Characterisation of ten NS2B-NS3 proteases: Paving the way for pan-flavivirus drugs.

Flaviviruses can cause severe illness in humans. Effective and safe vaccines are available for some species; however, for many flaviviruses disease prevention or specific treatments remain unavailable. The viral replication cycle depends on the proteolytic activity of the NS2B-NS3 protease, which releases functional viral proteins from a non-functional polyprotein precursor, rendering the protease a promising drug target. In this study, we characterised recombinant NS2B-NS3 proteases from ten flaviviruses including three unreported proteases from the Usutu, Kyasanur forest disease and Powassan viruses. All protease constructs comprise a covalent Gly4-Ser-Gly4 linker connecting the NS3 serine protease domain with its cofactor NS2B. We conducted a comprehensive cleavage site analysis revealing areas of high conversion. While all proteases were active in enzymatic assays, we noted a 1000-fold difference in catalytic efficiency across proteases from different flaviviruses. Two bicyclic peptide inhibitors displayed anti-pan-flaviviral protease activity with inhibition constants ranging from 10 to 1000 nM.

The Inhibition of NS2B/NS3 Protease: A New Therapeutic Opportunity to Treat Dengue and Zika Virus Infection.

In the global pandemic scenario, dengue and zika viruses (DENV and ZIKV, respectively), both mosquito-borne members of the flaviviridae family, represent a serious health problem, and considering the absence of specific antiviral drugs and available vaccines, there is a dire need to identify new targets to treat these types of viral infections. Within this drug discovery process, the protease NS2B/NS3 is considered the primary target for the development of novel anti-flavivirus drugs. The NS2B/NS3 is a serine protease that has a dual function both in the viral replication process and in the elusion of the innate immunity. To date, two main classes of NS2B/NS3 of DENV and ZIKV protease inhibitors have been discovered: those that bind to the orthosteric site and those that act at the allosteric site. Therefore, this perspective article aims to discuss the main features of the use of the most potent NS2B/NS3 inhibitors and their impact at the social level.

Unraveling the mechanisms of Sofosbuvir resistance in HCV NS3/4A protease: Structural and molecular simulation-based insights.

Current management of HCV infection is based on Direct-Acting Antiviral Drugs (DAAs). However, resistance-associated mutations, especially in the NS3 and NS5B regions are gradually decreasing the efficacy of DAAs. Among the most effective HCV NS3/4A protease drugs, Sofosbuvir also develops resistance due to mutations in the NS3 and NS5B regions. Four mutations at positions A156Y, L36P, Q41H, and Q80K are classified as high-level resistance mutations. The resistance mechanism of HCV NS3/4A protease toward Sofosbuvir caused by these mutations is still unclear, as there is less information available regarding the structural and functional effects of the mutations against Sofosbuvir. In this work, we combined molecular dynamics simulation, molecular mechanics/Generalized-Born surface area calculation, principal component analysis, and free energy landscape analysis to explore the resistance mechanism of HCV NS3/4A protease due to these mutations, as well as compare interaction changes in wild-type. Subsequently, we identified that the mutant form of HCV NS3/4A protease affects the activity of Sofosbuvir. In this study, the resistance mechanism of Sofosbuvir at the atomic level is proposed. The proposed drug-resistance mechanism will provide valuable guidance for the design of HCV drugs.

Dual-wield NTPases: A novel protein family mined from AlphaFold DB.

AlphaFold protein structure database (AlphaFold DB) archives a vast number of predicted models. We conducted systematic data mining against AlphaFold DB and discovered an uncharacterized P-loop NTPase family. The structure of the protein family was surprisingly novel, showing an atypical topology for P-loop NTPases, noticeable twofold symmetry, and two pairs of independent putative active sites. Our findings show that structural data mining is a powerful approach to identifying undiscovered protein families.

Contrasting functions of ATP hydrolysis by MDA5 and LGP2 in viral RNA sensing.

