unlike the low-risk HPVs, which cause benign lesions. E6 oncoproteins from the high-risk strains are essential for cell proliferation and transformation in HPV-induced cancers. We report that a cellular deubiquitinase, USP46, is selectively recruited by the E6 of high-risk, but not low-risk, HPV to deubiqutinate and stabilize Cdt2/DTL. Stabilization of Cdt2, a component of the CRL4^Cdt2 E3 ubiquitin ligase, limits the level of Set8, an epigenetic writer, and promotes cell proliferation. USP46 is essential for the proliferation of HPV-transformed cells, but not of cells without HPV. Cdt2 is elevated in human cervical cancers and knockdown of USP46 inhibits HPV-transformed tumor growth in xenografts. Recruitment of a cellular deubiquitinase to stabilize key cellular proteins is an important activity of oncogenic E6, and the importance of E6-USP46-Cdt2-Set8 pathway in HPV-induced cancers makes USP46 a target for the therapy of such cancers.

NOTCH1 regulates the viability of cholangiocarcinoma cells via 14-3-3 theta

Notch signaling has been reported to correlate with tumor progression and metastasis in several types of cancer. In cholangiocarcinoma (CCA), it has recently been shown that NOTCH1 is overexpressed in both nucleus and cytoplasm of CCA cells; however, the complete understanding of Notch signaling in CCA is still lacking. Here, we aimed to understand the functions of NOTCH1 in CCA cells and the molecular mechanisms that underlie those functions. We used retroviral vectors to overexpress active forms of NOTCH1, the NOTCH1 intracellular domain (N1ICD) and N1ICD that lacks the RBP-J-associated module (RAM), in human CCA cell lines RMCCA-1 and HuCCA-1. Our results showed that activation of Notch signaling by both N1ICD variants enhanced CCA cell proliferation and survival via upregulation of pro-survival protein Mcl-1 and Bcl-xL. Moreover, our LC-MS/MS proteomic studies demonstrated that NOTCH1 may cooperate with 14-3-3 theta to promote CCA cell survival. Knockdown of 14-3-3 theta in RMCCA-1 cells overexpressing N1ICD, diminished pro-survival effects of N1ICD under gemcitabine treatment. In conclusion, these data demonstrated that NOTCH1 plays a role in CCA cell proliferation and survival via the regulation of 14-3-3 theta in a RAM-independent fashion.

Regulation of neuronal morphogenesis by 14-3-3epsilon (Ywhae) via the microtubule binding protein, doublecortin

17p13.3 microduplication syndrome is a newly identified genetic disorder characterized by duplications in the 17p13.3 chromosome locus, resulting in a variety of disorders including autism spectrum disorder (ASD). Importantly, a minimum duplication region has been defined, and this region exclusively contains the gene encoding 14-3-3ε. Furthermore, duplication of this minimum region is strongly associated with the appearance of ASD in human patients, thus implicating the overexpression of 14-3-3ε in ASD. Using in vitro and in vivo techniques, we have found that 14-3-3ε binds to the microtubule binding protein doublecortin preventing its degradation. We also found that 14-3-3ε overexpression disrupts neurite formation by preventing the invasion of microtubules into primitive neurites, which can be rescued by the knockdown of doublecortin. To analyse the function of 14-3-3ε in neurite formation, we used 14-3-3ε flox mice and found that 14-3-3ε deficiency results in an increase in neurite formation. Our findings provide the first evidence of cellular pathology in 17p13.3 microduplication syndrome.

Mutations at multiple CDK phosphorylation consensus sites on Cdt2 increase the affinity of CRL4Cdt2 for PCNA and its ubiquitination activity in S phase

CRL4^Cdt2 ubiquitin ligase plays an important role maintaining genome integrity during the cell cycle. A recent report suggested that Cdk1 negatively regulates CRL4^Cdt2 activity through phosphorylation of its receptor, Cdt2, but the involvement of phosphorylation remains unclear. To address this, we mutated all CDK consensus phosphorylation sites located in the C-terminal half region of Cdt2 (Cdt2-18A) and examined the effect on substrate degradation. We show that both cyclinA/Cdk2 and cyclinB/Cdk1 phosphorylated Cdt2 in vitro and that phosphorylation was reduced by the 18A mutation both in vitro and in vivo. The 18A mutation increased the affinity of Cdt2 to PCNA, and a high amount of Cdt2-18A was colocalized with PCNA foci during S phase in comparison with Cdt2-WT. Poly-ubiquitination activity to Cdt1 was concomitantly enhanced in cells expressing Cdt2-18A. Other CRL4^Cdt2 substrates, Set8 and thymine DNA glycosylase, begin to accumulate around late S phase to G2 phase, but the accumulation was prevented in Cdt2-18A cells. Furthermore, mitotic degradation of Cdt1 after UV irradiation was induced in these cells. Our results suggest that CDK-mediated phosphorylation of Cdt2 inactivates its ubiquitin ligase activity by reducing its affinity to PCNA, an important strategy for regulating the levels of key proteins in the cell cycle.

