RecA-like motor ATPases—lessons from structures

The cluster model of energy transduction in biological systems

The central problem in transduction is to explain how the energy caught from sunlight by chloroplasts becomes biological work. Or to express it in different terms: how does the energy remain trapped in the biological network and not get lost through thermalization into the environment? The pathway consists of an immensely large number of steps crossing hierarchical levels - some upwards, to larger assemblies, others downwards into energy rich molecules - before fuelling an action potential or a contracting cell. Accepting the assumption that steps are executed by protein domains, we expect that transduction mechanisms are the result of conformational changes, which in turn involve rearrangements of the bonds responsible for the protein fold. But why are these essential changes so difficult to detect? In this presentation, the metabolic pathway is viewed as equivalent to an energy conduit composed of equally sized units - the protein domains - rather than a row of catalysts. The flow of energy through them occurs by the same mechanism as through the cytoplasmic medium (water). This mechanism is based on the cluster-wave model of water structure, which successfully explains the transfer of energy through the liquid medium responsible for the build up of osmotic pressure. The analogy to the line of balls called "Newton's cradle" provides a useful comparison, since there the transfer is also invisible to us because the intermediate balls are motionless. It is further proposed that the spatial arrangements of the H-bonds of the α and β secondary structures support wave motion, with the linear and lateral forms of the groups of bonds belonging to the helices and sheets executing the longitudinal and transverse modes, respectively.

Structural and functional investigation of GajB protein in Gabija anti-phage defense

Bacteriophages (phages) are viruses that infect bacteria and archaea. To fend off invading phages, the hosts have evolved a variety of anti-phage defense mechanisms. Gabija is one of the most abundant prokaryotic antiviral systems and consists of two proteins, GajA and GajB. GajA has been characterized experimentally as a sequence-specific DNA endonuclease. Although GajB was previously predicted to be a UvrD-like helicase, its function is unclear. Here, we report the results of structural and functional analyses of GajB. The crystal structure of GajB revealed a UvrD-like domain architecture, including two RecA-like core and two accessory subdomains. However, local structural elements that are important for the helicase function of UvrD are not conserved in GajB. In functional assays, GajB did not unwind or bind various types of DNA substrates. We demonstrated that GajB interacts with GajA to form a heterooctameric Gabija complex, but GajB did not exhibit helicase activity when bound to GajA. These results advance our understanding of the molecular mechanism underlying Gabija anti-phage defense and highlight the role of GajB as a component of a multi-subunit antiviral complex in bacteria.

Everything OLD is new again: How structural, functional, and bioinformatic advances have redefined a neglected nuclease family

Overcoming lysogenization defect (OLD) proteins are a conserved family of ATP-powered nucleases that function in anti-phage defense. Recent bioinformatic, genetic, and crystallographic studies have yielded new insights into the structure, function, and evolution of these enzymes. Here we review these developments and propose a new classification scheme to categorize OLD homologs that relies on gene neighborhoods, biochemical properties, domain organization, and catalytic machinery. This taxonomy reveals important similarities and differences between family members and provides a blueprint to contextualize future in vivo and in vitro findings. We also detail how OLD nucleases are related to PARIS and Septu anti-phage defense systems and discuss important mechanistic questions that remain unanswered.

Thermodynamic and mechanistic analysis of the functional properties of dengue virus NS3 helicase

The Dengue Virus (DENV) non-structural protein 3 (NS3) is a multi-functional protein critical in the viral life cycle. The DENV NS3 is comprised of a serine protease domain and a helicase domain. The helicase domain itself acts as a molecular motor, either translocating in a unidirectional manner along single-stranded RNA or unwinding double-stranded RNA, processes fueled by the hydrolysis of nucleoside triphosphates. In this brief review, we summarize our contributions and ongoing efforts to uncover the thermodynamic and mechanistic functional properties of the DENV NS3 as an NTPase and helicase.

TcpA from the Clostridiumperfringens plasmid pCW3 is more closely related to the DNA translocase FtsK than to coupling proteins

Conjugative DNA transfer is a major factor in the dissemination of antibiotic resistance and virulence genes. In the Gram-positive pathogen Clostridium perfringens, the majority of conjugative plasmids share the conserved tcp locus that governs the assembly of the transfer system. Here, we describe multiple structures of the coupling protein TcpA, an essential ATPase that is suggested to provide the mechanical force to propel the DNA through the transfer apparatus. The structures of TcpA in the presence and absence of nucleotides revealed conformational rearrangements and highlight a crucial role for the unstructured C terminus. Our findings reveal that TcpA shares most structural similarity with the FtsK DNA translocase, a central component of the bacterial cell division machinery. Our structural data suggest that conjugation in C. perfringens may have evolved from the bacterial chromosome segregation system and, accordingly, suggest the possibility that double-stranded DNA is transferred through the Tcp conjugation apparatus.

The Switch and Reciprocating Models for the Function of ABC Multidrug Exporters: Perspectives on Recent Research

ATP-binding cassette (ABC) transporters comprise a large superfamily of primary active transporters, which are integral membrane proteins that couple energy to the uphill vectorial transport of substrates across cellular membranes, with concomitant hydrolysis of ATP. ABC transporters are found in all living organisms, coordinating mostly import in prokaryotes and export in eukaryotes. Unlike the highly conserved nucleotide binding domains (NBDs), sequence conservation in the transmembrane domains (TMDs) is low, with their divergent nature likely reflecting a need to accommodate a wide range of substrate types in terms of mass and polarity. An explosion in high resolution structural analysis over the past decade and a half has produced a wealth of structural information for ABCs. Based on the structures, a general mechanism for ABC transporters has been proposed, known as the Switch or Alternating Access Model, which holds that the NBDs are widely separated, with the TMDs and NBDs together forming an intracellular-facing inverted "V" shape. Binding of two ATPs and the substrate to the inward-facing conformation induces a transition to an outward conformation. Despite this apparent progress, certainty around the transport mechanism for any given ABC remains elusive. How substrate binding and transport is coupled to ATP binding and hydrolysis is not known, and there is a large body of biochemical and biophysical data that is at odds with the widely separated NBDs being a functional physiological state. An alternative Constant Contact model has been proposed in which the two NBSs operate 180 degrees out of phase with respect to ATP hydrolysis, with the NBDs remaining in close proximity throughout the transport cycle and operating in an asymmetric allosteric manner. The two models are discussed in the light of recent nuclear magnetic resonance and hydrogen-deuterium exchange mass spectrometry analyses of three ABC exporters.

