Role of CCT chaperonin in the disassembly of mitotic checkpoint complexes

Epsilon subunit of T-complex protein-1 from Leishmania donovani: A tetrameric chaperonin

The cytosolic T-complex protein-1 ring complex (TRiC), also referred as chaperonin containing TCP-1(CCT), comprising eight different subunits stacked in double toroidal rings, binds to around 10 % of newly synthesized polypeptides and facilitates their folding in ATP dependent manner. In Leishmania, among five subunits of TCP1 complex, identified either by transcriptome or by proteome analysis, only LdTCP1γ has been well characterized. It forms biologically active homo-oligomeric complex and plays role in protein folding and parasite survival. Lack of information regarding rest of the TCP1 subunits and its structural configuration laid down the necessity to study individual subunits and their role in parasite pathogenicity. The present study involves the cloning, expression and biochemical characterization of TCP1ε subunit (LdTCP1ε) of Leishmania donovani, the causative agent of visceral leishmaniasis. LdTCP1ε exhibited significant difference in primary structure as compared to LdTCP1γ and was evolutionary close to LdTCP1 zeta subunit. Recombinant protein (rLdTCP1ε) exhibited two major bands of 132 kDa and 240 kDa on native-PAGE that corresponds to the dimeric and tetrameric assembly of the epsilon subunit, which showed the chaperonin activity (ATPase and luciferase refolding activity). LdTCP1ε also displayed an increased expression upto 2.7- and 1.8-fold in the late log phase and stationary phase promastigotes and exhibited majorly vesicular localization. The study, thus for the first time, provides an insight for the presence of highly diverge but functionally active dimeric/tetrameric TCP1 epsilon subunit in Leishmania parasite.

Role of TRIP13 in human cancer development

As an AAA + ATPase, thyroid hormone receptor interacting protein 13 (TRIP13) primarily functions in DNA double-strand break repair, chromosome recombination, and cell cycle checkpoint regulation; aberrant expression of TRIP13 can result in chromosomal instability (CIN). According to recent research, TRIP13 is aberrantly expressed in a variety of cancers, and a patient's poor prognosis and tumor stage are strongly correlated with high expression of TRIP13. Tumor cell and subcutaneous xenograft growth can be markedly inhibited by TRIP13 knockdown or TRIP13 inhibitor administration. In the initiation and advancement of human malignancies, TRIP13 seems to function as an oncogene. Based on available data, TRIP13 may function as a biological target and biomarker for cancer. The creation of inhibitors that specifically target TRIP13 may present novel approaches to treating cancer.

TRIP13 - a potential drug target in cancer pharmacotherapy

ATPases Associated with Diverse Cellular Activity (AAA+ATPases) are important enzymatic functional proteins in human cells. Thyroid Hormone Receptor Interacting Protein-13 (TRIP13) is a member of this protein superfamily, that partly regulates DNA repair pathways and spindle assembly checkpoints during mitosis. TRIP13 is reported as an oncogene involving multiple pathways in many human malignancies, including multiple myeloma, brain tumors, etc. The structure of TRIP13 reveals the mechanisms for ATP binding and how TRIP13 recognizes the Mitotic Arrest Deficiency-2 (MAD2) protein, with p31comet acting as an adapter protein. DCZ0415, TI17, DCZ5417, and DCZ5418 are the reported small-molecule inhibitors of TRIP13, which have been demonstrated to inhibit TRIP13's biological functions significantly and effective in suppressing various types of malignant cells, indicating that TRIP13 is a significant anticancer drug target. Currently, no systematic reviews are cutting across the functions, structure, and novel inhibitors of TRIP13. This review provides a comprehensive overview of TRIP13's biological functions, its roles in eighteen different cancers, four small molecule inhibitors, different underlying molecular mechanisms, and its functionality as a potential anticancer drug target.

The CCTδ subunit of the molecular chaperone CCT is required for correct localisation of p150Glued to spindle poles during mitosis

Chaperonin Containing Tailless complex polypeptide 1 (CCT) is a molecular chaperone composed of eight distinct subunits that can exist as individual monomers or as components of a double oligomeric ring, which is essential for the folding of actin and tubulin and other substrates. Here we assess the role of CCT subunits in the context of cell cycle progression by individual subunit depletions upon siRNA treatment in mammalian cells. The depletion of individual CCT subunits leads to variation in the distribution of cell cycle phases and changes in mitotic index. Mitotic defects, such as unaligned chromosomes occur when CCTδ is depleted, concurrent with a reduction in spindle pole-localised p150Glued, a component of the dynactin complex and a binding partner of monomeric CCTδ. In CCTδ-depleted cells, changes in the elution profile of p150Glued are observed consistent with altered conformations and or assembly states with the dynactin complex. Addition of monomeric CCTδ, in the form of GFP-CCTδ, restores correct p150Glued localisation to the spindle poles and rescues the mitotic segregation defects that occur when CCTδ is depleted. This study demonstrates a requirement for CCTδ in its monomeric form for correct chromosome segregation via a mechanism that promotes the correct localisation of p150Glued, thus revealing further complexities to the interplay between CCT, tubulin folding and microtubule dynamics.

Revisiting the chaperonin T-complex protein-1 ring complex in human health and disease: A proteostasis modulator and beyond

BACKGROUND

Disrupted protein homeostasis (proteostasis) has been demonstrated to facilitate the progression of various diseases. The cytosolic T-complex protein-1 ring complex (TRiC/CCT) was discovered to be a critical player in orchestrating proteostasis by folding eukaryotic proteins, guiding intracellular localisation and suppressing protein aggregation. Intensive investigations of TRiC/CCT in different fields have improved the understanding of its role and molecular mechanism in multiple physiological and pathological processes.

MAIN BODY

In this review, we embark on a journey through the dynamic protein folding cycle of TRiC/CCT, unraveling the intricate mechanisms of its substrate selection, recognition, and intriguing folding and assembly processes. In addition to discussing the critical role of TRiC/CCT in maintaining proteostasis, we detail its involvement in cell cycle regulation, apoptosis, autophagy, metabolic control, adaptive immunity and signal transduction processes. Furthermore, we meticulously catalogue a compendium of TRiC-associated diseases, such as neuropathies, cardiovascular diseases and various malignancies. Specifically, we report the roles and molecular mechanisms of TRiC/CCT in regulating cancer formation and progression. Finally, we discuss unresolved issues in TRiC/CCT research, highlighting the efforts required for translation to clinical applications, such as diagnosis and treatment.

CONCLUSION

This review aims to provide a comprehensive view of TRiC/CCT for researchers to inspire further investigations and explorations of potential translational possibilities.