Herpesvirus deconjugases inhibit the IFN response by promoting TRIM25 autoubiquitination and functional inactivation of the RIG-I signalosome

The N-terminal domains of the herpesvirus large tegument proteins encode a conserved cysteine protease with ubiquitin- and NEDD8-specific deconjugase activity. The proteins are expressed during the productive virus cycle and are incorporated into infectious virus particles, being delivered to the target cells upon primary infection. Members of this viral enzyme family were shown to regulate different aspects of the virus life cycle and the innate anti-viral response. However, only few substrates have been identified and the mechanisms of these effects remain largely unknown. In order to gain insights on the substrates and signaling pathways targeted by the viral enzymes, we have used co-immunoprecipitation and mass spectrometry to identify cellular proteins that interact with the Epstein-Barr virus encoded homologue BPLF1. Several members of the 14-3-3-family of scaffold proteins were found amongst the top hits of the BPLF1 interactome, suggesting that, through this interaction, BPLF1 may regulate a variety of cellular signaling pathways. Analysis of the shared protein-interaction network revealed that BPLF1 promotes the assembly of a tri-molecular complex including, in addition to 14-3-3, the ubiquitin ligase TRIM25 that participates in the innate immune response via ubiquitination of cytosolic pattern recognition receptor, RIG-I. The involvement of BPLF1 in the regulation of this signaling pathway was confirmed by inhibition of the type-I IFN responses in cells transfected with a catalytically active BPLF1 N-terminal domain or expressing the endogenous protein upon reactivation of the productive virus cycle. We found that the active viral enzyme promotes the dimerization and autoubiquitination of TRIM25. Upon triggering of the IFN response, RIG-I is recruited to the complex but ubiquitination is severely impaired, which functionally inactivates the RIG-I signalosome. The capacity to bind to and functionally inactivate the RIG-I signalosome is shared by the homologues encoded by other human herpesviruses.

Affinity purification mass spectrometry analysis of PD-1 uncovers SAP as a new checkpoint inhibitor

Programmed cell death-1 (PD-1) is an essential inhibitory receptor in T cells. Antibodies targeting PD-1 elicit durable clinical responses in patients with multiple tumor indications. Nevertheless, a significant proportion of patients do not respond to anti-PD-1 treatment, and a better understanding of the signaling pathways downstream of PD-1 could provide biomarkers for those whose tumors respond and new therapeutic approaches for those whose tumors do not. We used affinity purification mass spectrometry to uncover multiple proteins associated with PD-1. Among these proteins, signaling lymphocytic activation molecule-associated protein (SAP) was functionally and mechanistically analyzed for its contribution to PD-1 inhibitory responses. Silencing of SAP augmented and overexpression blocked PD-1 function. T cells from patients with X-linked lymphoproliferative disease (XLP), who lack functional SAP, were hyperresponsive to PD-1 signaling, confirming its inhibitory role downstream of PD-1. Strikingly, signaling downstream of PD-1 in purified T cell subsets did not correlate with PD-1 surface expression but was inversely correlated with intracellular SAP levels. Mechanistically, SAP opposed PD-1 function by acting as a molecular shield of key tyrosine residues that are targets for the tyrosine phosphatase SHP2, which mediates PD-1 inhibitory properties. Our results identify SAP as an inhibitor of PD-1 function and SHP2 as a potential therapeutic target in patients with XLP.