Biochemical and Structural Analyses Shed Light on the Mechanisms of RadD DNA Binding and Its ATPase from Escherichia coli

DNA double-strand breaks (DSBs) are the most perilous and harmful type of DNA damage and can cause tumorigenesis or cell death if left repaired with an error or unrepaired. RadD, a member of the SF2 family, is a recently discovered DNA repair protein involved in the repair of DSBs after radiation or chemical damage. However, the function of RadD in DNA repair remains unclear. Here, we determined the crystal structures of RadD/ATPγS and RadD/ATP complexes and revealed the novel mechanism of RadD binding to DNA and ATP hydrolysis with biochemical data. In the RadD catalytic center, the Gly34 and Gly36 on the P-loop are key residues for ATP binding besides the conserved amino acids Lys37 and Arg343 in the SF2 family. If any of them mutate, then RadD loses ATPase activity. Asp117 polarizes the attacking water molecule, which then starts a nucleophilic reaction toward γ-phosphate, forming the transition state. Lys68 acts as a pocket switch to regulate substrate entry and product release. We revealed that the C-terminal peptide of single-stranded DNA-binding protein (SSB) binds the RadD C-terminal domain (CTD) and promotes the RadD ATPase activity. Our mutagenesis studies confirmed that the residues Arg428 on the zinc finger domain (ZFD) and Lys488 on the CTD of RadD are the key sites for binding branched DNA. Using the Coot software combined with molecular docking, we propose a RadD-binding DNA model for the DNA damage repair process.

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.

To unwind the biological knots: The DNA/RNA G-quadruplex resolvase RHAU (DHX36) in development and disease

The G-quadruplex (G4) sequences are short fragments of 4-interval triple guanine (G) with frequent and ubiquitous distribution in the genome and RNA transcripts. The G4 sequences are usually folded into secondary "knot" structure via Hoogsteen hydrogen bond to exert negative regulation on a variety of biological processes, including DNA replication and transcription, mRNA translation, and telomere maintenance. Recent structural biological and mouse genetics studies have demonstrated that RHAU (DHX36) can bind and unwind the G4 "knots" to modulate embryonic development and postnatal organ function. Deficiency of RHAU gives rise to embryonic lethality, impaired organogenesis, and organ dysfunction. These studies uncovered the pivotal G4 resolvase function of RHAU to release the G4 barrier, which plays fundamental roles in development and physiological homeostasis. This review discusses the latest advancements and findings in deciphering RHAU functions using animal models.

Insights into the product release mechanism of dengue virus NS3 helicase

The non-structural protein 3 helicase (NS3h) is a multifunctional protein that is critical in RNA replication and other stages in the flavivirus life cycle. NS3h uses energy from ATP hydrolysis to translocate along single stranded nucleic acid and to unwind double stranded RNA. Here we present a detailed mechanistic analysis of the product release stage in the catalytic cycle of the dengue virus (DENV) NS3h. This study is based on a combined experimental and computational approach of product-inhibition studies and free energy calculations. Our results support a model in which the catalytic cycle of ATP hydrolysis proceeds through an ordered sequential mechanism that includes a ternary complex intermediate (NS3h-Pi-ADP), which evolves releasing the first product, phosphate (Pi), and subsequently ADP. Our results indicate that in the product release stage of the DENV NS3h a novel open-loop conformation plays an important role that may be conserved in NS3 proteins of other flaviviruses as well.

The apo-form of the Vibrio cholerae replicative helicase DnaB is a labile and inactive planar trimer of dimers

To enable chromosomal replication, DNA is unwound by the ATPase molecular motor replicative helicase. The bacterial helicase DnaB is a ring-shaped homo-hexamer whose conformational dynamics are being studied through its different 3D structural states adopted along its functional cycle. Our findings describe a new crystal structure for the apo-DnaB from Vibrio cholerae, forming a planar hexamer with pseudo-symmetry, constituted by a trimer of dimers in which the C-terminal domains delimit a triskelion-shaped hole. This hexamer is labile and inactive. We suggest that it represents an intermediate state allowing the formation of the active NTP-bound hexamer from dimers.

An Adenosine Triphosphate- Dependent 5'-3' DNA Helicase From sk1-Like Lactococcus lactis F13 Phage

Here, we describe functional characterization of an early gene (gp46) product of a virulent Lactococcus lactis sk1-like phage, vB_Llc_bIBBF13 (abbr. F13). The GP46_F13 protein carries a catalytically active RecA-like domain belonging to the P-loop NTPase superfamily. It also retains features characteristic for ATPases forming oligomers. In order to elucidate its detailed molecular function, we cloned and overexpressed the gp46 gene in Escherichia coli. Purified GP46_F13 protein binds to DNA and exhibits DNA unwinding activity on branched substrates in the presence of adenosine triphosphate (ATP). Size exclusion chromatography with multi-angle light scattering (SEC-MALS) experiments demonstrate that GP46_F13 forms oligomers, and further pull-down assays show that GP46_F13 interacts with host proteins involved in replication (i.e., DnaK, DnaJ, topoisomerase I, and single-strand binding protein). Taking together the localization of the gene and the obtained results, GP46_F13 is the first protein encoded in the early-expressed gene region with helicase activity that has been identified among lytic L. lactis phages up to date.

