Solution structure of RING finger-like domain of retinoblastoma-binding protein-6 (RBBP6) suggests it functions as a U-box

The Vitis yeshanensis U-box E3 ubiquitin ligase VyPUB21 enhances resistance to powdery mildew by targeting degradation of NIM1-interacting (NIMIN) protein

KEY MESSAGE

VyPUB21 plays a key role during the defense against powdery mildew in grapes. Ubiquitin-ligating enzyme (E3), a type of protein widely found in plants, plays a key role in their resistance to disease. Yet how E3 participates in the disease-resistant response of Chinese wild grapevine (Vitis yeshanensis) remains unclear. Here we isolated and identified a U-box type E3 ubiquitin ligase, VyPUB21, from V. yeshanensis. This gene's expression level rose rapidly after induction by exogenous salicylic acid (SA), jasmonic acid (JA), and ethylene (ETH) and powdery mildew. In vitro ubiquitination assay results revealed VyPUB21 could produce ubiquitination bands after co-incubation with ubiquitin, ubiquitin-activating enzyme (E1), and ubiquitin-conjugating enzyme (E2); further, mutation of the conserved amino acid site in the U-box can inhibit the ubiquitination. Transgenic VyPUB21 Arabidopsis had low susceptibility to powdery mildew, and significantly fewer conidiophores and spores on its leaves. Expression levels of disease resistance-related genes were also augmented in transgenic Arabidopsis, and its SA concentration also significantly increased. VyPUB21 interacts with VyNIMIN and targets VyNIMIN protein hydrolysis through the 26S proteasome system. Thus, the repressive effect of the NIMIN-NPR complex on the late systemic acquired resistance (SAR) gene was attenuated, resulting in enhanced resistance to powdery mildew. These results indicate that VyPUB21 encoding ubiquitin ligase U-box E3 activates the SA signaling pathway, and VyPUB21 promotes the expression of late SAR gene by degrading the important protein VyNIMIN of SA signaling pathway, thus enhancing grape resistance to powdery mildew.

Zooming into the structure-function of RING finger proteins for anti-cancer therapeutic applications

Cancer is one of the most common and widely diagnosed diseases worldwide. With an increase in prevalence and incidence, many studies in cancer biology have been looking at the role pro-cancer proteins play. One of these proteins is the Really Interesting New Gene (RING), which has been studied extensively due to its structure and functions such as apoptosis, neddylation, and its role in ubiquitination. The RING domain is a cysteine-rich domain known to bind Cysteine and Histidine residues. It also binds two zinc ions that help stabilize the protein in various patterns, often with a 'cross-brace' topology. Different RING finger proteins have been studied and found to have suitable targets for developing anti-cancer therapeutics. These identified candidate proteins include Parkin, COP1, MDM2, BARD1, BRCA-1, PIRH2, c-CBL, SIAH1, RBX1 and RNF8. Inhibiting these candidate proteins provides opportunities for shutting down pathways associated with tumour development and metastasis.

Expression, purification, and characterisation of the p53 binding domain of Retinoblastoma binding protein 6 (RBBP6)

RBBP6 is a 250 kDa eukaryotic protein known to be a negative regulator of p53 and essential for embryonic development. Furthermore, RBBP6 is a critical element in carcinogenesis and has been identified as a potential biomarker for certain cancers. RBBP6's ability to interact with p53 and cause its degradation makes it a potential drug target in cancer therapy. Therefore, a better understating of the p53 binding domain of RBBP6 is needed. This study presents a three-part purification protocol for the polyhistidine-tagged p53 binding domain of RBBP6, expressed in Escherichia coli bacterial cells. The purified recombinant domain was shown to have structure and is functional as it could bind endogenous p53. We characterized it using clear native PAGE and far-UV CD and found that it exists in a single form, most likely monomer. We predict that its secondary structure is predominantly random coil with 19% alpha-helices, 9% beta-strand and 14% turns. When we exposed the recombinant domain to increasing temperature or known denaturants, our investigation suggested that the domain undergoes relatively small structural changes, especially with increased temperature. Moreover, we notice a high percentage recovery after returning the domain close to starting conditions. The outcome of this study is a pure, stable, and functional recombinant RBBP6-p53BD that is primarily intrinsically disordered.