14-3-3 Proteins in Brain Development: Neurogenesis, Neuronal Migration and Neuromorphogenesis

The 14-3-3 proteins are a family of highly conserved, multifunctional proteins that are highly expressed in the brain during development. Cumulatively, the seven 14-3-3 isoforms make up approximately 1% of total soluble brain protein. Over the last decade, evidence has accumulated implicating the importance of the 14-3-3 protein family in the development of the nervous system, in particular cortical development, and have more recently been recognized as key regulators in a number of neurodevelopmental processes. In this review we will discuss the known roles of each 14-3-3 isoform in the development of the cortex, their relation to human neurodevelopmental disorders, as well as the challenges and questions that are left to be answered. In particular, we focus on the 14-3-3 isoforms and their involvement in the three key stages of cortical development; neurogenesis and differentiation, neuronal migration and neuromorphogenesis and synaptogenesis.

Methylation of 14-3-3σ gene and prognostic significance of 14-3-3σ expression in non-small cell lung cancer

Loss of 14-3-3σ expression through DNA methylation has been associated with carcinogenesis and the prognosis for various cancer types. Detection of methylation of the gene in serum may be useful for diagnostic utility. The present study aimed to investigate the correlation between 14-3-3σ methylation level in 36 paired tumor tissues of non-small cell lung cancer (NSCLC) and matched serum using methylation-specific polymerase chain reaction. The prognostic significance of 14-3-3σ expression in 167 NSCLC was also evaluated using immunohistochemistry. Methylation of the 14-3-3σ gene was identified in all samples. The methylation level in the serum (mean 87.7%, range 64.6–100%) was higher compared with tumor (mean 46.7%, range 25.3–56.3%). However, no significant correlation between methylation levels in tissues and serums was observed (Spearman's correlation, −0.036; P=0.837). In the 167 tumor tissues, the majority of the cases (83.8%) exhibited negative expression. Adenocarcinoma is more likely to exhibit negative expression (91.4%) compared with squamous cell carcinoma (70.2%). No significant difference was identified in the overall survival according to 14-3-3σ expression status and 14-3-3σ expression did not demonstrated independent prognostic significance. In conclusion, NSCLC harbors certain levels of 14-3-3σ methylation in the tumor and the sera of patients. The clinical value of serum 14-3-3σ methylation should be further elucidated. Immunohistochemical expression 14-3-3σ protein has limited value on prognostic significance.

Structural analysis of phosphatidylinositol 4-kinase IIIβ (PI4KB) - 14-3-3 protein complex reveals internal flexibility and explains 14-3-3 mediated protection from degradation in vitro

Phosphatidylinositol 4-kinase IIIβ (PI4KB) is responsible for the synthesis of the Golgi and trans-Golgi network (TGN) pool of phosphatidylinositol 4-phospahte (PI4P). PI4P is the defining lipid hallmark of Golgi and TGN and also serves as a signaling lipid and as a precursor for higher phosphoinositides. In addition, PI4KB is hijacked by many single stranded plus RNA (+RNA) viruses to generate PI4P-rich membranes that serve as viral replication organelles. Given the importance of this enzyme in cells, it has to be regulated. 14-3-3 proteins bind PI4KB upon its phosphorylation by protein kinase D, however, the structural basis of PI4KB recognition by 14-3-3 proteins is unknown. Here, we characterized the PI4KB:14-3-3 protein complex biophysically and structurally. We discovered that the PI4KB:14-3-3 protein complex is tight and is formed with 2:2 stoichiometry. Surprisingly, the enzymatic activity of PI4KB is not directly modulated by 14-3-3 proteins. However, 14-3-3 proteins protect PI4KB from proteolytic degradation in vitro. Our structural analysis revealed that the PI4KB:14-3-3 protein complex is flexible but mostly within the disordered regions connecting the 14-3-3 binding site of the PI4KB with the rest of the PI4KB enzyme. It also predicted no direct modulation of PI4KB enzymatic activity by 14-3-3 proteins and that 14-3-3 binding will not interfere with PI4KB recruitment to the membrane by the ACBD3 protein. In addition, the structural analysis explains the observed protection from degradation; it revealed that several disordered regions of PI4KB become protected from proteolytical degradation upon 14-3-3 binding. All the structural predictions were subsequently biochemically validated.