Red blood cell proteomics reveal remnant protein biosynthesis and folding pathways in PIEZO1-related hereditary xerocytosis

Hereditary xerocytosis is a dominant red cell membrane disorder characterized by an increased leak of potassium from the inside to outside the red blood cell membrane, associated with loss of water leading to red cell dehydration and chronic hemolysis. 90% of cases are related to heterozygous gain of function mutations in PIEZO1, encoding a mechanotransductor that translates a mechanical stimulus into a biological signaling. Data are still required to understand better PIEZO1-HX pathophysiology. Recent studies identified proteomics as an accurate and high-input tool to study erythroid progenitors and circulating red cell physiology. Here, we isolated red blood cells from 5 controls and 5 HX patients carrying an identified and pathogenic PIEZO1 mutation and performed a comparative deep proteomic analysis. A total of 603 proteins were identified among which 56 were differentially expressed (40 over expressed and 16 under expressed) between controls and HX with a homogenous expression profile within each group. We observed relevant modifications in the protein expression profile related to PIEZO1 mutations, identifying two main "knots". The first contained both proteins of the chaperonin containing TCP1 complex involved in the assembly of unfolded proteins, and proteins involved in translation. The second contained proteins involved in ubiquitination. Deregulation of proteins involved in protein biosynthesis was also observed in in vitro-produced reticulocytes after Yoda1 exposure. Thus, our work identifies significant changes in the protein content of PIEZO1-HX erythrocytes, revealing a "PIEZO1 signature" and identifying potentially targetable pathways in this disease characterized by a heterogeneous clinical expression and contra-indication of splenectomy.

The I.3.2 developmental mutant has a single nucleotide deletion in the gene centromere identifier

The mutation I.3.2 was previously identified in a FLP/FRT screen of chromosome 2R for conditional growth regulators. Here we report the phenotypic characterization and genetic mapping of I.3.2 by undergraduate students participating in Fly-CURE, a pedagogical program that teaches the science of genetics through a classroom research experience. We find that creation of I.3.2 cell clones in the developing eye-antennal imaginal disc causes a headless adult phenotype, suggestive of both autonomous and non-autonomous effects on cell growth or viability. We also identify the I.3.2 mutation as a loss-of-function allele of the gene centromere identifier ( cid ), which encodes centromere-specific histone H3 variant critical for chromosomal segregation.

The TRiCky Business of Protein Folding in Health and Disease

Maintenance of the cellular proteome or proteostasis is an essential process that when deregulated leads to diseases like neurological disorders and cancer. Central to proteostasis are the molecular chaperones that fold proteins into functional 3-dimensional (3D) shapes and prevent protein aggregation. Chaperonins, a family of chaperones found in all lineages of organisms, are efficient machines that fold proteins within central cavities. The eukaryotic Chaperonin Containing TCP1 (CCT), also known as Tailless complex polypeptide 1 (TCP-1) Ring Complex (TRiC), is a multi-subunit molecular complex that folds the obligate substrates, actin, and tubulin. But more than folding cytoskeletal proteins, CCT differs from most chaperones in its ability to fold proteins larger than its central folding chamber and in a sequential manner that enables it to tackle proteins with complex topologies or very large proteins and complexes. Unique features of CCT include an asymmetry of charges and ATP affinities across the eight subunits that form the hetero-oligomeric complex. Variable substrate binding capacities endow CCT with a plasticity that developed as the chaperonin evolved with eukaryotes and acquired functional capacity in the densely packed intracellular environment. Given the decades of discovery on the structure and function of CCT, much remains unknown such as the scope of its interactome. New findings on the role of CCT in disease, and potential for diagnostic and therapeutic uses, heighten the need to better understand the function of this essential molecular chaperone. Clues as to how CCT causes cancer or neurological disorders lie in the early studies of the chaperonin that form a foundational knowledgebase. In this review, we span the decades of CCT discoveries to provide critical context to the continued research on the diverse capacities in health and disease of this essential protein-folding complex.

Role of ubiquitin-protein ligase UBR5 in the disassembly of mitotic checkpoint complexes

The mitotic checkpoint system is essential for the prevention of mistakes in the segregation of chromosomes in mitosis. As long as chromosomes are not attached correctly to the mitotic spindle, a mitotic checkpoint complex (MCC) is assembled and inhibits the action of ubiquitin ligase APC/C (anaphase-promoting complex/cyclosome) to initiate anaphase. When the checkpoint is turned off, MCC is disassembled, allowing anaphase initiation. The mechanisms of MCC disassembly have been studied, but the regulation of this process remained obscure. We found that a second ubiquitin ligase, UBR5 (ubiquitin-protein ligase N-recognin 5), ubiquitylates MCC components and stimulates the disassembly of MCC from APC/C, as well as the dissociation of a subcomplex of MCC.

The mitotic (or spindle assembly) checkpoint system ensures accurate chromosome segregation in mitosis by preventing the onset of anaphase until correct bipolar attachment of sister chromosomes to the mitotic spindle is attained. It acts by promoting the assembly of a mitotic checkpoint complex (MCC), composed of mitotic checkpoint proteins BubR1, Bub3, Mad2, and Cdc20. MCC binds to and inhibits the action of ubiquitin ligase APC/C (anaphase-promoting complex/cyclosome), which targets for degradation regulators of anaphase initiation. When the checkpoint system is satisfied, MCCs are disassembled, allowing the recovery of APC/C activity and initiation of anaphase. Many of the pathways of the disassembly of the different MCCs have been elucidated, but the mode of their regulation remained unknown. We find that UBR5 (ubiquitin-protein ligase N-recognin 5) is associated with the APC/C*MCC complex immunopurified from extracts of nocodazole-arrested HeLa cells. UBR5 binds to mitotic checkpoint proteins BubR1, Bub3, and Cdc20 and promotes their polyubiquitylation in vitro. The dissociation of a Bub3*BubR1 subcomplex of MCC is stimulated by UBR5-dependent ubiquitylation, as suggested by observations that this process in mitotic extracts requires UBR5 and α−β bond hydrolysis of adenosine triphosphate. Furthermore, a system reconstituted from purified recombinant components carries out UBR5- and ubiquitylation-dependent dissociation of Bub3*BubR1. Immunodepletion of UBR5 from mitotic extracts slows down the release of MCC components from APC/C and prolongs the lag period in the recovery of APC/C activity in the exit from mitotic checkpoint arrest. We suggest that UBR5 may be involved in the regulation of the inactivation of the mitotic checkpoint.

CCT4 suppression inhibits tumor growth in hepatocellular carcinoma by interacting with Cdc20

BACKGROUND

The chaperonin containing t-complex (CCT) proteins play an important role in cell cycle-related protein degradation in yeast and mammals. The role of the chaperonin containing t-complex 4 (CCT4), one subtype of CCT proteins, in the progress of hepatocellular carcinoma (HCC) was not fully elucidated. Here, we aimed to explore the mechanisms of CCT4 in HCC.