Molecular interaction between small nuclear ribonucleoprotein polypeptide G and heat shock protein 70.14: a microscale thermophoresis exposition towards developing anti-cancer drugs

BACKGROUND

Targeting protein-protein interactions (PPIs) linked to protein quality control (PQC) pathways as potential anti-cancer drug targets have unanimously widened biological insights and the therapeutic potential of PPIs as smart-drug discovery tools in cancer. PPIs between disease-relevant proteins associated with protein homeostasis in PQC pathways have been linked to improved mechanistic understanding associated with conformational abnormalities and impairment, cellular proteotoxicity, induced apoptosis, and pathogenesis in different types of cancers. In this context, PPIs between small nuclear ribonucleoprotein polypeptide G (SNRPG) and heat shock protein 70.14 (Hsp70.14) have attracted attention as potential smart drug discovery tools in cancer diagnostics and therapeutics. Validated evidence of high-quality biological data has shown the presence of the two proteins in different types of cancers including breast cancer. The links between SNRPG and Hsp70.14 in cancer-cell networks remain elusive, overlooked, and uncharacterized.

METHODOLOGY

In this study, we explored the interaction between the two oncogenic proteins using the MST-based assays.

RESULTS

The results revealed a low K_D in the nanomolar concentration range of 2.4673 × 10^-7 demonstrating a great affinity for SNRPG binding to Hsp70.14.

CONCLUSIONS

The results suggest a possible involvement between the two proteins in hostile tumour microenvironments. Furthermore, these findings offer a different therapeutic perspective that could pave the way for the creation of novel small molecule inhibitors as drugs for the treatment of cancer.

Reconstitution of 3' end processing of mammalian pre-mRNA reveals a central role of RBBP6

The 3' ends of almost all eukaryotic mRNAs are generated in an essential two-step processing reaction: endonucleolytic cleavage of an extended precursor followed by the addition of a poly(A) tail. By reconstituting the reaction from overproduced and purified proteins, we provide a minimal list of 14 polypeptides that are essential and two that are stimulatory for RNA processing. In a reaction depending on the polyadenylation signal AAUAAA, the reconstituted system cleaves pre-mRNA at a single preferred site corresponding to the one used in vivo. Among the proteins, cleavage factor I stimulates cleavage but is not essential, consistent with its prominent role in alternative polyadenylation. RBBP6 is required, with structural data showing it to contact and presumably activate the endonuclease CPSF73 through its DWNN domain. The C-terminal domain of RNA polymerase II is dispensable. ATP, but not its hydrolysis, supports RNA cleavage by binding to the hClp1 subunit of cleavage factor II with submicromolar affinity.

Microscale thermophoresis analysis of the molecular interaction between small nuclear ribonucleoprotein polypeptide G and the RING finger domain of RBBP6 towards anti-cancer drug discovery

Regulatory core-splicing proteins are now becoming highly promising therapeutic targets for the development of anti-cancer drugs. SNRPG and RBBP6 are two good examples of regulatory core-splicing proteins involved in tumorigenesis and tumor development whose multi-functional role is primarily mediated by protein-protein interactions. Over the years, skepticism abutting from the two onco-proteins has been mounting. Suggestive evidence using yeast 2-hybrid technique observed possible involvement between SNRPG and the RING finger domain of RBBP6. However, the putative interaction remains elusive and yet to be characterized. In this study, we developed the first MST-based assay to confirm the interaction between SNRPG and the RING finger domain of RBBP6. The results demonstrated a strong binding affinity between SNRPG and the RING finger domain of RBBP6 with a K_D in the low nanomolar concentration range of 3.1596 nM. The results are congruent with previous findings suggesting possible involvement between the two proteins in cancer-cell networks, thereby providing a new mechanistic insight into the interaction between SNRPG and the RING finger domain of RBBP6. The interaction is therapeutically relevant and represents a great milestone in the anti-cancer drug discovery space. Identification of small molecule inhibitors to modulate the binding affinity between the two proteins would therefore be a major breakthrough in the development of new PPI-focused anti-cancer drugs.