Cytosolic long dsRNA, among the most potent proinflammatory signals, is recognized by melanoma differentiation-associated protein 5 (MDA5). MDA5 binds dsRNA cooperatively forming helical filaments. ATP hydrolysis by MDA5 fulfills a proofreading function by promoting dissociation of shorter endogenous dsRNs from MDA5 while allowing longer viral dsRNAs to remain bound leading to activation of interferon-ß responses. Here, we show that adjacent MDA5 subunits in MDA5-dsRNA filaments hydrolyze ATP cooperatively, inducing cooperative filament disassembly. Consecutive rounds of ATP hydrolysis amplify the filament footprint, displacing tightly bound proteins from dsRNA. Our electron microscopy and biochemical assays show that LGP2 binds to dsRNA at internal binding sites through noncooperative ATP hydrolysis. Unlike MDA5, LGP2 has low nucleic acid selectivity and can hydrolyze GTP and CTP as well as ATP. Binding of LGP2 to dsRNA promotes nucleation of MDA5 filament assembly resulting in shorter filaments. Molecular modeling identifies an internally bound MDA5-LGP2-RNA complex, with the LGP2 C-terminal tail forming the key contacts with MDA5. These contacts are specifically required for NTP-dependent internal RNA binding. We conclude that NTPase-dependent binding of LGP2 to internal dsRNA sites complements NTPase-independent binding to dsRNA ends, via distinct binding modes, to increase the number and signaling output of MDA5-dsRNA complexes.

NLRP14 deficiency causes female infertility with oocyte maturation defects and early embryonic arrest by impairing cytoplasmic UHRF1 abundance.

Oocyte maturation is vital to attain full competence required for fertilization and embryogenesis. NLRP14 is preferentially expressed in mammalian oocytes and early embryos. Yet, the role and molecular mechanism of NLRP14 in oocyte maturation and early embryogenesis are poorly understood, and whether NLRP14 deficiency accounts for human infertility is unknown. Here, we found that maternal loss of Nlrp14 resulted in sterility with oocyte maturation defects and early embryonic arrest (EEA). Nlrp14 ablation compromised oocyte competence due to impaired cytoplasmic and nuclear maturation. Importantly, we revealed that NLRP14 maintained cytoplasmic UHRF1 abundance by protecting it from proteasome-dependent degradation and anchoring it from nuclear translocation in the oocyte. Furthermore, we identified compound heterozygous NLRP14 variants in women affected by infertility with EEA, which interrupted the NLRP14-UHRF1 interaction and decreased UHRF1 levels. Our data demonstrate NLRP14 as a cytoplasm-specific regulator of UHRF1 during oocyte maturation, providing insights into genetic diagnosis for female infertility.

NLRP14 Safeguards Calcium Homeostasis via Regulating the K27 Ubiquitination of Nclx in Oocyte-to-Embryo Transition.

Sperm-induced Ca2+ rise is critical for driving oocyte activation and subsequent embryonic development, but little is known about how lasting Ca2+ oscillations are regulated. Here it is shown that NLRP14, a maternal effect factor, is essential for keeping Ca2+ oscillations and early embryonic development. Few embryos lacking maternal NLRP14 can develop beyond the 2-cell stage. The impaired developmental potential of Nlrp14-deficient oocytes is mainly caused by disrupted cytoplasmic function and calcium homeostasis due to altered mitochondrial distribution, morphology, and activity since the calcium oscillations and development of Nlrp14-deficient oocytes can be rescued by substitution of whole cytoplasm by spindle transfer. Proteomics analysis reveal that cytoplasmic UHRF1 (ubiquitin-like, containing PHD and RING finger domains 1) is significantly decreased in Nlrp14-deficient oocytes, and Uhrf1-deficient oocytes also show disrupted calcium homeostasis and developmental arrest. Strikingly, it is found that the mitochondrial Na+ /Ca2+ exchanger (NCLX) encoded by Slc8b1 is significantly decreased in the Nlrp14mNull oocyte. Mechanistically, NLRP14 interacts with the NCLX intrinsically disordered regions (IDRs) domain and maintain its stability by regulating the K27-linked ubiquitination. Thus, the study reveals NLRP14 as a crucial player in calcium homeostasis that is important for early embryonic development.