METHODS

In this study, we used the UALCAN platform to analyze the relationship between CCT4 and HCC, and the association of CCT4 with the overall survival (OS) of HCC patients was also analyzed. CCT4 expression in HCC tumor tissues and normal tissues was also determined by western blot (WB) assay. Lentivirus vector was used to knock down the CCT4 expression, and quantitative polymerase chain reaction and WB were used to determine the level of CCT4 in HCC cell lines. Cell counting kit-8 (CCK-8) and 5-ethynyl-2'-deoxyuridine (EdU) assays were used to detect the cell proliferation, and flow cytometry (FCM) was performed to evaluate the effect of CCT4 on the apoptosis of HCC cells. Co-immunoprecipitation (co-IP) assay and WB were used to explore the mechanisms of CCT4 regulating the growth of HCC. Data were calculated from at least three replicate experiments and expressed as mean ± standard deviation. Student's t test, paired t test, and Kaplan-Meier analysis were used to compare across different groups.

RESULTS

We found CCT4 was upregulated in HCC tissues compared with normal tissues, and its high expression was associated with poor prognosis (P < 0.001). CCT4 was significantly increased in HCC tumor tissues compared with normal tissues (0.98 ± 0.12 vs. 0.23 ± 0.05, t = 7.73, P < 0.001). After being transfected with CCT4 short-hairpin RNA (shRNA), CCT4 was decreased in mRNA level and protein level in both Huh7 (mRNA level: 0.41 ± 0.07 vs. 1.01 ± 0.11, t = 8.09, P = 0.001; protein level: 0.61 ± 0.03 vs. 0.93 ± 0.07, t = 7.19, P = 0.002) and Hep3b cells (mRNA level: 0.55 ± 0.11 vs. 1.04 ± 0.15, t = 4.51, P = 0.011; protein level: 0.64 ± 0.10 vs. 0.95 ± 0.08, t = 4.32, P = 0.012). CCK8 assay indicated that CCT4 knockdown inhibited cell proliferation in both Huh7 (OD value of 3 days: 0.60 ± 0.14 vs. 0.97 ± 0.16, t = 3.13, P = 0.036; OD value of 4 days: 1.03 ± 0.07 vs. 1.50 ± 0.12, t = 5.97, P = 0.004) and Hep3b (OD value of 3 days: 0.69 ± 0.14 vs. 1.10 ± 0.11, t = 3.91, P = 0.017; OD value of 4 days: 1.12 ± 0.12 vs. 1.48 ± 0.13, t = 3.55, P = 0.024) cells. EdU assay showed that CCT4 knockdown inhibited the cell proliferation in both Huh7 (EdU positive rate: [31.25 ± 3.41]% vs. [58.72 ± 3.78]%, t = 9.34, P = 0.001) and Hep3b cells (EdU positive rate: [44.13 ± 7.02]% vs. [61.79 ± 3.96]%, t = 3.79, P = 0.019). FCM assay suggested that CCT4 knockdown induced apoptosis in HCC cells (apoptosis rate of Huh7: [9.10 ± 0.80]% vs. [3.66 ± 0.64]%, t = -9.18, P = 0.001; apoptosis rate of Hep3b: [6.69 ± 0.72]% vs. [4.20 ± 0.86]%, t = -3.84, P = 0.018). We also found that CCT4 could regulate anaphase-promoting complex (APC)Cdc20 activity via interacting with Cdc20. Furthermore, CCT4 knockdown induced securin (0.65 ± 0.06 vs. 0.44 ± 0.05, t = -4.69, P = 0.009) and B-cell lymphoma-2 (Bcl-2) interacting mediator of cell death (Bim; 0.96 ± 0.06 vs. 0.61 ± 0.09, t = -5.65, P = 0.005) accumulation. The upregulation of securin inhibited cell growth by downregulating cyclin D1 (0.65 ± 0.05 vs. 1.04 ± 0.07, t = 8.12, P = 0.001), and the accumulation of Bim inhibited Bcl-2 (0.77 ± 0.04 vs. 0.87 ± 0.04, t = 3.00, P = 0.040) and activated caspase 9 (caspase 9: 0.77 ± 0.04 vs. 0.84 ± 0.05, t = 1.81, P = 0.145; cleaved caspase 9: 0.64 ± 0.06 vs. 0.16 ± 0.07, t = 1.81, P = 0.001), which led to elevated apoptosis.

CONCLUSIONS

Overall, these results showed that CCT4 played an important role in HCC pathogenesis through, at least partly, interacting with Cdc20.

Polycomb Requires Chaperonin Containing TCP-1 Subunit 7 for Maintaining Gene Silencing in Drosophila

In metazoans, heritable states of cell type-specific gene expression patterns linked with specialization of various cell types constitute transcriptional cellular memory. Evolutionarily conserved Polycomb group (PcG) and trithorax group (trxG) proteins contribute to the transcriptional cellular memory by maintaining heritable patterns of repressed and active expression states, respectively. Although chromatin structure and modifications appear to play a fundamental role in maintenance of repression by PcG, the precise targeting mechanism and the specificity factors that bind PcG complexes to defined regions in chromosomes remain elusive. Here, we report a serendipitous discovery that uncovers an interplay between Polycomb (Pc) and chaperonin containing T-complex protein 1 (TCP-1) subunit 7 (CCT7) of TCP-1 ring complex (TRiC) chaperonin in Drosophila. CCT7 interacts with Pc at chromatin to maintain repressed states of homeotic and non-homeotic targets of PcG, which supports a strong genetic interaction observed between Pc and CCT7 mutants. Depletion of CCT7 results in dissociation of Pc from chromatin and redistribution of an abundant amount of Pc in cytoplasm. We propose that CCT7 is an important modulator of Pc, which helps Pc recruitment at chromatin, and compromising CCT7 can directly influence an evolutionary conserved epigenetic network that supervises the appropriate cellular identities during development and homeostasis of an organism.

Novel Binding Partners for CCT and PhLP1 Suggest a Common Folding Mechanism for WD40 Proteins with a 7-Bladed Beta-Propeller Structure

This study investigates whether selected WD40 proteins with a 7-bladed β-propeller structure, similar to that of the β subunit of the G protein heterotrimer, interact with the cytosolic chaperonin CCT and its known binding partner, PhLP1. Previous studies have shown that CCT is required for the folding of the Gβ subunit and other WD40 proteins. The role of PhLP1 in the folding of Gβ has also been established, but it is unknown if PhLP1 assists in the folding of other Gβ-like proteins. The binding of three Gβ-like proteins, TBL2, MLST8 and CDC20, to CCT and PhLP1, was demonstrated in this study. Co-immunoprecipitation assays identified one novel binding partner for CCT and three new interactors for PhLP1. All three of the studied proteins interact with CCT and PhLP1, suggesting that these proteins may have a folding machinery in common with that of Gβ and that the well-established Gβ folding mechanism may have significantly broader biological implications than previously thought. These findings contribute to continuous efforts to determine common traits and unique differences in the folding mechanism of the WD40 β-propeller protein family, and the role PhLP1 has in this process.

