Biochimica et Biophysica Acta (BBA) - General Subjects

The insulin-linked polymorphic region (ILPR) is a variable number tandem repeat located in the promoter of the human insulin gene. This G-rich sequence can fold into four-stranded G-quadruplex DNA structures, while its complementary C-rich strand forms i-motifs. The ILPR varies in repeat number and sequence composition, but the relationship between sequence diversity, DNA structure, and insulin gene regulation remains poorly understood. Although both G-quadruplexes and i-motifs have been implicated in transcriptional control, their relative contributions, particularly when formed on complementary strands of the same locus, are unclear. Here, we characterised the structure and stability of nine ILPR-based sequences using biophysical techniques and luciferase reporter assays. We demonstrate that transcriptional activation in response to high glucose occurs only when both G-quadruplex and i-motif structures can form. Other combinations of structures do not induce transcription. Moreover, promoter activity correlated positively with i-motif stability, but not with G-quadruplex stability. These results suggest a model in which G-quadruplexes function as an on/off switch, while i-motifs act as modulators of gene expression. Our findings underscore the importance of treating G-quadruplexes and i-motifs as a dynamic, interdependent system in both the regulation of gene expression and also the potential of these structures as therapeutic targets. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=145 SRC="FIGDIR/small/651924v1_ufig1.gif" ALT="Figure 1"> View larger version (19K): org.highwire.dtl.DTLVardef@16ae322org.highwire.dtl.DTLVardef@65e893org.highwire.dtl.DTLVardef@884ab3org.highwire.dtl.DTLVardef@1e0311e_HPS_FORMAT_FIGEXP M_FIG C_FIG

Apurinic/apyrimidinic endonuclease 1 (APE1) is involved in DNA replication, repair, and transcriptional regulation mechanisms. This multifunctional activity of APE1 should be supported by specific structural properties of APE1 that have not yet been elucidated. Here we applied atomic force microscopy (AFM) to characterize the interactions of APE1 with DNA. Complexes of APE1 with DNA containing G-rich segments were visualized, and analysis of the complexes revealed the affinity of APE1 to G-rich DNA sequences. Furthermore, loops in the DNA-APE1 complexes were visualized, and their yield was as high as 53 %. However, the loops were non-specific, with quantitative analysis revealing the yield of loops bridging two G-rich DNA segments to be 41%. Analysis of protein size in various complexes was performed, and these data showed that loops are formed by APE1 monomer, suggesting that APE1 has two DNA binding sites. The data lead us to a model for the interaction of APE1 with DNA that describes its molecular site search mechanism. The new properties of APE1 in organizing DNA, by bringing two distant sites together, may be important for facilitating the scanning for damage and coordinating repair and transcription.

Protein-protein interactions involving 14-3-3 proteins regulate various cellular activities in normal and pathological conditions. These interactions have mostly been reported to be phosphorylation-dependent, but the 14-3-3 proteins also interact with unphosphorylated proteins. In this work, we investigated whether phosphorylation is required, or, alternatively, whether negative charges are sufficient for 14-3-3{varepsilon} binding. We substituted the pThr residue of pT(502-510) peptide by residues with varying number of negative charges, and investigated binding of the peptides to 14-3-3{varepsilon} using MD simulations and biophysical methods. We demonstrated that at least one negative charge is required for the peptides to bind 14-3-3{varepsilon} while phosphorylation is not necessary, and that two negative charges are preferable for high affinity binding.

The search for peptides that can specifically bind to regulatory regions in DNA is a necessary step for creating drugs that can regulate gene expression. The study is dedicated to the peculiarities of binding of a model peptide, which carries an ionic self-complementary motif and can form amyloid-like fibrils [1], with model double-stranded DNAs. The stoichiometric ratios of the components of the complex were found using the retardation method in agarose gel. Using microscale thermophoresis, it was shown that the peptide in the amyloid-like state is capable of binding to model 45-bp double-stranded DNA, with a micromolar equilibrium dissociation constant. Using cryo-electron, transmission electron, and atomic force microscopy, the morphology of peptide-DNA complexes was studied. Using dynamic light scattering and nanoparticle tracking analysis, as well as small-angle neutron scattering, the spatial parameters of the resulting DNA-peptide complexes were characterized. Molecular dynamics simulations showed that the arginine side chains of the peptide are prone to interact with guanine nitrogenous bases. It was shown that the formation of peptide-dsDNA complexes interferes with the operation of restriction endonucleases that have guanine-cytosine pairs in the recognition center, which is consistent with the results of prediction of interaction sites obtained using computer modeling. The results of the work can be used in the development of peptides capable of interacting with functional regions of DNA, as well as in the development of new carriers for transfection of DNA constructs.