Atypical low-copy number plasmid segregation systems, all in one?

Plasmid families harbor different maintenances functions, depending on their size and copy number. Low copy number plasmids rely on active partition systems, organizing a partition complex at specific centromere sites that is actively positioned using NTPase proteins. Some low copy number plasmids lack an active partition system, but carry atypical intracellular positioning systems using a single protein that binds to the centromere site but without an associated NTPase. These systems have been studied in the case of the Escherichia coli R388 and of the Staphylococcus aureus pSK1 plasmids. Here we review these two systems, which appear to be unrelated but share common features, such as their distribution on plasmids of medium size and copy number, certain activities of their centromere-binding proteins, StbA and Par, respectively, as well as their mode of action, which may involve dynamic interactions with the nucleoid-packed chromosome of their hosts.

SARS-CoV-2 nsp13 Restricts Episomal DNA Transcription without Affecting Chromosomal DNA.

Nonstructural protein 13 (nsp13), the helicase of SARS-CoV-2, has been shown to possess multiple functions that are essential for viral replication, and is considered an attractive target for the development of novel antivirals. We were initially interested in the interplay between nsp13 and interferon (IFN) signaling, and found that nsp13 inhibited reporter signal in an IFN-ß promoter assay. Surprisingly, the ectopic expression of different components of the RIG-I/MDA5 pathway, which were used to stimulate IFN-ß promoter, was also mitigated by nsp13. However, endogenous expression of these genes was not affected by nsp13. Interestingly, nsp13 restricted the expression of foreign genes originating from plasmid transfection, but failed to inhibit them after chromosome integration. These data, together with results from a runoff transcription assay and RNA sequencing, suggested a specific inhibition of episomal but not chromosomal gene transcription by nsp13. By using different truncated and mutant forms of nsp13, we demonstrated that its NTPase and helicase activities contributed to the inhibition of episomal DNA transcription, and that this restriction required direct interaction with episomal DNA. Based on these findings, we developed an economical and convenient high-throughput drug screening method targeting nsp13. We evaluated the inhibitory effects of various compounds on nsp13 by the expression of reporter gene plasmid after co-transfection with nsp13. In conclusion, we found that nsp13 can specifically inhibit episomal DNA transcription and developed a high-throughput drug screening method targeting nsp13 to facilitate the development of new antiviral drugs. IMPORTANCE To combat COVID-19, we need to understand SARS-CoV-2 and develop effective antiviral drugs. In our study, we serendipitously found that SARS-CoV-2 nsp13 could suppress episomal DNA transcription without affecting chromosomal DNA. Detailed characterization revealed that nsp13 suppresses episomal gene expression through its NTPase and helicase functions following DNA binding. Furthermore, we developed a high-throughput drug screening system targeting SARS-CoV-2 nsp13. Compared to traditional SARS-CoV-2 drug screening methods, our system is more economical and convenient, facilitating the development of more potent and selective nsp13 inhibitors and enabling the discovery of new antiviral therapies.

Extended DNA threading through a dual-engine motor module of the activating signal co-integrator 1 complex.

Activating signal co-integrator 1 complex (ASCC) subunit 3 (ASCC3) supports diverse genome maintenance and gene expression processes, and contains tandem Ski2-like NTPase/helicase cassettes crucial for these functions. Presently, the molecular mechanisms underlying ASCC3 helicase activity and regulation remain unresolved. We present cryogenic electron microscopy, DNA-protein cross-linking/mass spectrometry as well as in vitro and cellular functional analyses of the ASCC3-TRIP4 sub-module of ASCC. Unlike the related spliceosomal SNRNP200 RNA helicase, ASCC3 can thread substrates through both helicase cassettes. TRIP4 docks on ASCC3 via a zinc finger domain and stimulates the helicase by positioning an ASC-1 homology domain next to the C-terminal helicase cassette of ASCC3, likely supporting substrate engagement and assisting the DNA exit. TRIP4 binds ASCC3 mutually exclusively with the DNA/RNA dealkylase, ALKBH3, directing ASCC3 for specific processes. Our findings define ASCC3-TRIP4 as a tunable motor module of ASCC that encompasses two cooperating NTPase/helicase units functionally expanded by TRIP4.