Interaction with the CCT chaperonin complex limits APOBEC3A cytidine deaminase cytotoxicity

The APOBEC3 cytidine deaminases are implicated as the cause of a prevalent somatic mutation pattern found in cancer genomes. The APOBEC3 enzymes act as viral restriction factors by mutating viral genomes. Mutation of the cellular genome is presumed to be an off-target activity of the enzymes, although the regulatory measures for APOBEC3 expression and activity remain undefined. It is therefore difficult to predict circumstances that enable APOBEC3 interaction with cellular DNA that leads to mutagenesis. The APOBEC3A (A3A) enzyme is the most potent deaminase of the family. Using proteomics, we evaluate protein interactors of A3A to identify potential regulators. We find that A3A interacts with the chaperonin-containing TCP-1 (CCT) complex, a cellular machine that assists in protein folding and function. Importantly, depletion of CCT results in A3A-induced DNA damage and cytotoxicity. Evaluation of cancer genomes demonstrates an enrichment of A3A mutational signatures in cancers with silencing mutations in CCT subunit genes. Together, these data suggest that the CCT complex interacts with A3A, and that disruption of CCT function results in increased A3A mutational activity.

Chaperonin point mutation enhances cadmium endurance in Saccharomyces cerevisiae

OBJECTIVE

To study the effect of the mutation in conserved G412E in Cct7p subunit of CCT complex on its cellular fate.

RESULTS

TriC/CCT is a dynamic multimeric protein that assists in protein folding in an energy-dependent manner. A point mutation in the ATP binding pocket in the equatorial domain of the Cct7p subunit delays the doubling time. The cell size was twice the wild type, and the formation of protein aggregates suggests disturbed folding of the proteins. Upon growing in stressful conditions of arsenous acid and cadmium chloride, the mutant was lethal in As³⁺ but grew well in Cd²⁺ with 10.5 µg cadmium uptake mg^-1 compared to the wild type. The increased expression of vacuole transporters YCF1 and BPT1 by ten-fold and two-fold in mutant indicates the metal transportation to the vacuole.

CONCLUSION

CCT complex was vulnerable to the mutation in G412E in the Cct7p subunit of protein folding molecular machinery. Interestingly, already stressed cells provided robustness against oxidative stress and cadmium sequestration in the vacuole.

Intricate Regulatory Mechanisms of the Anaphase-Promoting Complex/Cyclosome and Its Role in Chromatin Regulation

The ubiquitin (Ub)-proteasome system is vital to nearly every biological process in eukaryotes. Specifically, the conjugation of Ub to target proteins by Ub ligases, such as the Anaphase-Promoting Complex/Cyclosome (APC/C), is paramount for cell cycle transitions as it leads to the irreversible destruction of cell cycle regulators by the proteasome. Through this activity, the RING Ub ligase APC/C governs mitosis, G1, and numerous aspects of neurobiology. Pioneering cryo-EM, biochemical reconstitution, and cell-based studies have illuminated many aspects of the conformational dynamics of this large, multi-subunit complex and the sophisticated regulation of APC/C function. More recent studies have revealed new mechanisms that selectively dictate APC/C activity and explore additional pathways that are controlled by APC/C-mediated ubiquitination, including an intimate relationship with chromatin regulation. These tasks go beyond the traditional cell cycle role historically ascribed to the APC/C. Here, we review these novel findings, examine the mechanistic implications of APC/C regulation, and discuss the role of the APC/C in previously unappreciated signaling pathways.

Human Papillomavirus infection requires the CCT Chaperonin Complex

Human papillomavirus (HPV) infection is a multi-step process that implies complex interactions of the viral particles with cellular proteins. The HPV capsid includes the two structural proteins L1 and L2, that play crucial roles on infectious viral entry. L2 is particularly relevant for the intracellular trafficking of the viral DNA towards the nucleus. Here, using proteomic studies we identified CCT proteins as novel interaction partners of HPV-16 L2. The CCT multimeric complex is an essential chaperonin which interacts with a large number of protein targets. We analysed the binding of different components of the CCT complex to L2. We confirmed the interaction of this structural viral protein with the CCT subunit 3 (CCT3) and we found that this interaction requires the N-terminal region of L2. Defects in HPV-16 pseudoviral particle (PsVs) infection were revealed by siRNA-mediated knockdown of some CCT subunits. While a substantial drop in the viral infection was associated with the ablation of CCT component 2, even more pronounced effects on infectivity were observed upon depletion of CCT component 3. Using confocal immunofluorescence assays, CCT3 co-localised with HPV PsVs at early times after infection, with L2 being required for this to occur. Further analysis showed the colocalization of several other subunits of CCT with the PsVs. Moreover, we observed a defect in capsid uncoating and a change in PsVs intracellular normal processing when ablating CCT3. Taken together, these studies demonstrate the importance of CCT chaperonin during HPV infectious entry.ImportanceSeveral of the mechanisms that function during the infection of target cells by HPV particles have been previously described. However, many aspects of this process remain unknown. In particular, the role of cellular proteins functioning as molecular chaperones during HPV infections has been only partially investigated. To the best of our knowledge, we describe here for the first time, a requirement of the CCT chaperonin for HPV infection. The role of this cellular complex seems to be determined by the binding of its component 3 to the viral structural protein L2. However, CCT's effect on HPV infection most probably comprises the whole chaperonin complex. Altogether, these studies define an important role for the CCT chaperonin in the processing and intracellular trafficking of HPV particles and in subsequent viral infectious entry.

Gene Co-Expression Analysis of Human RNASEH2A Reveals Functional Networks Associated with DNA Replication, DNA Damage Response, and Cell Cycle Regulation

Simple Summary

RNASEH2A is the catalytic subunit of the ribonuclease (RNase) H2 ternary complex that plays an important role in maintaining DNA stability in cells. Recent studies have shown that the RNASEH2A subunit alone is highly expressed in certain cancer cell types. Via a series of bioinformatics approaches, we found that RNASEH2A is highly expressed in human proliferative tissues and many cancers. Our analyses reveal a possible involvement of RNASEH2A in cell cycle regulation in addition to its well established role in DNA replication and DNA repair. Our findings underscore that RNASEH2A could serve as a biomarker for cancer diagnosis and a therapeutic target.

Abstract

Ribonuclease (RNase) H2 is a key enzyme for the removal of RNA found in DNA-RNA hybrids, playing a fundamental role in biological processes such as DNA replication, telomere maintenance, and DNA damage repair. RNase H2 is a trimer composed of three subunits, RNASEH2A being the catalytic subunit. RNASEH2A expression levels have been shown to be upregulated in transformed and cancer cells. In this study, we used a bioinformatics approach to identify RNASEH2A co-expressed genes in different human tissues to underscore biological processes associated with RNASEH2A expression. Our analysis shows functional networks for RNASEH2A involvement such as DNA replication and DNA damage response and a novel putative functional network of cell cycle regulation. Further bioinformatics investigation showed increased gene expression in different types of actively cycling cells and tissues, particularly in several cancers, supporting a biological role for RNASEH2A but not for the other two subunits of RNase H2 in cell proliferation. Mass spectrometry analysis of RNASEH2A-bound proteins identified players functioning in cell cycle regulation. Additional bioinformatic analysis showed that RNASEH2A correlates with cancer progression and cell cycle related genes in Cancer Cell Line Encyclopedia (CCLE) and The Cancer Genome Atlas (TCGA) Pan Cancer datasets and supported our mass spectrometry findings.