The Rous sarcoma virus (RSV) is an onco-retrovirus that infects avian species such as the chicken (Gallus gallus). RSV is the first oncovirus to be described and the oncogenic activity of this virus is related to the expression of a tyrosine kinase that induces carcinogenic transformation. Interestingly, we have noted that the RSV genome contains various potential G4 forming sequences. Among these, two sequences located in the GAG and POL genes, respectively, show high G4 forming potential. Additionally, the SRC oncogene also harbours a putative G4 forming sequence. In this study, we have characterised the G4 formation and topology in these three loci in the RSV-DNA. We have found that these sequences form dynamic G4 structures in physiological conditions and such dynamicity may be associated with their cellular functions. Further, we have also established that these G4s are recognized by G4 interacting small-molecule ligands and the G4-stabilizing protein nucleolin. Binding of these ligands induces structural shifts in the G4 leading to change in structure and stability. Thus, the RSV-DNA G4s may be further studied as targets to control its infection and oncogenic effects.

Non-canonical secondary structures in DNA are increasingly being revealed as critical players in DNA metabolism, including modulating the accessibility and activity of promoters. These structures comprise the so-called G-quadruplexes (G4s) that are formed from sequences rich in guanine bases. Using a well-defined transcriptional reporter system, we sought to systematically investigate the impact of the presence of G4 structures on transcription in yeast S. cerevisiae. To this aim, different G4 prone sequences were modeled to vary the chance of intramolecular G4 formation, analyzed in vitro by Thioflavin T binding test and circular dichroism and then placed at the yeast ADE2 locus on chromosome XV, downstream and adjacent to a P53 response element (RE) and upstream from a minimal CYC1 promoter and Luciferase 1 (LUC1) reporter gene in isogenic strains. While the minimal CYC1 promoter provides for basal reporter activity, the P53 RE enables LUC1 transactivation under the control of the human P53 family proteins expressed under the inducible GAL1 promoter. Thus, the impact of the different G4 prone sequences on both basal and P53 family proteins dependent expression was measured after shifting the yeast cells onto galactose containing medium. The results showed that the presence of G4 prone sequences upstream of a yeast minimal promoter can increase its basal activity proportionally to their potential to form intramolecular G4 structures; consequently, this improved accessibility, when present near the target binding site of P53 family transcription factors can be exploited in order to regulate the transcriptional activity of P53, P63 and P73 proteins.

The histone-like protein from E. coli strain U93 (HU) is an abundant nucleoid-associated protein that contributes to the compaction of the bacterial genome as well as to the regulation of many of its transactions. Despite many years of investigations, the way and extent to which HU binding alters the DNA double helix and/or generates hierarchical structures using DNA as a scaffold is not completely understood. Here we combined single-molecule magnetic measurements with circular dichroism studies to monitor structural changes in the DNA-HU fiber as HU concentration was increased from 0 to 1000 nM under low and physiological monovalent salt conditions. We confirmed that DNA compaction correlated with HU concentration in a biphasic manner but DNA unwinding varied monotonically with HU concentration in 100 mM KCl. Instead, in more physiological 200 mM salt conditions, DNA compaction was monotonic while HU-induced DNA unwinding was negligible. Differential compaction and unwinding of DNA may be part of the response of bacteria to large variations in salt concentrations.