Norovirus MLKL-like protein initiates cell death to induce viral egress.

Non-enveloped viruses require cell lysis to release new virions from infected cells, suggesting that these viruses require mechanisms to induce cell death. Noroviruses are one such group of viruses, but there is no known mechanism that causes norovirus infection-triggered cell death and lysis1-3. Here we identify a molecular mechanism of norovirus-induced cell death. We found that the norovirus-encoded NTPase NS3 contains an N-terminal four-helix bundle domain homologous to the membrane-disruption domain of the pseudokinase mixed lineage kinase domain-like (MLKL). NS3 has a mitochondrial localization signal and thus induces cell death by targeting mitochondria. Full-length NS3 and an N-terminal fragment of the protein bound the mitochondrial membrane lipid cardiolipin, permeabilized the mitochondrial membrane and induced mitochondrial dysfunction. Both the N-terminal region and the mitochondrial localization motif of NS3 were essential for cell death, viral egress from cells and viral replication in mice. These findings suggest that noroviruses have acquired a host MLKL-like pore-forming domain to facilitate viral egress by inducing mitochondrial dysfunction.

Characterization of Human Norovirus Nonstructural Protein NS1.2 Involved in the Induction of the Filamentous Endoplasmic Reticulum, Enlarged Lipid Droplets, LC3 Recruitment, and Interaction with NTPase and NS4.

Human noroviruses (HuNVs) are the leading cause of gastroenteritis worldwide. NS1.2 is critical for HuNV pathogenesis, but the function is still unclear. The GII NS1.2 of HuNVs, unlike GI NS1.2, was localized to the endoplasmic reticulum (ER) and lipid droplets (LDs) and is accompanied by a distorted-filamentous ER morphology and aggregated-enlarged LDs. LC3 was recruited to the NS1.2-localized membrane through an autophagy-independent pathway. NS1.2, expressed from a cDNA clone of GII.4 norovirus, formed complexes with NTPase and NS4, which exhibited aggregated vesicle-like structures that were also colocalized with LC3 and LDs. NS1.2 is structurally divided into three domains from the N terminus: an inherently disordered region (IDR), a region that contains a putative hydrolase with the H-box/NC catalytic center (H-box/NC), and a C-terminal 251-330 a.a. region containing membrane-targeting domain. All three functional domains of NS1.2 were required for the induction of the filamentous ER. The IDR was essential for LC3 recruitment by NS1.2. Both the H-Box/NC and membrane-targeting domains are required for the induction of aggregated-enlarged LDs, NS1.2 self-assembly, and interaction with NTPase. The membrane-targeting domain was sufficient to interact with NS4. The study characterized the NS1.2 domain required for membrane targeting and protein-protein interactions, which are crucial for forming a viral replication complex.

The alphavirus nonstructural protein 2 NTPase induces a host translational shut-off through phosphorylation of eEF2 via cAMP-PKA-eEF2K signaling.