Markers of acute toxicity of DDT exposure in pancreatic beta-cells determined by a proteomic approach

Many compounds have the potential to harm pancreatic beta-cells; organochlorine pollutants belong to those compounds. In this work, we aimed to find markers of acute toxicity of p,p'-DDT exposure among proteins expressed in NES2Y human pancreatic beta-cells employing 2-D electrophoresis. We exposed NES2Y cells to a high concentration (150 μM, LC96 after 72 hours) of p,p'-DDT for 24 and 30 hours and determined proteins with changed expression using 2-D electrophoresis. We have found 22 proteins that changed their expression. They included proteins involved in ER stress (GRP78, and endoplasmin), mitochondrial proteins (GRP75, ECHM, IDH3A, NDUS1, and NDUS3), proteins involved in the maintenance of the cell morphology (EFHD2, TCPA, NDRG1, and ezrin), and some other proteins (HNRPF, HNRH1, K2C8, vimentin, PBDC1, EF2, PCNA, biliverdin reductase, G3BP1, FRIL, and HSP27). The proteins we have identified may serve as indicators of p,p'-DDT toxicity in beta-cells in future studies, including long-term exposure to environmentally relevant concentrations.

Upregulation of CCT-3 Induces Breast Cancer Cell Proliferation Through miR-223 Competition and Wnt/β-Catenin Signaling Pathway Activation

The clinical significance and the function of chaperonin-containing TCP1 complex 3 (CCT-3) in breast cancer remain unknown. In this study, we found that CCT-3 was markedly overexpressed in breast cancer tissues. Statistical analysis revealed a significant correlation of CCT-3 expression with advanced breast cancer clinical stage and poorer survival. Ablation of CCT-3 knocked down the proliferation and the tumorigenicity of breast cancer cells in vitro and in vivo. CCT-3 may regulate breast cancer cell proliferation through a ceRNA network between miR-223 and β-catenin, thus affecting Wnt/β-catenin signaling pathway activation. We also validated that CCT-3 and β-catenin are novel direct targets of tumor suppressor miR-223. Our results suggest that both mRNA and the protein levels of CCT-3 are potential diagnosis biomarkers and therapeutic targets for breast cancer.

Homeostatic Control of Meiotic Prophase Checkpoint Function by Pch2 and Hop1

Checkpoint cascades link cell cycle progression with essential chromosomal processes. During meiotic prophase, recombination and chromosome synapsis are monitored by what are considered distinct checkpoints. In budding yeast, cells that lack the AAA+ ATPase Pch2 show an impaired cell cycle arrest in response to synapsis defects. However, unperturbed pch2Δ cells are delayed in meiotic prophase, suggesting paradoxical roles for Pch2 in cell cycle progression. Here, we provide insight into the checkpoint roles of Pch2 and its connection to Hop1, a HORMA domain-containing client protein. Contrary to current understanding, we find that Pch2 (together with Hop1) is crucial for checkpoint function in response to both recombination and synapsis defects, thus revealing a shared meiotic checkpoint cascade. Meiotic checkpoint responses are transduced by DNA break-dependent phosphorylation of Hop1. Based on our data and on the described effect of Pch2 on HORMA topology, we propose that Pch2 promotes checkpoint proficiency by catalyzing the availability of signaling-competent Hop1. Conversely, we demonstrate that Pch2 can act as a checkpoint silencer, also in the face of persistent DNA repair defects. We establish a framework in which Pch2 and Hop1 form a homeostatic module that governs general meiotic checkpoint function. We show that this module can-depending on the cellular context-fuel or extinguish meiotic checkpoint function, which explains the contradictory roles of Pch2 in cell cycle control. Within the meiotic prophase checkpoint, the Pch2-Hop1 module thus operates analogous to the Pch2/TRIP13-Mad2 module in the spindle assembly checkpoint that monitors chromosome segregation.

The role of Anaphase Promoting Complex activation, inhibition and substrates in cancer development and progression

The Anaphase Promoting Complex (APC), a multi-subunit ubiquitin ligase, facilitates mitotic and G1 progression, and is now recognized to play a role in maintaining genomic stability. Many APC substrates have been observed overexpressed in multiple cancer types, such as CDC20, the Aurora A and B kinases, and Forkhead box M1 (FOXM1), suggesting APC activity is important for cell health. We performed BioGRID analyses of the APC coactivators CDC20 and CDH1, which revealed that at least 69 proteins serve as APC substrates, with 60 of them identified as playing a role in tumor promotion and 9 involved in tumor suppression. While these substrates and their association with malignancies have been studied in isolation, the possibility exists that generalized APC dysfunction could result in the inappropriate stabilization of multiple APC targets, thereby changing tumor behavior and treatment responsiveness. It is also possible that the APC itself plays a crucial role in tumorigenesis through its regulation of mitotic progression. In this review the connections between APC activity and dysregulation will be discussed with regards to cell cycle dysfunction and chromosome instability in cancer, along with the individual roles that the accumulation of various APC substrates may play in cancer progression.

Investigating Chaperonin-Containing TCP-1 subunit 2 as an essential component of the chaperonin complex for tumorigenesis

Chaperonin-containing TCP-1 (CCT or TRiC) is a multi-subunit complex that folds many of the proteins essential for cancer development. CCT is expressed in diverse cancers and could be an ideal therapeutic target if not for the fact that the complex is encoded by eight distinct genes, complicating the development of inhibitors. Few definitive studies addressed the role of specific subunits in promoting the chaperonin’s function in cancer. To this end, we investigated the activity of CCT2 (CCTβ) by overexpressing or depleting the subunit in breast epithelial and breast cancer cells. We found that increasing total CCT2 in cells by 1.3-1.8-fold using a lentiviral system, also caused CCT3, CCT4, and CCT5 levels to increase. Likewise, silencing cct2 gene expression by ~50% caused other CCT subunits to decrease. Cells expressing CCT2 were more invasive and had a higher proliferative index. CCT2 depletion in a syngeneic murine model of triple negative breast cancer (TNBC) prevented tumor growth. These results indicate that the CCT2 subunit is integral to the activity of the chaperonin and is needed for tumorigenesis. Hence CCT2 could be a viable target for therapeutic development in breast and other cancers.