Cyclic-di-nucleotide based secondary messengers regulate various physiological processes including the stress responses in bacteria. In the past decade, cyclic diadenosine monophosphate (c-di-AMP) has emerged as a crucial second messenger, implicated in fatty acid metabolism, antibiotic resistance, biofilm formation, virulence, DNA repair, ion homeostasis, sporulation etc. The level of c-di-AMP is maintained in the cell by the action of two opposing enzymes, namely diadenylate cyclase (DAC) and phosphodiesterase (PDE). In mycobacteria, this molecule is essential for its regulatory role in bacterial physiology and host-pathogen interactions. However, such modulation of c-di-AMP remains to be explored in Mycobacterium smegmatis. Here, we systematically characterised the c-di-AMP synthase (MsDisA) and a hydrolase (MsPDE) from M. smegmatis at different pH and osmolytic conditions in vitro. Our biochemical assays show that the MsDisA activity is enhanced during the alkaline stress and c-di-AMP is readily produced without any intermediates. At pH 9.4, the MsDisA promoter activity in vivo increases significantly, strengthening this observation. However, under physiological conditions, the activity of MsDisA was moderate with the formation of intermediates. To get further insights into the structural characteristics, we determined the cryo-EM structure of the MsDisA, revealing some interesting features. Biochemical analysis of individual domains shows that the N-terminal minimal region alone can form a functional octamer. Altogether, our results reveal the biochemical and structural regulation of mycobacterial c-di-AMP in response to various environmental stress.

CHT7 is a regulator of quiescence repression and TAG degradation between the nitrogen deprived and the nitrogen replenished states in Chlamydomonas reinhardtii. Initially it was thought that the CHT7s repression activity is managed by its DNA binding CXC domain which is a tandem repeat of two cysteine rich subdomains. Later, it was found that the CXC (CHT7_CXC) domain is effectively dispensable for CHT7s activities. Rather, CHT7s predicted protein binding domains are proposed to be involved in gene regulation activities by binding through other repressors in the cell. Yet, it remains unclear why and how CHT7 manages to refrain its own CXC domain from participating in any transcriptional activities. The question becomes more intriguing, because CXC binding regions are available in promoter regions of some of the misregulated genes in the CHT7 mutant (cht7). Through the combination of biophysical experiments and molecular dynamics approaches, we have studied the DNA recognition behavior of CHT7_CXC. The results show that CHT7_CXC domain is highly selective towards DNA sequences and this selectivity is imparted due to the differential binding abilities of the CXC subdomains. Further, to understand if the case is - that CXC looses its DNA binding capabilities in the vicinity of other repressor molecules, we carried out CHT7_CXCs DNA binding stability test by simulating the spatial constraint conditions using the AsLOV2- CXC fusion. Our test results show limited ability of CHT7_CXC to withstand steric forces and provide insights to why and how algal cells may hold back CHT7_CXCs indulgence in quiescence repression. SignificanceMicroalgae, under nutrient rich conditions, provide biomass. Whereas, nutrient deprivation leads to accumulation of biofuel feedstock, but cells enter quiescence. Net enhancement in feedstock, therefore relies on the precision of the quiescence regulator. In Chlamydomonas reinhardtii, CHT7 is a central regulator of quiescence. Surprisingly, rather than using its own DNA binding domain (DBD) for the regulatory activities, CHT7 recruits external transcriptional regulators using its non DBDs. To ensure smooth functioning, CHT7s DBD must rapidly switch to inactive form. Modifications in DNA binding profiles of DBDs due to non DBDs are seen in transcription factors of many organisms. The switching mechanism discussed could therefore be a generic approach of timely regulation of individual components of the complex transcriptional machineries.

DNA molecule is the storage of genetic information in all living organisms. Its integrity is critical to life. However, due to the exposure to various environmental factors and endogenous agents, double-strand DNA breaks occur. Bacteria are capable to restore their genome integrity through a process called the SOS response. The key protein of SOS response is the RecA recombinase. Also critical for DNA repair is the SMC-like RecN protein, which helps RecA to find the homologous DNA template. Currently, its functions and mechanism of action remain poorly understood. In this work, using optical tweezers, we show predominant binding of RecN to ssDNA and also demonstrate a weak binding of dsDNA causing a condition similar to DNA loops formation.