Chikungunya virus (CHIKV) is a reemerging alphavirus. Since 2005, it has infected millions of people during outbreaks in Africa, Asia, and South/Central America. CHIKV replication depends on host cell factors at many levels and is expected to have a profound effect on cellular physiology. To obtain more insight into host responses to infection, stable isotope labeling with amino acids in cell culture and liquid chromatography-tandem mass spectrometry were used to assess temporal changes in the cellular phosphoproteome during CHIKV infection. Among the ~3,000 unique phosphorylation sites analyzed, the largest change in phosphorylation status was measured on residue T56 of eukaryotic elongation factor 2 (eEF2), which showed a >50-fold increase at 8 and 12 h p.i. Infection with other alphaviruses (Semliki Forest, Sindbis and Venezuelan equine encephalitis virus (VEEV)) triggered a similarly strong eEF2 phosphorylation. Expression of a truncated form of CHIKV or VEEV nsP2, containing only the N-terminal and NTPase/helicase domains (nsP2-NTD-Hel), sufficed to induce eEF2 phosphorylation, which could be prevented by mutating key residues in the Walker A and B motifs of the NTPase domain. Alphavirus infection or expression of nsP2-NTD-Hel resulted in decreased cellular ATP levels and increased cAMP levels. This did not occur when catalytically inactive NTPase mutants were expressed. The wild-type nsP2-NTD-Hel inhibited cellular translation independent of the C-terminal nsP2 domain, which was previously implicated in directing the virus-induced host shut-off for Old World alphaviruses. We hypothesize that the alphavirus NTPase activates a cellular adenylyl cyclase resulting in increased cAMP levels, thus activating PKA and subsequently eukaryotic elongation factor 2 kinase. This in turn triggers eEF2 phosphorylation and translational inhibition. We conclude that the nsP2-driven increase of cAMP levels contributes to the alphavirus-induced shut-off of cellular protein synthesis that is shared between Old and New World alphaviruses. MS Data are available via ProteomeXchange with identifier PXD009381.

The NTPase activity of the double FYVE domain-containing protein 1 regulates lipid droplet metabolism.

Lipid droplets (LDs) are transient lipid storage organelles that can be readily tapped to resupply cells with energy or lipid building blocks and therefore play a central role in cellular metabolism. However, the molecular factors and underlying mechanisms that regulate the growth and degradation of LDs are poorly understood. It has emerged that proteins that establish contacts between LDs and the endoplasmic reticulum play a critical role in regulating LD metabolism. Recently, the autophagy-related protein, double FYVE domain-containing protein 1 (DFCP1/ZFYVE1) was shown to reside at the interface of the endoplasmic reticulum and LDs, however, little is known about the involvement of DFCP1 in autophagy and LD metabolism. Here, we show that DFCP1 is a novel NTPase that regulates free fatty acid metabolism. Specifically, we show that DFPC1-knockdown, particularly during starvation, increases cellular free fatty acids and decreases the levels of cellular TAGs, resulting in accumulated small LDs. Using selective truncations, we demonstrate that DFCP1 accumulation on LDs in cells and in vitro is regulated by a previously unknown NTPase domain. Using spectroscopic approaches, we show that this NTPase domain can dimerize and can hydrolyze both ATP and GTP. Furthermore, mutations in DFCP1 that either impact nucleotide hydrolysis or dimerization result in changes in the accumulation of DFCP1 on LDs, changes in LD density and size, and colocalization of LDs to autophagosomes. Collectively, our findings suggest that DFCP1 is an NTPase that modulates the metabolism of LDs in cells.

Genetic Diversity and Characterization of Circular Replication (Rep)-Encoding Single-Stranded (CRESS) DNA Viruses.