Regulation of the Ysh1 endonuclease of the mRNA cleavage/polyadenylation complex by ubiquitin-mediated degradation

Mutation of the essential yeast protein Ipa1 has previously been demonstrated to cause defects in pre-mRNA 3' end processing and growth, but the mechanism underlying these defects was not clear. In this study, we show that the ipa1-1 mutation causes a striking depletion of Ysh1, the evolutionarily conserved endonuclease subunit of the 19-subunit mRNA Cleavage/Polyadenylation (C/P) complex, but does not decrease other C/P subunits. YSH1 overexpression rescues both the growth and 3' end processing defects of the ipa1-1 mutant. YSH1 mRNA level is unchanged in ipa1-1 cells, and proteasome inactivation prevents Ysh1 loss and causes accumulation of ubiquitinated Ysh1. Ysh1 ubiquitination is mediated by the Ubc4 ubiquitin-conjugating enzyme and Mpe1, which in addition to its function in C/P, is also a RING ubiquitin ligase. In summary, Ipa1 affects mRNA processing by controlling the availability of the C/P endonuclease and may represent a regulatory mechanism that could be rapidly deployed to facilitate reprogramming of cellular responses.

The oncogenic potential of small nuclear ribonucleoprotein polypeptide G: a comprehensive and perspective view

Small nuclear ribonucleoprotein polypeptide G (SNRPG), often referred to as Smith protein G (SmG), is an indispensable component in the biogenesis of spliceosomal uridyl-rich small nuclear ribonucleoprotein particles (U snRNPs; U1, U2, U4 and U5), which are precursors of both the major and minor spliceosome. SNRPG has attracted significant attention because of its implicated roles in tumorigenesis and tumor development. Suggestive evidence of its varying expression levels has been reported in different types of cancers, which include breast cancer, lung cancer, prostate cancer and colon cancer. The accumulating evidence suggests that the splicing machinery component plays a significant role in the initiation and progression of cancers. SNRPG has a wide interaction network, and its functions are predominantly mediated by protein-protein interactions (PPIs), making it a promising anti-cancer therapeutic target in PPI-focused drug technology. Understanding its roles in tumorigenesis and tumor development is an indispensable arsenal in the development of molecular-targeted therapies. Several antitumor drugs linked to splicing machinery components have been reported in different types of cancers and some have already entered the clinic. However, targeting SNRPG as a drug development tool has been an overlooked and underdeveloped strategy in cancer therapy. In this article, we present a comprehensive and perspective view on the oncogenic potential of SNRPG in PPI-focused drug discovery.

Human BRCA1-BARD1 ubiquitin ligase activity counteracts chromatin barriers to DNA resection

The opposing activities of 53BP1 and BRCA1 influence pathway choice in DNA double-strand-break repair. How BRCA1 counteracts the inhibitory effect of 53BP1 on DNA resection and homologous recombination is unknown. Here we identify the site of BRCA1-BARD1 required for priming ubiquitin transfer from E2∼ubiquitin and demonstrate that BRCA1-BARD1's ubiquitin ligase activity is required for repositioning 53BP1 on damaged chromatin. We confirm H2A ubiquitination by BRCA1-BARD1 and show that an H2A-ubiquitin fusion protein promotes DNA resection and repair in BARD1-deficient cells. BRCA1-BARD1's function in homologous recombination requires the chromatin remodeler SMARCAD1. SMARCAD1 binding to H2A-ubiquitin and optimal localization to sites of damage and activity in DNA repair requires its ubiquitin-binding CUE domains. SMARCAD1 is required for 53BP1 repositioning, and the need for SMARCAD1 in olaparib or camptothecin resistance is alleviated by 53BP1 loss. Thus, BRCA1-BARD1 ligase activity and subsequent SMARCAD1-dependent chromatin remodeling are critical regulators of DNA repair.