Caseinolytic Proteins (Clp) in the Genus Klebsiella: Special Focus on ClpK

Caseinolytic proteins (Clp), which are present in both prokaryotes and eukaryotes, play a major role in cell protein quality control and survival of bacteria in harsh environmental conditions. Recently, a member of this protein family, ClpK was identified in a pathogenic strain of Klebsiella pneumoniae which was responsible for nosocomial infections. ClpK is linked to the thermal stress survival of this pathogen. The genome wide analysis of Clp proteins in Klebsiella spp. indicates that ClpK is present in only 34% of the investigated strains. This suggests that the uptake of the clpk gene is selective and may only be taken up by a pathogen that needs to survive harsh environmental conditions. In silico analyses and molecular dynamic simulations show that ClpK is mainly α-helical and is highly dynamic. ClpK was successfully expressed and purified to homogeneity using affinity and anion exchange chromatography. Biophysical characterization of ClpK showed that it is predominantly alpha-helical, and this is in agreement with in silico analysis of the protein structure. Furthermore, the purified protein is biologically active and hydrolyses ATP in a concentration- dependent manner.

Structural basis for activation of Swi2/Snf2 ATPase RapA by RNA polymerase

RapA is a bacterial RNA polymerase (RNAP)-associated Swi2/Snf2 ATPase that stimulates RNAP recycling. The ATPase activity of RapA is autoinhibited by its N-terminal domain (NTD) but activated with RNAP bound. Here, we report a 3.4-Å cryo-EM structure of Escherichia coli RapA-RNAP elongation complex, in which the ATPase active site of RapA is structurally remodeled. In this process, the NTD of RapA is wedged open by RNAP β' zinc-binding domain (ZBD). In addition, RNAP β flap tip helix (FTH) forms extensive hydrophobic interactions with RapA ATPase core domains. Functional assay demonstrates that removing the ZBD or FTH of RNAP significantly impairs its ability to activate the ATPase activity of RapA. Our results provide the structural basis of RapA ATPase activation by RNAP, through the active site remodeling driven by the ZBD-buttressed large-scale opening of NTD and the direct interactions between FTH and ATPase core domains.

Structure and efflux mechanism of the yeast pleiotropic drug resistance transporter Pdr5

Pdr5, a member of the extensive ABC transporter superfamily, is representative of a clinically relevant subgroup involved in pleiotropic drug resistance. Pdr5 and its homologues drive drug efflux through uncoupled hydrolysis of nucleotides, enabling organisms such as baker’s yeast and pathogenic fungi to survive in the presence of chemically diverse antifungal agents. Here, we present the molecular structure of Pdr5 solved with single particle cryo-EM, revealing details of an ATP-driven conformational cycle, which mechanically drives drug translocation through an amphipathic channel, and a clamping switch within a conserved linker loop that acts as a nucleotide sensor. One half of the transporter remains nearly invariant throughout the cycle, while its partner undergoes changes that are transmitted across inter-domain interfaces to support a peristaltic motion of the pumped molecule. The efflux model proposed here rationalises the pleiotropic impact of Pdr5 and opens new avenues for the development of effective antifungal compounds.

Pdr5 is an ABC transporter conferring multidrug resistance to pathogenic fungi. Here, structural analysis of Pdr5 provides insights into the transport mechanism featuring asymmetric movements of Pdr5 domain and enabling efflux of a broad spectrum of compounds.

Autophosphorylation of the KaiC-like protein ArlH inhibits oligomerization and interaction with ArlI, the motor ATPase of the archaellum

Motile archaea are propelled by the archaellum, whose motor complex consists of the membrane protein ArlJ, the ATPase ArlI, and the ATP-binding protein ArlH. Despite its essential function and the existence of structural and biochemical data on ArlH, the role of ArlH in archaellum assembly and function remains elusive. ArlH is a structural homolog of KaiC, the central component of the cyanobacterial circadian clock. Since autophosphorylation and dephosphorylation of KaiC are central properties for the function of KaiC, we asked whether autophosphorylation is also a property of ArlH proteins. We observed that both ArlH from the euryarchaeon Pyrococcus furiosus (PfArlH) and from the crenarchaeon Sulfolobus acidocaldarius (SaArlH) have autophosphorylation activity. Using a combination of single-molecule fluorescence measurements and biochemical assays, we show that autophosphorylation of ArlH is closely linked to its oligomeric state when bound to hexameric ArlI. These experiments also strongly suggest that ArlH is a hexamer in its ArlI-bound state. Mutagenesis of the putative catalytic residue (Glu-57 in SaArlH) in ArlH results in a reduced autophosphorylation activity and abolished archaellation and motility in S. acidocaldarius, indicating that optimum phosphorylation activity of ArlH is essential for archaellation and motility.

Flagellar export apparatus and ATP synthetase: Homology evidenced by synteny predating the Last Universal Common Ancestor

We report evidence further supporting homology between proteins in the F₁ F_O -ATP synthetase and the bacterial flagellar motor (BFM). BFM proteins FliH, FliI, and FliJ have been hypothesized to be homologous to F_O -b + F₁ -δ, F₁ -α/β, and F₁ -γ, with similar structure and interactions. We conduct a further test by constructing a gene order dataset, examining the order of fliH, fliI, and fliJ genes across the phylogenetic breadth of flagellar and nonflagellar type 3 secretion systems, and comparing this to published surveys of gene order in the F₁ F_O -ATP synthetase, its N-ATPase relatives, and the bacterial/archaeal V- and A-type ATPases. Strikingly, the fliHIJ gene order was deeply conserved, with the few exceptions appearing derived, and exactly matching the widely conserved F-ATPase gene order atpFHAG, coding for subunits b-δ-α-γ. The V/A-type ATPases have a similar conserved gene order. Our results confirm homology between these systems, and suggest a rare case of synteny conserved over billions of years, predating the Last Universal Common Ancestor (LUCA).