The CRESS-DNA viruses are the ubiquitous virus detected in almost all eukaryotic life trees and play an essential role in the maintaining ecosystem of the globe. Still, their genetic diversity is not fully understood. Here, we bring to light the genetic diversity of replication (Rep) and capsid (Cap) proteins of CRESS-DNA viruses. We divided the Rep protein of the CRESS-DNA virus into 10 clusters using CLANS and phylogenetic analyses. Also, most of the Rep protein in Rep cluster 1 (R1) and R2 (Circoviridae, Smacoviridae, Nanoviridae, and CRESSV1-5) contain the Viral_Rep superfamily and P-loop_NTPase superfamily domains, while the Rep protein of viruses in other clusters has no such characterized functional domain. The Circoviridae, Nanoviridae, and CRESSV1-3 viruses contain two domains, such as Viral_Rep and P-loop_NTPase; the CRESSV4 and CRESSV5 viruses have only the Viral_Rep domain; most of the sequences in the pCRESS-related group have only P-loop_NTPase; and Smacoviridae do not have these two domains. Further, we divided the Cap protein of the CRESS-DNA virus into 20 clusters using CLANS and phylogenetic analyses. The Rep and Cap proteins of Circoviridae and Smacoviridae are grouped into a specific cluster. Cap protein of CRESS-DNA viruses grouped with one cluster and Rep protein with another cluster. Further, our study reveals that selection pressure plays a significant role in the evolution of CRESS-DNA viruses' Rep and Cap genes rather than mutational pressure. We hope this study will help determine the genetic diversity of CRESS-DNA viruses as more sequences are discovered in the future. IMPORTANCE The genetic diversity of CRESS-DNA viruses is not fully understood. CRESS-DNA viruses are classified as CRESSV1 to CRESSV6 using only Rep protein. This study revealed that the Rep protein of the CRESS-DNA viruses is classified as CRESSV1 to CRESSV6 groups and the new Smacoviridae-related, CRESSV2-related, pCRESS-related, Circoviridae-related, and 1 to 4 outgroups, according to the Viral_Rep and P-loop_NTPase domain organization, CLANS, and phylogenetic analysis. Furthermore, for the first time in this study, the Cap protein of CRESS-DNA viruses was classified into 20 distinct clusters by CLANS and phylogenetic analysis. Through this classification, the genetic diversity of CRESS-DNA viruses clarifies the possibility of recombinations in Cap and Rep proteins. Finally, it has been shown that selection pressure plays a significant role in the evolution and genetic diversity of Cap and Rep proteins. This study explains the genetic diversity of CRESS-DNA viruses and hopes that it will help classify future detected viruses.

Common Patterns of Hydrolysis Initiation in P-loop Fold Nucleoside Triphosphatases.

The P-loop fold nucleoside triphosphate (NTP) hydrolases (also known as Walker NTPases) function as ATPases, GTPases, and ATP synthases, are often of medical importance, and represent one of the largest and evolutionarily oldest families of enzymes. There is still no consensus on their catalytic mechanism. To clarify this, we performed the first comparative structural analysis of more than 3100 structures of P-loop NTPases that contain bound substrate Mg-NTPs or their analogues. We proceeded on the assumption that structural features common to these P-loop NTPases may be essential for catalysis. Our results are presented in two articles. Here, in the first, we consider the structural elements that stimulate hydrolysis. Upon interaction of P-loop NTPases with their cognate activating partners (RNA/DNA/protein domains), specific stimulatory moieties, usually Arg or Lys residues, are inserted into the catalytic site and initiate the cleavage of gamma phosphate. By analyzing a plethora of structures, we found that the only shared feature was the mechanistic interaction of stimulators with the oxygen atoms of gamma-phosphate group, capable of causing its rotation. One of the oxygen atoms of gamma phosphate coordinates the cofactor Mg ion. The rotation must pull this oxygen atom away from the Mg ion. This rearrangement should affect the properties of the other Mg ligands and may initiate hydrolysis according to the mechanism elaborated in the second article.

Common Mechanism of Activated Catalysis in P-loop Fold Nucleoside Triphosphatases-United in Diversity.

To clarify the obscure hydrolysis mechanism of ubiquitous P-loop-fold nucleoside triphosphatases (Walker NTPases), we analysed the structures of 3136 catalytic sites with bound Mg-NTP complexes or their analogues. Our results are presented in two articles; here, in the second of them, we elucidated whether the Walker A and Walker B sequence motifs-common to all P-loop NTPases-could be directly involved in catalysis. We found that the hydrogen bonds (H-bonds) between the strictly conserved, Mg-coordinating Ser/Thr of the Walker A motif ([Ser/Thr]WA) and aspartate of the Walker B motif (AspWB) are particularly short (even as short as 2.4 ångströms) in the structures with bound transition state (TS) analogues. Given that a short H-bond implies parity in the pKa values of the H-bond partners, we suggest that, in response to the interactions of a P-loop NTPase with its cognate activating partner, a proton relocates from [Ser/Thr]WA to AspWB. The resulting anionic [Ser/Thr]WA alkoxide withdraws a proton from the catalytic water molecule, and the nascent hydroxyl attacks the gamma phosphate of NTP. When the gamma-phosphate breaks away, the trapped proton at AspWB passes by the Grotthuss relay via [Ser/Thr]WA to beta-phosphate and compensates for its developing negative charge that is thought to be responsible for the activation barrier of hydrolysis.