¹¹³Cd NMR experiments reveal an unusual metal cluster in the solution structure of the yeast splicing protein Bud31p

Establishing the binding topology of structural zinc ions in proteins is an essential part of their structure determination by NMR spectroscopy. Using (113)Cd NMR experiments with (113)Cd-substituted samples is a useful approach but has previously been limited mainly to very small protein domains. Here we used (113)Cd NMR spectroscopy during structure determination of Bud31p, a 157-residue yeast protein containing an unusual Zn3Cys9 cluster, demonstrating that recent hardware developments make this approach feasible for significantly larger systems.

113Cd NMR Experiments Reveal an Unusual Metal Cluster in the Solution Structure of the Yeast Splicing Protein Bud31p

Establishing the binding topology of structural zinc ions in proteins is an essential part of their structure determination by NMR spectroscopy. Using ¹¹³Cd NMR experiments with ¹¹³Cd‐substituted samples is a useful approach but has previously been limited mainly to very small protein domains. Here we used ¹¹³Cd NMR spectroscopy during structure determination of Bud31p, a 157‐residue yeast protein containing an unusual Zn₃Cys₉ cluster, demonstrating that recent hardware developments make this approach feasible for significantly larger systems.

Efficient mRNA polyadenylation requires a ubiquitin-like domain, a zinc knuckle, and a RING finger domain, all contained in the Mpe1 protein

Almost all eukaryotic mRNAs must be polyadenylated at their 3' ends to function in protein synthesis. This modification occurs via a large nuclear complex that recognizes signal sequences surrounding a poly(A) site on mRNA precursor, cleaves at that site, and adds a poly(A) tail. While the composition of this complex is known, the functions of some subunits remain unclear. One of these is a multidomain protein called Mpe1 in the yeast Saccharomyces cerevisiae and RBBP6 in metazoans. The three conserved domains of Mpe1 are a ubiquitin-like (UBL) domain, a zinc knuckle, and a RING finger domain characteristic of some ubiquitin ligases. We show that mRNA 3'-end processing requires all three domains of Mpe1 and that more than one region of Mpe1 is involved in contact with the cleavage/polyadenylation factor in which Mpe1 resides. Surprisingly, both the zinc knuckle and the RING finger are needed for RNA-binding activity. Consistent with a role for Mpe1 in ubiquitination, mutation of Mpe1 decreases the association of ubiquitin with Pap1, the poly(A) polymerase, and suppressors of mpe1 mutants are linked to ubiquitin ligases. Furthermore, an inhibitor of ubiquitin-mediated interactions blocks cleavage, demonstrating for the first time a direct role for ubiquitination in mRNA 3'-end processing.

The RBBP6/ZBTB38/MCM10 axis regulates DNA replication and common fragile site stability

Faithful DNA replication is essential for the maintenance of genome integrity. Incomplete genome replication leads to DNA breaks and chromosomal rearrangements, which are causal factors in cancer and other human diseases. Despite their importance, the molecular mechanisms that control human genome stability are incompletely understood. Here, we report a pathway that is required for human genome replication and stability. This pathway has three components: an E3 ubiquitin ligase, a transcriptional repressor, and a replication protein. The E3 ubiquitin ligase RBBP6 ubiquitinates and destabilizes the transcriptional repressor ZBTB38. This repressor negatively regulates transcription and levels of the MCM10 replication factor on chromatin. Cells lacking RBBP6 experience reduced replication fork progression and increased damage at common fragile sites due to ZBTB38 accumulation and MCM10 downregulation. Our results uncover a pathway that ensures genome-wide DNA replication and chromosomal stability.

The C. elegans Homolog of RBBP6 (RBPL-1) regulates fertility through controlling cell proliferation in the germline and nutrient synthesis in the intestine