The functional significance of 14-3-3 proteins in cancer: focus on lung cancer

The 14-3-3 family proteins are phosphoserine/phosphothreonine binding proteins constituting a conserved class of proteins which are detected in all eukaryotic cells. In mammalians, 14-3-3 proteins have seven distinct isoforms (β, γ, ε, η, ζ, σ and τ/θ) which are involved in various cellular processes including signal transduction, cell cycle, cell proliferation, apoptosis, differentiation and survival. 14-3-3 proteins do not have a distinct catalytic activity and often regulate the activity, stability, subcellular localization and interactions of other proteins. The 14-3-3 family proteins function through interacting with their client proteins or facilitating the interaction of other proteins likely as adaptor proteins. The versatile functions of these proteins in the regulation of cell growth, cell division, cell death and cell migration make them candidate proteins for which an important role in cancer development could be envisioned. Indeed, analysis of cancer cell lines and tumor-derived tissues have indicated the differential abundance or post-translational modification of some 14-3-3 isoforms. In this review, we aimed to show how deregulation of 14-3-3 proteins contributes to initiation, establishment and progression of cancers with a particular emphasis on lung cancer. The role of these proteins in cancer-relevant processes including cell cycle, cell migration, cell-cell communication and programmed cell death will be discussed in detail.

An account of fungal 14-3-3 proteins

14-3-3s are a group of relatively low molecular weight, acidic, dimeric, protein(s) conserved from single-celled yeast to multicellular vertebrates including humans. Despite lacking catalytic activity, these proteins have been shown to be involved in multiple cellular processes. Apart from their role in normal cellular physiology, recently these proteins have been implicated in various medical consequences. In this present review, fungal 14-3-3 protein localization, interactions, transcription, regulation, their role in the diverse cellular process including DNA duplication, cell cycle, protein trafficking or secretion, apoptosis, autophagy, cell viability under stress, gene expression, spindle positioning, role in carbon metabolism have been discussed. In the end, I also highlighted various roles of yeasts 14-3-3 proteins in tabular form. Thus this review with primary emphasis on yeast will help in appreciating the significance of 14-3-3 proteins in cell physiology.

p70S6K1 (S6K1)-mediated Phosphorylation Regulates Phosphatidylinositol 4-Phosphate 5-Kinase Type I γ Degradation and Cell Invasion

Phosphatidylinositol 4-phosphate 5-kinase type I γ (PIPKIγ90) ubiquitination and subsequent degradation regulate focal adhesion assembly, cell migration, and invasion. However, it is unknown how upstream signals control PIPKIγ90 ubiquitination or degradation. Here we show that p70S6K1 (S6K1), a downstream target of mechanistic target of rapamycin (mTOR), phosphorylates PIPKIγ90 at Thr-553 and Ser-555 and that S6K1-mediated PIPKIγ90 phosphorylation is essential for cell migration and invasion. Moreover, PIPKIγ90 phosphorylation is required for the development of focal adhesions and invadopodia, key machineries for cell migration and invasion. Surprisingly, substitution of Thr-553 and Ser-555 with Ala promoted PIPKIγ90 ubiquitination but enhanced the stability of PIPKIγ90, and depletion of S6K1 also enhanced the stability of PIPKIγ90, indicating that PIPKIγ90 ubiquitination alone is insufficient for its degradation. These data suggest that S6K1-mediated PIPKIγ90 phosphorylation regulates cell migration and invasion by controlling PIPKIγ90 degradation.

Recombinant protein of Haemonchus contortus 14-3-3 isoform 2 (rHcftt-2) decreased the production of IL-4 and suppressed the proliferation of goat PBMCs in vitro

14-3-3 proteins have been found to be an excreted/secreted antigen and assumed to be released into the host-parasite interface and described in several unicellular and multicellular parasites. However, little is known about the immunomodulatory effects of H. controtus 14-3-3 protein on host cell. In present study, 14-3-3 isoform 2 gene, designated as Hcftt-2, was amplified by reverse transcription-polymerase chain reaction (RT-PCR) from the adult H. contortus cDNA and cloned into expression plasmid pET32a (+) and expression of the recombinant protein (rHcftt-2) was induced by IPTG. Binding activity of rHcftt-2 to goat peripheral blood mononuclear cells (PBMCs) was confirmed by immunofluorescence assay (IFA) and modulatory effects on cytokine production, cell proliferation, cell migration and nitric oxide (NO) production were observed by co-incubation of rHcftt-2 with goat PBMCs. Sequence analysis showed that it had significant homology with the known 14-3-3 protein isoform 2. Results of IFA revealed that, the rHcftt-2 was bound to the cell surface. We found that, the productions of IL10, IL-17, IFN-γ and cell migration of PBMCs were increased after the cells were incubated with rHCftt-2. However, the productions of IL-4, NO and cell proliferation of the PBMCs were significantly decreased in dose depended manner. Our results showed that the Hcftt-2 played important suppressive regulatory effects on the goat PBMCs.