Multidrug Resistance in Mammals and Fungi-From MDR to PDR: A Rocky Road from Atomic Structures to Transport Mechanisms

Multidrug resistance (MDR) can be a serious complication for the treatment of cancer as well as for microbial and parasitic infections. Dysregulated overexpression of several members of the ATP-binding cassette transporter families have been intimately linked to MDR phenomena. Three paradigm ABC transporter members, ABCB1 (P-gp), ABCC1 (MRP1) and ABCG2 (BCRP) appear to act as brothers in arms in promoting or causing MDR in a variety of therapeutic cancer settings. However, their molecular mechanisms of action, the basis for their broad and overlapping substrate selectivity, remains ill-posed. The rapidly increasing numbers of high-resolution atomic structures from X-ray crystallography or cryo-EM of mammalian ABC multidrug transporters initiated a new era towards a better understanding of structure-function relationships, and for the dynamics and mechanisms driving their transport cycles. In addition, the atomic structures offered new evolutionary perspectives in cases where transport systems have been structurally conserved from bacteria to humans, including the pleiotropic drug resistance (PDR) family in fungal pathogens for which high resolution structures are as yet unavailable. In this review, we will focus the discussion on comparative mechanisms of mammalian ABCG and fungal PDR transporters, owing to their close evolutionary relationships. In fact, the atomic structures of ABCG2 offer excellent models for a better understanding of fungal PDR transporters. Based on comparative structural models of ABCG transporters and fungal PDRs, we propose closely related or even conserved catalytic cycles, thus offering new therapeutic perspectives for preventing MDR in infectious disease settings.

Autophosphorylation of the KaiC-like protein ArlH inhibits oligomerisation and interaction with ArlI, the motor ATPase of the archaellum.. [DOI: 10.1101/2021.03.19.436134] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Motile archaea are propelled by the archaellum, whose motor complex consists of the membrane protein ArlJ, the ATPase ArlI, and the ATP-binding protein ArlH. Despite its essential function and the existence of structural and biochemical data on ArlH, the role of ArlH in archaellum assembly and function remains elusive. ArlH is a structural homolog of KaiC, the central component of the cyanobacterial circadian clock. Similar to KaiC, ArlH exhibits autophosphorylation activity, which was observed for both ArlH of the euryarchaeonPyrococcus furiosus (PfArlH)and the crenarchaeonSulfolobus acidocaldarius(SaArlH). Using a combination of single molecule fluorescence measurements and biochemical assays, it is shown that autophosphorylation of ArlH is closely linked to the oligomeric state of ArlH bound to ArlI. These experiments also strongly suggest that ArlH is a hexamer in its functional ArlI bound state. Mutagenesis of the putative catalytic residue Glu-57 inSaArlH results in a reduced autophosphorylation activity and abolished archaellation and motility, suggesting that optimum phosphorylation activity of ArlH is essential for both archaellation and motility.

Sequence and functional differences in the ATPase domains of CHD3 and SNF2H promise potential for selective regulability and drugability

Chromatin remodelers use the energy of ATP hydrolysis to regulate chromatin dynamics. Their impact for development and disease requires strict enzymatic control. Here, we address the differential regulability of the ATPase domain of hSNF2H and hCHD3, exhibiting similar substrate affinities and enzymatic activities. Both enzymes are comparably strongly inhibited in their ATP hydrolysis activity by the competitive ATPase inhibitor ADP. However, the nucleosome remodeling activity of SNF2H is more strongly affected than that of CHD3. Beside ADP, also IP₆ inhibits the nucleosome translocation of both enzymes to varying degrees, following a competitive inhibition mode at CHD3, but not at SNF2H. Our observations are further substantiated by mutating conserved Q- and K-residues of ATPase domain motifs. The variants still bind both substrates and exhibit a wild-type similar, basal ATP hydrolysis. Apart from three CHD3 variants, none of the variants can translocate nucleosomes, suggesting for the first time that the basal ATPase activity of CHD3 is sufficient for nucleosome remodeling. Together with the ADP data, our results propose a more efficient coupling of ATP hydrolysis and remodeling in CHD3. This aspect correlates with findings that CHD3 nucleosome translocation is visible at much lower ATP concentrations than SNF2H. We propose sequence differences between the ATPase domains of both enzymes as an explanation for the functional differences and suggest that aa interactions, including the conserved Q- and K-residues distinctly regulate ATPase-dependent functions of both proteins. Our data emphasize the benefits of remodeler ATPase domains for selective drugability and/or regulability of chromatin dynamics.

Primordial bioenergy sources: The two facets of adenosine triphosphate

Life requires energy to exist, to reproduce and to survive. Two major hypotheses have been put forward concerning the source of this energy at the very early stages of life evolution: (i) abiotic organics either brought to Earth by comets and/or meteorites, or produced at its atmosphere, and (ii) mineral surface-dependent bioinorganic catalytic reactions. Considering the latter possibility, I propose that, besides being a precursor of nucleic acids, adenosine triphosphate (ATP), which probably was used very early to improve the fidelity of nucleic acid polymerization, played an essential role in the transition between mineral-bound protocells and their free counterparts. Indeed, phosphorylation by ATP renders carboxylate groups electrophilic enough to react with nucleophiles such as amines, an effect that, thanks to their Lewis acid character, also have dehydrated metal ions on mineral surfaces. Early ATP synthesis for metabolic processes most likely depended on substrate level phosphorylation. However, the exaptation of a hexameric helicase-like ATPase and a transmembrane H⁺ pump (which evolved to counteract the acidity caused by fermentation reactions within the protocell) generated a much more efficient membrane-bound ATP synthase that uses chemiosmosis to make ATP.