The TRiC/CCT Chaperonin and Its Role in Uncontrolled Proliferation

The cell cycle is a sophisticated space-time regulated mechanism where a wide variety of protein modules and complexes associate functioning in a concerted manner to regulate and transfer the genetic material to daughter cells. CCT (chaperonin containing TCP-1, also known as TRiC) is a molecular machine that forms a high molecular weight complex (1000 KDa). CCT is emerging as a key molecule during mitosis due to its essential role in the folding of many important proteins involved in cell division (Cdh1, Plk1, p27, Cdc20, PP2a regulatory subunits, tubulin or actin) suggesting its involvement in uncontrolled proliferation. The assembly is formed by eight different subunits called CCTα, β, γ, δ, ε, ζ, η and θ in mammals corresponding to CCT1-8 in yeast. CCT/TRiC is organized in a unique intra- and inter-ring arrangement. The chaperonin monomers share a common domain structure including an equatorial domain, which contains all the inter-ring contacts, most of the intra-ring contacts and the ATP binding site, whose binding and hydrolysis triggers the conformational changes that take place during the functional cycle. All chaperonins display an open substrate-receptive conformation, where the unfolded protein is recognized and trapped, and a closed conformation where the substrate is isolated from the bulk of the intracellular environment. In this chapter we discuss the complex set of intra- and inter-ring allosteric signals during chaperonin function.

Insights into a Crucial Role of TRIP13 in Human Cancer

Thyroid Hormone Receptor Interacting Protein 13 (TRIP13) plays a key role in regulating mitotic processes, including spindle assembly checkpoint and DNA repair pathways, which may account for Chromosome instability (CIN). As CIN is a predominant hallmark of cancer, TRIP13 may act as a tumor susceptibility locus. Amplification of TRIP13 has been observed in various human cancers and implicated in several aspects of malignant transformation, including cancer cell proliferation, drug resistance and tumor progression. Here, we discussed the functional significance of TRIP13 in cell progression, highlighted the recent findings on the aberrant expression in human cancers and emphasized its significance for the therapeutic potential.

TRIP13 Functions in the Establishment of the Spindle Assembly Checkpoint by Replenishing O-MAD2

The spindle assembly checkpoint (SAC) prevents premature segregation of chromosomes during mitosis. This process requires structural remodeling of MAD2 from O-MAD2 to C-MAD2 conformation. After the checkpoint is satisfied, C-MAD2 is reverted to O-MAD2 to allow anaphase-promoting complex/cyclosome (APC/C) to trigger anaphase. Recently, the AAA⁺-ATPase TRIP13 was shown to act in concert with p31^comet to catalyze C- to O-MAD2. Paradoxically, although C-MAD2 is present in TRIP13-deficient cells, the SAC cannot be activated. Using a degron-mediated system to uncouple TRIP13 from O- and C-MAD2 equilibrium, we demonstrated that the loss of TRIP13 did not immediately abolish the SAC, but the resulting C-MAD2-only environment was insufficient to enable the SAC. These results favor a model in which MAD2-CDC20 interaction is coupled directly to the conversion of O- to C-MAD2 instead of one that involves unliganded C-MAD2. TRIP13 replenishes the O-MAD2 pool for activation by unattached kinetochores.

Chaperonin TRiC/CCT supports mitotic exit and entry into endocycle in Drosophila

Endocycle is a commonly observed cell cycle variant through which cells undergo repeated rounds of genome DNA replication without mitosis. Endocycling cells arise from mitotic cells through a switch of the cell cycle mode, called the mitotic-to-endocycle switch (MES), to initiate cell growth and terminal differentiation. However, the underlying regulatory mechanisms of MES remain unclear. Here we used the Drosophila steroidogenic organ, called the prothoracic gland (PG), to study regulatory mechanisms of MES, which is critical for the PG to upregulate biosynthesis of the steroid hormone ecdysone. We demonstrate that PG cells undergo MES through downregulation of mitotic cyclins, which is mediated by Fizzy-related (Fzr). Moreover, we performed a RNAi screen to further elucidate the regulatory mechanisms of MES, and identified the evolutionarily conserved chaperonin TCP-1 ring complex (TRiC) as a novel regulator of MES. Knockdown of TRiC subunits in the PG caused a prolonged mitotic period, probably due to impaired nuclear translocation of Fzr, which also caused loss of ecdysteroidogenic activity. These results indicate that TRiC supports proper MES and endocycle progression by regulating Fzr folding. We propose that TRiC-mediated protein quality control is a conserved mechanism supporting MES and endocycling, as well as subsequent terminal differentiation.

The ATP-powered gymnastics of TRiC/CCT: an asymmetric protein folding machine with a symmetric origin story

The eukaryotic chaperonin TRiC/CCT is a large hetero-oligomeric complex that plays an essential role assisting cellular protein folding and suppressing protein aggregation. It consists of two rings, and each composed of eight different subunits; non-native polypeptides bind and fold in an ATP-dependent manner within their central chamber. Here, we review recent advances in our understanding of TRiC structure and mechanism enabled by application of hybrid structural methods including the integration of cryo-electron microscopy with distance constraints from crosslinking mass spectrometry. These new insights are revealing how the different TRiC/CCT subunits create asymmetry in its ATP-driven conformational cycle and its interaction with non-native polypeptides, which ultimately underlie its unique ability to fold proteins that cannot be folded by other chaperones.

TRiC/CCT chaperonins are essential for organ growth by interacting with insulin/TOR signaling in Drosophila

Organ size is regulated by intercellular signaling for cell growth and proliferation. The TOR pathway mediates a key signaling mechanism for controlling cell size and number in organ growth. Chaperonin containing TCP-1 (CCT) is a complex that assists protein folding and function, but its role in animal development is largely unknown. Here we show that the CCT complex is required for organ growth by interacting with the TOR pathway in Drosophila. Reduction of CCT4 results in growth defects by affecting both cell size and proliferation. Loss of CCT4 causes preferential cell death anterior to the morphogenetic furrow in the eye disc and within wing pouch in the wing disc. Depletion of any CCT subunit in the eye disc results in headless phenotype. Overgrowth by active TOR signaling is suppressed by CCT RNAi. The CCT complex physically interacts with TOR signaling components including TOR, Rheb, and S6K. Loss of CCT leads to decreased phosphorylation of S6K and S6 while increasing phosphorylation of Akt. Insulin/TOR signaling is also necessary and sufficient for promoting CCT complex transcription. Our data provide evidence that the CCT complex regulates organ growth by directly interacting with the TOR signaling pathway.