Adenylate cyclase toxin (ACT) is one of the main virulence factors of Bordetella pertussis, with crucial role in colonization of human respiratory tract. ACT toxicity on target phagocytes results from translocation of its adenylate cyclase domain and production of high cAMP levels and from pore formation. Recently, we unveiled in ACT four cholesterol-recognition motifs involved in specific interaction with membrane cholesterol, which might stabilize membrane topology of critical helices for ACT activity. Here we explore an amphipathic peptide corresponding to ACT residues 454 to 487 containing one of such CRAC motifs. We show that P454-487 penetrates into DOPC vesicles as a long and tilted -helix, while in cholesterol presence experiments conformational changes that critically depend on the CRAC Phe-485 residue. Moreover, P454-487 is capable of blocking ACT toxicity on cells by outcompeting with the full-length toxin for membrane binding. We anticipate P454-487 may have potential clinical applicability in controlling Bordetella infection.

Antimicrobial peptides (AMPs) have emerged as exciting alternatives to the alarming increase of multiresistant bacteria due to their high activity against them through mechanisms that are thought to largely avoid resistance in the long term. Buforin II (BUFII) is an antibacterial peptide hypothesized to kill bacteria by crossing their membranes to interact with intracellular molecules and interrupt key processes for survival. In particular, interactions with DNA have been considered crucial for triggering cell death mechanisms. However, such interactions are still unknown, and thus far, no reports are available describing BUFII-DNA complexes. Here, we describe a complete biophysical study of the interaction between BUFII and Escherichia coli gDNA via spectrofluorimetric, spectroscopic, and microscopic techniques, complemented with whole-genome sequencing. The E. colis DNA-BUFII interactome was isolated by an in vitro pull-down method aided by BUFII-magnetite nanobioconjugates. Our results demonstrated that DNA-BUFII formed round-shape nanoscale complexes by strong electrostatic interactions, likely occurring nonspecifically throughout the entire bacterial genome. Further sequencing of the isolated DNA fragments corroborated this notion and led to hypothesize that BUFII is possibly responsible for inducing DNAs supercoiling. Other evidence for this idea was provided by the significant DNA conformational changes observed upon interaction with BUFII. Even though the evidence found fails to describe the complete action mechanism of BUFII in vivo, our findings pave the way to engineer DNA-peptide supramolecular complexes very precisely, which might find application in the field of gene therapy delivery.

Calcium is crucial in insulin biology and Ca2+ sensor proteins enforced in insulin release and signalling. The neuronal calcium sensor proteins (NCS) such as NCS-1 and VILIP are shown to be involved in insulin secretion from {beta}-pancreatic cells. However, the expression of different NCS proteins in the pancreas and their functional significance and role in pathologies remained unexplored. The present work, through different biophysical methods, presented that NCS proteins interact with insulin. NCS-1, the founder member of NCS family proteins interacts with insulin in a Ca2+ independent manner and Ca2+ enhances the affinity of the interaction. The evolutionarily conserved cryptic EF-hand in NCS proteins was found to be an essential commodity for binding insulin. The presence of Ca2+ binding first EF-hand abolishes the interaction with insulin and suggests the significance of non-functional EF-hand. The fluorescence and circular dichroism (CD) spectroscopy show that insulin interaction induces structural changes in NCS-1, which is demonstrated by size exclusion chromatography and analytical ultra-centrifugation. The autism mutant NCS-1-R102Q relatively retained insulin binding properties but with a significant difference in binding thermodynamics. Considering substantial sequence similarity among different NCS proteins and localisation in the pancreas, we examined the insulin interaction with the neurocalcin delta (NCALD). The NCALD shows metal ion-independent insulin binding and contrary to NCS-1, the Ca2+ abolishes the insulin binding. This highlights the differential regulation of Ca2+ towards insulin interaction in NCS protein. Conclusively the present work highlight that NCS proteins interact with insulin and further investigation would aid to understand the significance of NCS proteins in insulin physiology/pathophysiology and possible new molecular targets in diabetes.