ABCG: a new fold of ABC exporters and a whole new bag of riddles!

ATP-binding cassette (ABC) superfamily comprises membrane transporters that power the active transport of substrates across biological membranes. These proteins harness the energy of nucleotide binding and hydrolysis to fuel substrate translocation via an alternating-access mechanism. The primary structural blueprint is relatively conserved in all ABC transporters. A transport-competent ABC transporter is essentially made up of two nucleotide-binding domains (NBDs) and two transmembrane domains (TMDs). While the NBDs are conserved in their primary sequence and form at their interface two nucleotide-binding sites (NBSs) for ATP binding and hydrolysis, the TMDs are variable among different families and form the translocation channel. Transporters catalyzing the efflux of substrates from the cells are called exporters. In humans, they range from A to G subfamilies, with the B, C and G subfamilies being involved in chemoresistance. The recently elucidated structures of ABCG5/G8 followed by those of ABCG2 highlighted a novel structural fold that triggered extensive research. Notably, suppressor genetics in the orthologous yeast Pleiotropic Drug Resistance (PDR) subfamily proteins have pointed to a crosstalk between TMDs and NBDs modulating substrate export. Considering the structural information provided by their neighbors from the G subfamily, these studies provide mechanistic keys and posit a functional role for the non-hydrolytic NBS found in several ABC exporters. The present chapter provides an overview of structural and functional aspects of ABCG proteins with a special emphasis on the yeast PDR systems.

Discovery of COVID-19 Inhibitors Targeting the SARS-CoV-2 Nsp13 Helicase

The raging COVID-19 pandemic caused by SARS-CoV-2 has infected tens of millions of people and killed several hundred thousand patients worldwide. Currently, there are no effective drugs or vaccines available for treating coronavirus infections. In this study, we have focused on the SARS-CoV-2 helicase (Nsp13), which is critical for viral replication and the most conserved nonstructural protein within the coronavirus family. Using homology modeling that couples published electron-density with molecular dynamics (MD)-based structural refinements, we generated structural models of the SARS-CoV-2 helicase in its apo- and ATP/RNA-bound conformations. We performed virtual screening of ∼970 000 chemical compounds against the ATP-binding site to identify potential inhibitors. Herein, we report docking hits of approved human drugs targeting the ATP-binding site. Importantly, two of our top drug hits have significant activity in inhibiting purified recombinant SARS-CoV-2 helicase, providing hope that these drugs can be potentially repurposed for the treatment of COVID-19.

Structural Characterization of the Essential Cell Division Protein FtsE and Its Interaction with FtsX in Streptococcus pneumoniae

Bacterial cell division is a central process that requires exquisite orchestration of both the cell wall biosynthetic and lytic machineries. The essential membrane complex FtsEX, widely conserved across bacteria, plays a central role by recruiting proteins to the divisome apparatus and by regulating periplasmic muralytic activity from the cytosol. FtsEX is a member of the type VII family of the ABC-superfamily, but instead of being a transporter, it couples the ATP hydrolysis catalyzed by FtsE to mechanically transduce a conformational signal that provokes the activation of peptidoglycan (PG) hydrolases. So far, no structural information is available for FtsE. Here, we provide the structural characterization of FtsE, confirming its ATPase nature and revealing regions with high structural plasticity which are key for FtsE binding to FtsX. The complementary binding region in FtsX has also been identified and validated in vivo. Our results provide evidence on how the difference between the ATP/ADP-bound states in FtsE would dramatically alter the interaction of FtsEX with the PG hydrolase PcsB in pneumococcal division.

FtsEX is a membrane complex widely conserved across diverse bacterial genera and involved in critical processes such as recruitment of division proteins and in spatial and temporal regulation of muralytic activity during cell division or sporulation. FtsEX is a member of the ABC transporter superfamily. The component FtsX is an integral membrane protein, whereas FtsE is an ATPase and is required for the transmission of a conformational signal from the cytosol through the membrane to regulate the activity of cell wall hydrolases in the periplasm. Both proteins are essential in the major human respiratory pathogenic bacterium Streptococcus pneumoniae, and FtsX interacts with the modular peptidoglycan hydrolase PcsB at the septum. Here, we report high-resolution structures of pneumococcal FtsE bound to different nucleotides. Structural analysis revealed that FtsE contains all the conserved structural motifs associated with ATPase activity and afforded interpretation of the in vivo dimeric arrangement in both the ADP and ATP states. Interestingly, three specific FtsE regions with high structural plasticity were identified that shape the cavity in which the cytosolic region of FtsX would be inserted. The residues corresponding to the FtsX coupling helix, responsible for contacting FtsE, were identified and validated by in vivo mutagenesis studies showing that this interaction is essential for cell growth and proper morphology.