The role of the molecular chaperone CCT in protein folding and mediation of cytoskeleton-associated processes: implications for cancer cell biology

The chaperonin-containing tailless complex polypeptide 1 (CCT) is required in vivo for the folding of newly synthesized tubulin and actin proteins and is thus intrinsically connected to all cellular processes that rely on the microtubule and actin filament components of the cytoskeleton, both of which are highly regulated and dynamic assemblies. In addition to CCT acting as a protein folding oligomer, further modes of CCT action mediated either by the CCT oligomer itself or via CCT subunits in their monomeric forms can influence processes associated with assembled actin filaments and microtubules. Thus, there is an extended functional role for CCT with regard to its major folding substrates with a complex interplay between CCT as folding machine for tubulin/actin and as a modulator of processes involving the assembled cytoskeleton. As cell division, directed cell migration, and invasion are major drivers of cancer development and rely on the microtubule and actin filament components of the cytoskeleton, CCT activity is fundamentally linked to cancer. Furthermore, the CCT oligomer also folds proteins connected to cell cycle progression and interacts with several other proteins that are linked to cancer such as tumor-suppressor proteins and regulators of the cytoskeleton, while CCT monomer function can influence cell migration. Thus, understanding CCT activity is important for many aspects of cancer cell biology and may reveal new ways to target tumor growth and invasion.

TRiC/CCT Chaperonin: Structure and Function

The eukaryotic group II chaperonin TRiC/CCT assists the folding of 10% of cytosolic proteins including many key structural and regulatory proteins. TRiC plays an essential role in maintaining protein homeostasis, and dysfunction of TRiC is closely related to human diseases including cancer and neurodegenerative diseases. TRiC consists of eight paralogous subunits, each of which plays a specific role in the assembly, allosteric cooperativity, and substrate recognition and folding of this complex macromolecular machine. TRiC-mediated substrate folding is regulated through its ATP-driven conformational changes. In recent years, progresses have been made on the structure, subunit arrangement, conformational cycle, and substrate folding of TRiC. Additionally, accumulating evidences also demonstrate the linkage between TRiC oligomer or monomer and diseases. In this review, we focus on the TRiC structure itself, TRiC assisted substrate folding, TRiC and disease, and the potential therapeutic application of TRiC in various diseases.

The structure and evolution of eukaryotic chaperonin-containing TCP-1 and its mechanism that folds actin into a protein spring

Actin is folded to its native state in eukaryotic cytosol by the sequential allosteric mechanism of the chaperonin-containing TCP-1 (CCT). The CCT machine is a double-ring ATPase built from eight related subunits, CCT1–CCT8. Non-native actin interacts with specific subunits and is annealed slowly through sequential binding and hydrolysis of ATP around and across the ring system. CCT releases a folded but soft ATP-G-actin monomer which is trapped 80 kJ/mol uphill on the folding energy surface by its ATP-Mg2+/Ca2+ clasp. The energy landscape can be re-explored in the actin filament, F-actin, because ATP hydrolysis produces dehydrated and more compact ADP-actin monomers which, upon application of force and strain, are opened and closed like the elements of a spring. Actin-based myosin motor systems underpin a multitude of force generation processes in cells and muscles. We propose that the water surface of F-actin acts as a low-binding energy, directional waveguide which is recognized specifically by the myosin lever-arm domain before the system engages to form the tight-binding actomyosin complex. Such a water-mediated recognition process between actin and myosin would enable symmetry breaking through fast, low energy initial binding events. The origin of chaperonins and the subsequent emergence of the CCT–actin system in LECA (last eukaryotic common ancestor) point to the critical role of CCT in facilitating phagocytosis during early eukaryotic evolution and the transition from the bacterial world. The coupling of CCT-folding fluxes to the cell cycle, cell size control networks and cancer are discussed together with directions for further research.

Genomic, Pathway Network, and Immunologic Features Distinguishing Squamous Carcinomas

This integrated, multiplatform PanCancer Atlas study co-mapped and identified distinguishing molecular features of squamous cell carcinomas (SCCs) from five sites associated with smoking and/or human papillomavirus (HPV). SCCs harbor 3q, 5p, and other recurrent chromosomal copy-number alterations (CNAs), DNA mutations, and/or aberrant methylation of genes and microRNAs, which are correlated with the expression of multi-gene programs linked to squamous cell stemness, epithelial-to-mesenchymal differentiation, growth, genomic integrity, oxidative damage, death, and inflammation. Low-CNA SCCs tended to be HPV(+) and display hypermethylation with repression of TET1 demethylase and FANCF, previously linked to predisposition to SCC, or harbor mutations affecting CASP8, RAS-MAPK pathways, chromatin modifiers, and immunoregulatory molecules. We uncovered hypomethylation of the alternative promoter that drives expression of the ΔNp63 oncogene and embedded miR944. Co-expression of immune checkpoint, T-regulatory, and Myeloid suppressor cells signatures may explain reduced efficacy of immune therapy. These findings support possibilities for molecular classification and therapeutic approaches.

TRiC controls transcription resumption after UV damage by regulating Cockayne syndrome protein A

Transcription-blocking DNA lesions are removed by transcription-coupled nucleotide excision repair (TC-NER) to preserve cell viability. TC-NER is triggered by the stalling of RNA polymerase II at DNA lesions, leading to the recruitment of TC-NER-specific factors such as the CSA–DDB1–CUL4A–RBX1 cullin–RING ubiquitin ligase complex (CRL^CSA). Despite its vital role in TC-NER, little is known about the regulation of the CRL^CSA complex during TC-NER. Using conventional and cross-linking immunoprecipitations coupled to mass spectrometry, we uncover a stable interaction between CSA and the TRiC chaperonin. TRiC’s binding to CSA ensures its stability and DDB1-dependent assembly into the CRL^CSA complex. Consequently, loss of TRiC leads to mislocalization and depletion of CSA, as well as impaired transcription recovery following UV damage, suggesting defects in TC-NER. Furthermore, Cockayne syndrome (CS)-causing mutations in CSA lead to increased TRiC binding and a failure to compose the CRL^CSA complex. Thus, we uncover CSA as a TRiC substrate and reveal that TRiC regulates CSA-dependent TC-NER and the development of CS.

An integrated network of chaperones and protein degradation machineries called the proteostasis network (PN) is required to maintain protein homeostasis. Here the authors show that one of the components of the PN, the chaperonin TRiC, interacts with the core transcription-coupled nucleotide excision repair protein CSA to ensure its assembly into the CRLCSA complex.

Whole exome sequencing in three families segregating a pediatric case of sarcoidosis

BACKGROUND

Sarcoidosis (OMIM 181000) is a multi-systemic granulomatous disorder of unknown origin. Despite multiple genome-wide association (GWAS) studies, no major pathogenic pathways have been identified to date. To find out relevant sarcoidosis predisposing genes, we searched for de novo and recessive mutations in 3 young probands with sarcoidosis and their healthy parents using a whole-exome sequencing (WES) methodology.

METHODS

From the SARCFAM project based on a national network collecting familial cases of sarcoidosis, we selected three families (trios) in which a child, despite healthy parents, develop the disease before age 15 yr. Each trio was genotyped by WES (Illumina HiSEQ 2500) and we selected the gene variants segregating as 1) new mutations only occurring in affected children and 2) as recessive traits transmitted from each parents. The identified coding variants were compared between the three families. Allelic frequencies and in silico functional results were analyzed using ExAC, SIFT and Polyphenv2 databases. The clinical and genetic studies were registered by the ClinicalTrials.gov - Protocol Registration and Results System (PRS) ( https://clinicaltrials.gov ) receipt under the reference NCT02829853 and has been approved by the ethical committee (CPP LYON SUD EST - 2 - REF IRB 00009118 - September 21, 2016).