The EccC enzyme of M. tuberculosis ESX-1 system is a promising target for antivirulence drug development. The EccC enzyme comprises two polypeptides (i) EccCa1, a membrane bound enzyme having two ATPase domains D2 & D3 (ii) cytosolic EccCb, which contains two ATPase domains. In current study, we have analyzed the low-resolution structure of EccCb1, performed ATPase activity and EsxAB substrate binding analysis. The EccCb1 enzyme eluted as oligomer from size exclusion column and small angle X-ray scattering analysis revealed the double hexameric structure in solution. The EccCb1 enzyme showed catalytic efficiency (kcat/KM)[~] 0.020{+/-}0.005 M-1 min-1, however [~] 3.7 fold lower than its D2 and [~]1.7 fold lower than D3 domains respectively. The D2 and D3 domains exhibited the ATPase activity and mutation of residues involved in ATP+Mg2+ binding have yielded 56-94% reduction in catalytic efficiency for both D2 and D3 domains. The EccCb1 binds the EsxAB substrate with KD [~] 11.4{+/-}3.4 nM via specific groove located at C-terminal region of D3 domain. ATP binding to EccCb1 enhanced the EsxAB substrate binding by [~] 3 fold, indicating ATPase energy involvement in EsxAB substrate translocation. We modeled the dodecameric EccCb1+EsxAB+ ATP+Mg2+ complex, which showed the binding pockets involved in ATP+Mg2+ and EsxAB substrate binding. The enzyme dynamics involved in ATP+Mg2+ and EsxAB substrate recognition were identified and showed the enhanced stability of EccCb1 enzyme as a result of ligand binding. Overall, our structural and biochemical analysis showed the low-resolution structure and mechanism involved in ATPase activity and EsxAB substrate binding and dynamics involved in EsxAB substrate and ATP+Mg2+ recognition. Overall, our structural and biochemical data on EccCb1 will contribute significantly in development of antivirulence inhibitors, which will prevent virulence factor secretion by M. tuberculosis ESX-1 system.

Mycobacterium tuberculosis uses the ESAT-6 system-1/type VII (ESX-1) system for secretion of virulence proteins into the host cell, however the mechanism of virulence proteins secretion, molecular components and regulation of ESX-1 system are only partly understood. In the current study, we have analyzed the biological function and recognition mechanism between ESX-1 virulence EspC and EccA1 ATPase proteins. The EspC enters into A549 human lung carcinoma cells and exhibited cytotoxicity, as observed in MTT Assay. To understand the recognition mechanism between EspC and EccA1 ATPase, the EspC and EccA1 mutants were generated based on EspC~EccA1 interactions, as observed in molecular modeling. Binding analysis shows that EspC export arm interacts specifically to the {beta}-hairpin insertion motif of the TPR domain of EccA1 ATPase. Mutations in these epitopes lead to significant decrease/or abolish the binding between EspC and EccA1 ATPase. Our study provides insight into biological function and recognition mechanism between EspC and EccA1 ATPase, which can be used as target to prevent EspC secretion/ or in general virulence factor secretion by mycobacterial ESX-1 system.

Membrane proteins are critical elements of numerous therapeutic approaches ranging from cancer to bacterial infections. MsbA is a bacterial membrane protein that has received increasing attention as an antibacterial target for its role in the processing of Lipid A, a key precursor of lipopolysaccharide that is essential for bacterial growth. When employing nanodiscs it is possible to stabilize MsbA by providing a membrane-like environment that enhances its enzymatic activity. Taking advantage of this property we have carried out a fragment screening using the biophysical method of surface plasmon resonance. This approach identified several compounds that bind specifically to MsbA. In particular, one of these fragment molecules not only binds to the target, but also inhibits the ATPase activity of the MsbA protein. The similarity of this fragment to the adenine moiety of ATP points at a route to generate stronger and more potent inhibitors for MsbA and even other proteins of its family of ABC transporters. Collectively, our study reveals biophysical approaches that facilitate the identification of fragment candidates inhibiting the activity of membrane proteins.

CRISPR-Cas12d is a distinct V-D type system discovered in the metagenomes of Candidate Phyla Radiation bacteria. It stands out from most closely related systems due to its 17-19 nucleotide short spacer region and specialized stabilizing scoutRNAs. We made significant improvements to this system by modifying its scoutRNA to create sgRNA, which greatly simplifies its use. We found mutations in the RuvC domain of the effector protein KbCas12d that resulted in loss of nuclease activity. We obtained two catalytically inactive dKbCas12d variants: D827A and E913A. Using the optical tweezers technique, we demonstrated the high specificity of dKbCas12d in binding targets on individual DNA molecules. Engineered sgRNA and catalytically inactive dKbCas12d variants have promising applications in biotechnology for the precise regulation of gene expression and molecular diagnostics.