Structure and Multitasking of the c-di-GMP-Sensing Cellulose Secretion Regulator BcsE

Most bacteria respond to surfaces by biogenesis of intracellular c-di-GMP, which inhibits motility and induces secretion of biofilm-promoting adherence factors. Bacterial cellulose is a widespread biofilm component whose secretion in Gram-negative species requires an inner membrane, c-di-GMP-dependent synthase tandem (BcsAB), an outer membrane porin (BcsC), and various accessory subunits that regulate synthase assembly and function as well as the exopolysaccharide's chemical composition and mechanical properties. We recently showed that in Escherichia coli, most Bcs proteins form a megadalton-sized secretory nanomachine, but the role and structure of individual regulatory components remained enigmatic. Here, we demonstrate that essential-for-secretion BcsR and BcsQ regulate each other's folding and stability and are recruited to the inner membrane via c-di-GMP-sensing BcsE and its intraoperon partner BcsF. Crystallographic and solution-based data show that BcsE's predicted GIL domain is a degenerate receiver-GGDEF domain tandem (BcsE^REC*^-GGDEF*), where the divergent diguanylate cyclase module binds both dimeric c-di-GMP and BcsQ through mutually independent interfaces. In addition, we reveal that a third N-terminal domain (BcsE^NTD) determines the protein's homooligomerization and targeting of BcsERQ to the membrane as well as previously unreported interactions with transcription antitermination complex components. Together, the data suggest that BcsE acts on multiple levels to fine-tune bacterial cellulose secretion, from the early stages of secretion system assembly to the maintenance of a membrane-proximal pool of dimeric c-di-GMP for processive synthase activation.IMPORTANCE Bacterial cellulose is a widespread biofilm component that can modulate microbial fitness and virulence both in the environment and infected hosts. Whereas its secretion generally involves an inner membrane c-di-GMP-dependent synthase tandem (BcsAB) across the bacterial domain of life, enterobacteria feature sophisticated Escherichia coli-like Bcs secretion systems, where multiple additional subunits are either required for secretion or contribute to the maximal production of the polysaccharide in vivo. Here, we demonstrate that essential-for-secretion BcsR and BcsQ regulate each other's folding and stability and are recruited to the inner membrane via c-di-GMP-sensing BcsE and its intraoperon partner, BcsF. Crystallographic and functional data reveal that BcsE features unexpected domain architecture and likely acts on multiple levels to fine-tune bacterial cellulose production, from the early stages of secretion system assembly to the maintenence of a membrane-proximal pool of dimeric c-di-GMP for processive synthase activation.

Discovery of COVID-19 Inhibitors Targeting the SARS-CoV2 Nsp13 Helicase

The raging COVID-19 pandemic caused by SARS-CoV2 has infected millions of people and killed several hundred thousand patients worldwide. Currently, there are no effective drugs or vaccines available for treating coronavirus infections. In this study, we have focused on the SARS-CoV2 helicase (Nsp13), which is critical for viral replication and the most conserved non-structural protein within the coronavirus family. Using homology modeling and molecular dynamics approaches, we generated structural models of the SARS-CoV2 helicase in its apo- and ATP/RNA-bound conformations. We performed virtual screening of ~970,000 chemical compounds against the ATP binding site to identify potential inhibitors. Herein, we report docking hits of approved human drugs targeting the ATP binding site. Importantly, two of our top drug hits have significant activity in inhibiting purified recombinant SARS-CoV-2 helicase, providing hope that these drugs can be potentially repurposed for the treatment of COVID-19.

FtsK in motion reveals its mechanism for double-stranded DNA translocation

FtsK protein contains a fast DNA motor that is involved in bacterial chromosome dimer resolution. During cell division, FtsK translocates double-stranded DNA until both dif recombination sites are placed at mid cell for subsequent dimer resolution. Here, we solved the 3.6-Å resolution electron cryo-microscopy structure of the motor domain of FtsK while translocating on its DNA substrate. Each subunit of the homo-hexameric ring adopts a unique conformation and one of three nucleotide states. Two DNA-binding loops within four subunits form a pair of spiral staircases within the ring, interacting with the two DNA strands. This suggests that simultaneous conformational changes in all ATPase domains at each catalytic step generate movement through a mechanism related to filament treadmilling. While the ring is only rotating around the DNA slowly, it is instead the conformational states that rotate around the ring as the DNA substrate is pushed through.

The full-length structure of Thermus scotoductus OLD defines the ATP hydrolysis properties and catalytic mechanism of Class 1 OLD family nucleases

OLD family nucleases contain an N-terminal ATPase domain and a C-terminal Toprim domain. Homologs segregate into two classes based on primary sequence length and the presence/absence of a unique UvrD/PcrA/Rep-like helicase gene immediately downstream in the genome. Although we previously defined the catalytic machinery controlling Class 2 nuclease cleavage, degenerate conservation of the C-termini between classes precludes pinpointing the analogous residues in Class 1 enzymes by sequence alignment alone. Our Class 2 structures also provide no information on ATPase domain architecture and ATP hydrolysis. Here we present the full-length structure of the Class 1 OLD nuclease from Thermus scotoductus (Ts) at 2.20 Å resolution, which reveals a dimerization domain inserted into an N-terminal ABC ATPase fold and a C-terminal Toprim domain. Structural homology with genome maintenance proteins identifies conserved residues responsible for Ts OLD ATPase activity. Ts OLD lacks the C-terminal helical domain present in Class 2 OLD homologs yet preserves the spatial organization of the nuclease active site, arguing that OLD proteins use a conserved catalytic mechanism for DNA cleavage. We also demonstrate that mutants perturbing ATP hydrolysis or DNA cleavage in vitro impair P2 OLD-mediated killing of recBC-Escherichia coli hosts, indicating that both the ATPase and nuclease activities are required for OLD function in vivo.