RBBP6 (retinoblastoma binding protein 6, also known as PACT or P2P-R in humans) is a multi-domain protein that functions in multiple processes, such as mitosis, cell differentiation, and cell apoptosis. RBBP6 is evolutionarily conserved and is present in unicellular organisms to mammals. Studies of RBBP6 have mostly focused on its RB- and p53-binding domains, which are found exclusively in mammals. Here, we investigated the C. elegans homolog of RBBP6 to explore the functional roles of its other domains. We found that RBPL-1, the homolog of RBBP6 in C. elegans, is indispensable for worm development. RNAi silencing of rbpl-1 led to embryonic lethality, as well as defects in oocyte production and intestine development. rbpl-1 RNAi worms showed defects in germ cell proliferation, suggesting that RBPL-1 regulates mitosis. Moreover, RNAi silencing of rbpl-1 inhibited nutrient synthesis in the worm intestine. RBPL-1, as a nucleolus protein, was found to be expressed in diverse tissues and necessary for both germline and soma development. Using microarray analysis, we identified ≈700 genes whose expression levels were changed at least 10-fold in rbpl-1 worms. We propose that RBPL-1, like its yeast homolog, may regulate gene expression as an mRNA cleavage and polyadenylation factor. Taken together, the findings from this study reveal that RBPL-1 plays a pivotal role in C. elegans germline and soma development, suggesting that the functions of RBBP6 are conserved in diverse eukaryotic species.

The global transcriptional response of isolated human islets of langerhans to glucagon-like Peptide-1 receptor agonist liraglutide

GLP-1 and its analog have been used in diabetes treatment; however, the direct alteration of gene expression profile in human islets induced by GLP-1 has not been reported. In present study, transcriptional gene expression in the liraglutide-treated human islets was analyzed with 12 human U133A chips including 23000 probe sets. The data compared between liraglutide and control groups showed a significant difference on glucose-induced insulin secretion, rather than viability. Microarray analysis identified 7000 genes expressed in human islets. Eighty genes were found to be modulated by liraglutide treatment. Furthermore, the products of these genes are proteins involved in binding capability, enzyme activity, transporter function, signal transduction, cell proliferation, apoptosis, and cell differentiation. Our data provides a set of information in the complex events, following the activation of the GLP-1 receptor in the islets of Langerhans.

Variation in genes related to cochlear biology is strongly associated with adult-onset deafness in border collies

Domestic dogs can suffer from hearing losses that can have profound impacts on working ability and quality of life. We have identified a type of adult-onset hearing loss in Border Collies that appears to have a genetic cause, with an earlier age of onset (3–5 years) than typically expected for aging dogs (8–10 years). Studying this complex trait within pure breeds of dog may greatly increase our ability to identify genomic regions associated with risk of hearing impairment in dogs and in humans. We performed a genome-wide association study (GWAS) to detect loci underlying adult-onset deafness in a sample of 20 affected and 28 control Border Collies. We identified a region on canine chromosome 6 that demonstrates extended support for association surrounding SNP Chr6.25819273 (p-value = 1.09×10⁻¹³). To further localize disease-associated variants, targeted next-generation sequencing (NGS) of one affected and two unaffected dogs was performed. Through additional validation based on targeted genotyping of additional cases (n = 23 total) and controls (n = 101 total) and an independent replication cohort of 16 cases and 265 controls, we identified variants in USP31 that were strongly associated with adult-onset deafness in Border Collies, suggesting the involvement of the NF-κB pathway. We found additional support for involvement of RBBP6, which is critical for cochlear development. These findings highlight the utility of GWAS–guided fine-mapping of genetic loci using targeted NGS to study hereditary disorders of the domestic dog that may be analogous to human disorders.

The domestic dog offers a unique opportunity to study complex disorders similar to those seen in humans, but within the context of the much simpler genetic backgrounds of pure breeds, which represent closed populations. We performed a whole-genome search for genetic risk factors of adult-onset deafness in the Border Collie, a breed of herding dog that relies on acute hearing to perceive and respond to commands while working. Adult-onset deafness in Border Collies typically begins in early adulthood and is similar to age-related hearing loss in humans. This earlier onset has particular impact on the utility of working Border Collies and the livelihoods of their owners, and it appears to have a genetic cause. We identified three genetic variants that were strongly associated with adult-onset deafness in a sample of 405 Border Collies. These variants are located in two genes that have previously been linked to deafness, one involved in ear development and another that appears to mitigate tissue damage in the ear. These results provide new insight regarding genetic risk factors for age-related hearing loss in both dogs and humans.