Integrate Omics Data and Molecular Dynamics Simulations toward Better Understanding of Human 14-3-3 Interactomes and Better Drugs for Cancer Therapy

The 14-3-3 protein family is among the most extensively studied, yet still largely mysterious protein families in mammals to date. As they are well recognized for their roles in apoptosis, cell cycle regulation, and proliferation in healthy cells, aberrant 14-3-3 expression has unsurprisingly emerged as instrumental in the development of many cancers and in prognosis. Interestingly, while the seven known 14-3-3 isoforms in humans have many similar functions across cell types, evidence of isoform-specific functions and localization has been observed in both healthy and diseased cells. The strikingly high similarity among 14-3-3 isoforms has made it difficult to delineate isoform-specific functions and for isoform-specific targeting. Here, we review our knowledge of 14-3-3 interactome(s) generated by high-throughput techniques, bioinformatics, structural genomics and chemical genomics and point out that integrating the information with molecular dynamics (MD) simulations may bring us new opportunity to the design of isoform-specific inhibitors, which can not only be used as powerful research tools for delineating distinct interactomes of individual 14-3-3 isoforms, but also can serve as potential new anti-cancer drugs that selectively target aberrant 14-3-3 isoform.

The antineoplastic properties of FTY720: evidence for the repurposing of fingolimod

Almost all drugs approved for use in humans possess potentially beneficial 'off-target' effects in addition to their principal activity. In some cases this has allowed for the relatively rapid repurposing of drugs for other indications. In this review we focus on the potential for re-purposing FTY720 (also known as fingolimod, Gilenya(™)), an immunomodulatory drug recently approved for the treatment of multiple sclerosis (MS). The therapeutic benefit of FTY720 in MS is largely attributed to the immunosuppressive effects that result from its modulation of sphingosine 1-phosphate receptor signalling. However, this drug has also been shown to inhibit other cancer-associated signal transduction pathways in part because of its structural similarity to sphingosine, and consequently shows efficacy as an anti-cancer agent both in vitro and in vivo. Here, we review the effects of FTY720 on signal transduction pathways and cancer-related cellular processes, and discuss its potential use as an anti-cancer drug.

Regulation of Unperturbed DNA Replication by Ubiquitylation

Posttranslational modification of proteins by means of attachment of a small globular protein ubiquitin (i.e., ubiquitylation) represents one of the most abundant and versatile mechanisms of protein regulation employed by eukaryotic cells. Ubiquitylation influences almost every cellular process and its key role in coordination of the DNA damage response is well established. In this review we focus, however, on the ways ubiquitylation controls the process of unperturbed DNA replication. We summarise the accumulated knowledge showing the leading role of ubiquitin driven protein degradation in setting up conditions favourable for replication origin licensing and S-phase entry. Importantly, we also present the emerging major role of ubiquitylation in coordination of the active DNA replication process: preventing re-replication, regulating the progression of DNA replication forks, chromatin re-establishment and disassembly of the replisome at the termination of replication forks.

CDK1-dependent inhibition of the E3 ubiquitin ligase CRL4CDT2 ensures robust transition from S Phase to Mitosis

Replication-coupled destruction of a cohort of cell cycle proteins ensures efficient and precise genome duplication. Three proteins destroyed during replication via the CRL4(CDT2) ubiquitin E3 ligase, CDT1, p21, and SET8 (PR-SET7), are also essential or important during mitosis, making their reaccumulation after S phase a critical cell cycle event. During early and mid-S phase and during DNA repair, proliferating cell nuclear antigen (PCNA) loading onto DNA (PCNA(DNA)) triggers the interaction between CRL4(CDT2) and its substrates, resulting in their degradation. We have discovered that, beginning in late S phase, PCNA(DNA) is no longer sufficient to trigger CRL4(CDT2)-mediated degradation. A CDK1-dependent mechanism that blocks CRL4(CDT2) activity by interfering with CDT2 recruitment to chromatin actively protects CRL4(CDT2) substrates. We postulate that deliberate override of replication-coupled destruction allows anticipatory accumulation in late S phase. We further show that (as for CDT1) de novo SET8 reaccumulation is important for normal mitotic progression. In this manner, CDK1-dependent CRL4(CDT2) inactivation contributes to efficient transition from S phase to mitosis.