Molecular Mechanism Underlying Inhibition of Intrinsic ATPase Activity in a Ski2-like RNA Helicase

RNA-dependent NTPases can act as RNA/RNA-protein remodeling enzymes and typically exhibit low NTPase activity in the absence of RNA/RNA-protein substrates. How futile intrinsic NTP hydrolysis is prevented is frequently not known. The ATPase/RNA helicase Brr2 belongs to the Ski2-like family of nucleic acid-dependent NTPases and is an integral component of the spliceosome. Comprehensive nucleotide binding and hydrolysis studies are not available for a member of the Ski2-like family. We present crystal structures of Chaetomium thermophilum Brr2 in the apo, ADP-bound, and ATPγS-bound states, revealing nucleotide-induced conformational changes and a hitherto unknown ATPγS binding mode. Our results in conjunction with Brr2 structures in other molecular contexts reveal multiple molecular mechanisms that contribute to the inhibition of intrinsic ATPase activity, including an N-terminal region that restrains the RecA-like domains in an open conformation and exclusion of an attacking water molecule, and suggest how RNA substrate binding can lead to ATPase stimulation.

Weaving DNA strands: structural insight on ATP hydrolysis in RecA-induced homologous recombination

Homologous recombination is a fundamental process in all living organisms that allows the faithful repair of DNA double strand breaks, through the exchange of DNA strands between homologous regions of the genome. Results of three decades of investigation and recent fruitful observations have unveiled key elements of the reaction mechanism, which proceeds along nucleofilaments of recombinase proteins of the RecA family. Yet, one essential aspect of homologous recombination has largely been overlooked when deciphering the mechanism: while ATP is hydrolyzed in large quantity during the process, how exactly hydrolysis influences the DNA strand exchange reaction at the structural level remains to be elucidated. In this study, we build on a previous geometrical approach that studied the RecA filament variability without bound DNA to examine the putative implication of ATP hydrolysis on the structure, position, and interactions of up to three DNA strands within the RecA nucleofilament. Simulation results on modeled intermediates in the ATP cycle bring important clues about how local distortions in the DNA strand geometries resulting from ATP hydrolysis can aid sequence recognition by promoting local melting of already formed DNA heteroduplex and transient reverse strand exchange in a weaving type of mechanism.

The ATPase mechanism of UvrA2 reveals the distinct roles of proximal and distal ATPase sites in nucleotide excision repair

The UvrA₂ dimer finds lesions in DNA and initiates nucleotide excision repair. Each UvrA monomer contains two essential ATPase sites: proximal (P) and distal (D). The manner whereby their activities enable UvrA₂ damage sensing and response remains to be clarified. We report three key findings from the first pre-steady state kinetic analysis of each site. Absent DNA, a P_2ATP-D_2ADP species accumulates when the low-affinity proximal sites bind ATP and enable rapid ATP hydrolysis and phosphate release by the high-affinity distal sites, and ADP release limits catalytic turnover. Native DNA stimulates ATP hydrolysis by all four sites, causing UvrA₂ to transition through a different species, P_2ADP-D_2ADP. Lesion-containing DNA changes the mechanism again, suppressing ATP hydrolysis by the proximal sites while distal sites cycle through hydrolysis and ADP release, to populate proximal ATP-bound species, P_2ATP-D_empty and P_2ATP-D_2ATP. Thus, damaged and native DNA trigger distinct ATPase site activities, which could explain why UvrA₂ forms stable complexes with UvrB on damaged DNA compared with weaker, more dynamic complexes on native DNA. Such specific coupling between the DNA substrate and the ATPase mechanism of each site provides new insights into how UvrA₂ utilizes ATP for lesion search, recognition and repair.

The way is the goal: how SecA transports proteins across the cytoplasmic membrane in bacteria

In bacteria, translocation of most soluble secreted proteins (and outer membrane proteins in Gram-negative bacteria) across the cytoplasmic membrane by the Sec machinery is mediated by the essential ATPase SecA. At its core, this machinery consists of SecA and the integral membrane proteins SecYEG, which form a protein conducting channel in the membrane. Proteins are recognised by the Sec machinery by virtue of an internally encoded targeting signal, which usually takes the form of an N-terminal signal sequence. In addition, substrate proteins must be maintained in an unfolded conformation in the cytoplasm, prior to translocation, in order to be competent for translocation through SecYEG. Recognition of substrate proteins occurs via SecA—either through direct recognition by SecA or through secondary recognition by a molecular chaperone that delivers proteins to SecA. Substrate proteins are then screened for the presence of a functional signal sequence by SecYEG. Proteins with functional signal sequences are translocated across the membrane in an ATP-dependent fashion. The current research investigating each of these steps is reviewed here.

New insights into structural and functional relationships between LonA proteases and ClpB chaperones

LonA proteases and ClpB chaperones are key components of the protein quality control system in bacterial cells. LonA proteases form a unique family of ATPases associated with diverse cellular activities (AAA+ ) proteins due to the presence of an unusual N-terminal region comprised of two domains: a β-structured N domain and an α-helical domain, including the coiled-coil fragment, which is referred to as HI(CC). The arrangement of helices in the HI(CC) domain is reminiscent of the structure of the H1 domain of the first AAA+ module of ClpB chaperones. It has been hypothesized that LonA proteases with a single AAA+ module may also contain a part of another AAA+ module, the full version of which is present in ClpB. Here, we established and tested the structural basis of this hypothesis using the known crystal structures of various fragments of LonA proteases and ClpB chaperones, as well as the newly determined structure of the Escherichia coli LonA fragment (235-584). The similarities and differences in the corresponding domains of LonA proteases and ClpB chaperones were examined in structural terms. The results of our analysis, complemented by the finding of a singular match in the location of the most conserved axial pore-1 loop between the LonA NB domain and the NB2 domain of ClpB, support our hypothesis that there is a structural and functional relationship between two coiled-coil fragments and implies a similar mechanism of engagement of the pore-1 loops in the AAA+ modules of LonAs and ClpBs.

SecA-Mediated Protein Translocation through the SecYEG Channel

In bacteria, the Sec translocase mediates the translocation of proteins into and across the cytoplasmic membrane. It consists of a protein conducting channel SecYEG, the ATP-dependent motor SecA, and the accessory SecDF complex. Here we discuss the function and structure of the Sec translocase.