Molecular Analysis of pSK1 par: A Novel Plasmid Partitioning System Encoded by Staphylococcal Multiresistance Plasmids.

The segregation of prokaryotic plasmids typically requires a centromere-like site and two proteins, a centromere-binding protein (CBP) and an NTPase. By contrast, a single 245 residue Par protein mediates partition of the prototypical staphylococcal multiresistance plasmid pSK1 in the absence of an identifiable NTPase component. To gain insight into centromere binding by pSK1 Par and its segregation function we performed structural, biochemical and in vivo studies. Here we show that pSK1 Par binds a centromere consisting of seven repeat elements. We demonstrate this Par-centromere interaction also mediates Par autoregulation. To elucidate the Par centromere binding mechanism, we obtained a structure of the Par N-terminal DNA-binding domain bound to centromere DNA to 2.25 Å. The pSK1 Par structure, which harbors a winged-helix-turn-helix (wHTH), is distinct from other plasmid CBP structures but shows homology to the B. subtilis chromosome segregation protein, RacA. Biochemical studies suggest the region C-terminal to the Par wHTH forms coiled coils and mediates oligomerization. Fluorescence microscopy analyses show that pSK1 Par enhances the separation of plasmids from clusters, driving effective segregation upon cell division. Combined the data provide insight into the molecular properties of a single protein partition system.

Characterization of the DNA Binding Domain of StbA, A Key Protein of A New Type of DNA Segregation System.

Low-copy-number plasmids require sophisticated genetic devices to achieve efficient segregation of plasmid copies during cell division. Plasmid R388 uses a unique segregation mechanism, based on StbA, a small multifunctional protein. StbA is the key protein in a segregation system not involving a plasmid-encoded NTPase partner, it regulates the expression of several plasmid operons, and it is the main regulator of plasmid conjugation. The mechanisms by which StbA, together with the centromere-like sequence stbS, achieves segregation, is largely uncharacterized. To better understand the molecular basis of R388 segregation, we determined the crystal structure of the conserved N-terminal domain of StbA to 1.9 Å resolution. It folds into an HTH DNA-binding domain, structurally related to that of the PadR subfamily II of transcriptional regulators. StbA is organized in two domains. Its N-terminal domain carries the specific stbS DNA binding activity. A truncated version of StbA, deleted of its C-terminal domain, displays only partial activities in vivo, indicating that the non-conserved C-terminal domain is required for efficient segregation and subcellular plasmid positioning. The structure of StbA DNA-binding domain also provides some insight into how StbA monomers cooperate to repress transcription by binding to the stbDR and to form the segregation complex with stbS.

The nonstructural protein 2C of Coxsackie B virus has RNA helicase and chaperoning activities.

RNA-remodeling proteins, including RNA helicases and chaperones, play vital roles in the remodeling of structured RNAs. During viral replication, viruses require RNA-remodeling proteins to facilitate proper folding and/or re-folding the viral RNA elements. Coxsackieviruses B3 (CVB3) and Coxsackieviruses B5 (CVB5), belonging to the genus Enterovirus in the family Picornaviridae, have been reported to cause various infectious diseases such as hand-foot-and-mouth disease, aseptic meningitis, and viral myocarditis. However, little is known about whether CVB3 and CVB5 encode any RNA remodeling proteins. In this study, we showed that 2C proteins of CVB3 and CVB5 contained the conserved SF3 helicase A, B, and C motifs, and functioned not only as RNA helicase that unwound RNA helix bidirectionally in an NTP-dependent manner, but also as RNA chaperone that remodeled structured RNAs and facilitated RNA strand annealing independently of NTP. In addition, we determined that the NTPase activity and RNA helicase activity of 2C proteins of CVB3 and CVB5 were dependent on the presence of divalent metallic ions. Our findings demonstrate that 2C proteins of CVBs possess RNA-remodeling activity and underline the functional importance of 2C protein in the life cycle of CVBs.