Mycobacterial GroELs namely GroEL1 and GroEL2 belong to the family of molecular chaperones, chaperonins. Chaperonins in Escherichia coli are termed as GroEL and GroES which are encoded by essential genes and are involved in cellular protein folding. GroEL1 has a characteristic Histidine-rich C-terminus contrary to its essential paralog GroEL2 and E. coli GroEL which have hydrophobic (GGM) repeats. Since Histidine richness is likely to be involved in metal binding, in this study we have attempted to decipher the role of GroEL1 protein in chelating metals and the consequent role on M. tuberculosis physiology. Using isothermal titration calorimetry (ITC), we found that GroEL1 binds copper, nickel and cobalt, with the highest binding affinity to copper. Since copper is known to be toxic at higher concentration, we cultured Wild Type M. tuberculosis H37Rv, groEL1 knock-out and groEL1-complemented strain with increasing concentrations of copper. We found that M. tuberculosis groEL1 knock out strain is more sensitive to copper than the wild type. Further hypothesizing that the probable mode of action of copper is by induction of oxidative stress, we attempted to understand the role of GroEL1 in redox silencing and hydroxyl radical mediated DNA damage. We interestingly found through our in vitro studies that GroEL1 is helpful in protection from copper stress through maintaining redox balance and free radical mediated DNA damage. Thus, these results indicate that the duplication of chaperonin genes in M. tuberculosis might have led to their evolutionary divergence and resulted in a functional divergence of chaperonins.

The process of protein phase separation, particularly in the context of intrinsically disordered proteins, has been extensively studied for its implications in several neuro-degenerative diseases. Although the mechanism of protein phase separation and the involved molecular grammar have been well explored under in vitro conditions, the focus is now shifting towards developing more complex models of phase separation in order to mimic the biological systems more closely. Here, we studied the phase separation of alpha synuclein (-syn), an intrinsically disordered protein whose aggregation is implicated in the pathology of Parkinsons Disease, (PD) inside yeast cells (Saccharomyces cerevisiae). Using a positively charged polymer; polyethylenimine (PEI), which binds presumably at the negatively charged C-terminal domain of -syn, we find that the aggregation of -syn inside yeast can be modulated by at least two pathways: one involving phase separation and the second one without phase separation. We find further that these two pathways lead to varying fibril characteristics and toxicities. We believe that this model can be used as a quick and convenient system to screen novel and repurposed small molecules against toxic protein droplets.

Vascular Endothelial Growth Factor-A (VEGF-A), a pluripotent cytokine and angiogenic growth factor mediates the switch to an angiogenic phenotype in cancer cells. The interaction of VEGF-A protein with the VEGF receptors (VEGFR-1and VEGFR-2) starts downstream effect that promotes angiogenesis by mediating migration and increasing the permeability of endothelial cells. A cis-regulatory elements consisting of a polypurine/polypyrimidine (pPu/pPy) tract in the proximal 36-bp region (-85 to -50), can participate in the formation of a stable higher order G-quadruplex structure (G4) which is essential for VEGF promoter activity. During cancer progression the VEGF-A G4 succumbs to cellular pressure and fails to maintain the stable structure. This shifts the balance to form duplex structure thereby increasing the rate of transcription. Earlier research has tried to develop small-molecule ligands to target and stabilize G4, however they either lack specificity or non-toxicity. Peptide on the other hand are very less studied. Here we used bioinformatics in-silico tool to develop peptides which can successfully bind and stabilize the VEGF-A G4 while reducing its gene expression. This further alters the expression fate of the VEGF-A signalling cascade and prevents angiogenesis in cancer cells. We used high resolution Nuclear magnetic resonance and molecular dynamics simulation to map the chemistry of the interaction while the qPCR and western blot allowed us to check the expression pattern of the molecules of VEGF-A signalling cascade. In this investigation, we navigate the complex interplay between peptides and quadruplex structures, unravelling valuable insights that can enhance the crafting of pharmacophores directed at the dynamic quadruplex structure. The outcomes of our study are promising, paving the way for progress in the realms of research, characterization, and optimization of peptides binding to G-quadruplexes, with potential implications for therapeutic applications.