RESULTS

We identified 37 genes sharing coding variants occurring either as recessive mutations in at least 2 trios or de novo mutations in one of the three affected children. The genes were classified according to their potential roles in immunity related pathways: 9 to autophagy and intracellular trafficking, 6 to G-proteins regulation, 4 to T-cell activation, 4 to cell cycle and immune synapse, 2 to innate immunity. Ten of the 37 genes were studied in a bibliographic way to evaluate the functional link with sarcoidosis.

CONCLUSIONS

Whole exome analysis of case-parent trios is useful for the identification of genes predisposing to complex genetic diseases as sarcoidosis. Our data identified 37 genes that could be putatively linked to a pediatric form of sarcoidosis in three trios. Our in-depth focus on 10 of these 37 genes may suggest that the formation of the characteristic lesion in sarcoidosis, granuloma, results from combined deficits in autophagy and intracellular trafficking (ex: Sec16A, AP5B1 and RREB1), G-proteins regulation (ex: OBSCN, CTTND2 and DNAH11), T-cell activation (ex: IDO2, IGSF3), mitosis and/or immune synapse (ex: SPICE1 and KNL1). The significance of these findings needs to be confirmed by functional tests on selected gene variants.

Mad2 Overexpression Uncovers a Critical Role for TRIP13 in Mitotic Exit

The mitotic checkpoint ensures proper segregation of chromosomes by delaying anaphase until all kinetochores are bound to microtubules. This inhibitory signal is composed of a complex containing Mad2, which inhibits anaphase progression. The complex can be disassembled by p31^comet and TRIP13; however, TRIP13 knockdown has been shown to cause only a mild mitotic delay. Overexpression of checkpoint genes, as well as TRIP13, is correlated with chromosomal instability (CIN) in cancer, but the initial effects of Mad2 overexpression are prolonged mitosis and decreased proliferation. Here, we show that TRIP13 overexpression significantly reduced, and TRIP13 reduction significantly exacerbated, the mitotic delay associated with Mad2 overexpression, but not that induced by microtubule depolymerization. The combination of Mad2 overexpression and TRIP13 loss reduced the ability of checkpoint complexes to disassemble and significantly inhibited the proliferation of cells in culture and tumor xenografts. These results identify an unexpected dependency on TRIP13 in cells overexpressing Mad2.

Role of ubiquitylation of components of mitotic checkpoint complex in their dissociation from anaphase-promoting complex/cyclosome

The mitotic checkpoint system ensures the fidelity of chromosome segregation in mitosis by preventing premature initiation of anaphase until correct bipolar attachment of chromosomes to the mitotic spindle is reached. It promotes the assembly of a mitotic checkpoint complex (MCC), composed of BubR1, Bub3, Cdc20, and Mad2, which inhibits the activity of the anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase. When the checkpoint is satisfied, anaphase is initiated by the disassembly of MCC. Previous studies indicated that the dissociation of APC/C-bound MCC requires ubiquitylation and suggested that the target of ubiquitylation is the Cdc20 component of MCC. However, it remained unknown how ubiquitylation causes the release of MCC from APC/C and its disassembly and whether ubiquitylation of additional proteins is involved in this process. We find that ubiquitylation causes the dissociation of BubR1 from Cdc20 in MCC and suggest that this may lead to the release of MCC components from APC/C. BubR1 in MCC is ubiquitylated by APC/C, although to a lesser degree than Cdc20. The extent of BubR1 ubiquitylation was markedly increased in recombinant MCC that contained a lysine-less mutant of Cdc20. Mutation of lysine residues to arginines in the N-terminal region of BubR1 partially inhibited its ubiquitylation and slowed down the release of MCC from APC/C, provided that Cdc20 ubiquitylation was also blocked. It is suggested that ubiquitylation of both Cdc20 and BubR1 may be involved in their dissociation from each other and in the release of MCC components from APC/C.

Genetic expansion of chaperonin-containing TCP-1 (CCT/TRiC) complex subunits yields testis-specific isoforms required for spermatogenesis in planarian flatworms

Chaperonin-containing Tail-less complex polypeptide 1 (CCT) is a highly conserved, hetero-oligomeric complex that ensures proper folding of actin, tubulin, and regulators of mitosis. Eight subunits (CCT1-8) make up this complex, and every subunit has a homolog expressed in the testes and somatic tissue of the planarian flatworm Schmidtea mediterranea. Gene duplications of four subunits in the genomes of S. mediterranea and other planarian flatworms created paralogs to CCT1, CCT3, CCT4, and CCT8 that are expressed exclusively in the testes. Functional analyses revealed that each CCT subunit expressed in the S. mediterranea soma is essential for homeostatic integrity and survival, whereas sperm elongation defects were observed upon knockdown of each individual testis-specific paralog (Smed-cct1B; Smed-cct3B; Smed-cct4A; and Smed-cct8B), regardless of potential redundancy with paralogs expressed in both testes and soma (Smed-cct1A; Smed-cct3A; Smed-cct4B; and Smed-cct8A). Yet, no detriment was observed in the number of adult somatic stem cells (neoblasts) that maintain differentiated tissue in planarians. Thus, expression of all eight CCT subunits is required to execute the essential functions of the CCT complex. Furthermore, expression of the somatic paralogs in planarian testes is not sufficient to complete spermatogenesis when testis-specific paralogs are knocked down, suggesting that the evolution of chaperonin subunits may drive changes in the development of sperm structure and that correct CCT subunit stoichiometry is crucial for spermiogenesis.

Dynamic ubiquitin signaling in cell cycle regulation

Gilberto and Peter discuss the role of ubiquitylation in the regulation of DNA replication and mitosis.

The cell division cycle is driven by a collection of enzymes that coordinate DNA duplication and separation, ensuring that genomic information is faithfully and perpetually maintained. The activity of the effector proteins that perform and coordinate these biological processes oscillates by regulated expression and/or posttranslational modifications. Ubiquitylation is a cardinal cellular modification and is long known for driving cell cycle transitions. In this review, we emphasize emerging concepts of how ubiquitylation brings the necessary dynamicity and plasticity that underlie the processes of DNA replication and mitosis. New studies, often focusing on the regulation of chromosomal proteins like DNA polymerases or kinetochore kinases, are demonstrating that ubiquitylation is a versatile modification that can be used to fine-tune these cell cycle events, frequently through processes that do not involve proteasomal degradation. Understanding how the increasing variety of identified ubiquitin signals are transduced will allow us to develop a deeper mechanistic perception of how the multiple factors come together to faithfully propagate genomic information. Here, we discuss these and additional conceptual challenges that are currently under study toward understanding how ubiquitin governs cell cycle regulation.