The big picture of chromatin biology by cryo-EM

Modifications of chromatin structure are one of the key mechanisms regulating epigenetic gene expression. Proteins involved in chromatin modification mainly function as large multi-subunit complexes, and each component in the complex contributes to the function and activity of the complex. However, little is known about the structures of whole complexes and the mechanisms by which the chromatin-modifying complexes function, the functional roles of each component in the complexes, and how the complexes recognize the central unit of chromatin, the nucleosome. This lack of information is partially due to the lack of structural information for whole complexes. Recent advances in cryo-EM have begun to reveal the structures of whole chromatin-modifying complexes that enable us to understand the big picture of chromatin biology. In this review, we discuss the recent discoveries related to the mechanisms of chromatin-modifying complexes.

Crystal structure of a novel ATPase RadD from Escherichia coli

The helicase superfamily 2 (SF2) proteins are involved in essentially every step in DNA and RNA metabolism. The radD (yejH) gene, which belongs to SF2, plays an important role in DNA repair. The RadD protein includes all seven conserved SF2 motifs and has shown ATPase activity. Here, we first reported the structure of RadD from Escherichia coli containing two RecA-like domains, a zinc finger motif, and a C-terminal domain. Based on the structure of RadD and other SF2 proteins, we then built a model of the RedD-ATP complex.

A structural view of bacterial DNA replication

DNA replication mechanisms are conserved across all organisms. The proteins required to initiate, coordinate, and complete the replication process are best characterized in model organisms such as Escherichia coli. These include nucleotide triphosphate-driven nanomachines such as the DNA-unwinding helicase DnaB and the clamp loader complex that loads DNA-clamps onto primer-template junctions. DNA-clamps are required for the processivity of the DNA polymerase III core, a heterotrimer of α, ε, and θ, required for leading- and lagging-strand synthesis. DnaB binds the DnaG primase that synthesizes RNA primers on both strands. Representative structures are available for most classes of DNA replication proteins, although there are gaps in our understanding of their interactions and the structural transitions that occur in nanomachines such as the helicase, clamp loader, and replicase core as they function. Reviewed here is the structural biology of these bacterial DNA replication proteins and prospects for future research.

Elucidation of the functional roles of the Q and I motifs in the human chromatin-remodeling enzyme BRG1

The Snf2 proteins, comprising 53 different enzymes in humans, belong to the SF2 family. Many Snf2 enzymes possess chromatin-remodeling activity, requiring a functional ATPase domain consisting of conserved motifs named Q and I-VII. These motifs form two recA-like domains, creating an ATP-binding pocket. Little is known about the function of the conserved motifs in chromatin-remodeling enzymes. Here, we characterized the function of the Q and I (Walker I) motifs in hBRG1 (SMARCA4). The motifs are in close proximity to the bound ATP, suggesting a role in nucleotide binding and/or hydrolysis. Unexpectedly, when substituting the conserved residues Gln⁷⁵⁸ (Q motif) or Lys⁷⁸⁵ (I motif) of both motifs, all variants still bound ATP and exhibited basal ATPase activity similar to that of wildtype BRG1 (wtBRG1). However, all mutants lost the nucleosome-dependent stimulation of the ATPase domain. Their chromatin-remodeling rates were impaired accordingly, but nucleosome binding was retained and still comparable with that of wtBRG1. Interestingly, a cancer-relevant substitution, L754F (Q motif), displayed defects similar to the Gln⁷⁵⁸ variant(s), arguing for a comparable loss of function. Because we excluded a mutual interference of ATP and nucleosome binding, we postulate that both motifs stimulate the ATPase and chromatin-remodeling activities upon binding of BRG1 to nucleosomes, probably via allosteric mechanisms. Furthermore, mutations of both motifs similarly affect the enzymatic functionality of BRG1 in vitro and in living cells. Of note, in BRG1-deficient H1299 cells, exogenously expressed wtBRG1, but not BRG1 Q758A and BRG1 K785R, exhibited a tumor suppressor-like function.

Control of mRNA Translation by Versatile ATP-Driven Machines

Translation is organized in a cycle that requires ribosomal subunits, mRNA, aminoacylated transfer RNAs, and myriad regulatory factors. As soon as translation reaches a stop codon or stall, a termination or surveillance process is launched via the release factors eRF1 or Pelota, respectively. The ATP-binding cassette (ABC) protein ABCE1 interacts with release factors and coordinates the recycling process in Eukarya and Archaea. After splitting, ABCE1 stays with the small ribosomal subunit and emerges as an integral part of translation initiation complexes. In addition, eEF3 and ABCF proteins control translation by binding at the E-site. In this review, we highlight advances in the fundamental role of ABC systems in mRNA translation in view of their collective inner mechanics.

Mapping Free Energy Pathways for ATP Hydrolysis in the E. coli ABC Transporter HlyB by the String Method

HlyB functions as an adenosine triphosphate (ATP)-binding cassette (ABC) transporter that enables bacteria to secrete toxins at the expense of ATP hydrolysis. Our previous work, based on potential energy profiles from combined quantum mechanical and molecular mechanical (QM/MM) calculations, has suggested that the highly conserved H-loop His residue H662 in the nucleotide binding domain (NBD) of E. coli HlyB may catalyze the hydrolysis of ATP through proton relay. To further test this hypothesis when entropic contributions are taken into account, we obtained QM/MM minimum free energy paths (MFEPs) for the HlyB reaction, making use of the string method in collective variables. The free energy profiles along the MFEPs confirm the direct participation of H662 in catalysis. The MFEP simulations of HlyB also reveal an intimate coupling between the chemical steps and a local protein conformational change involving the signature-loop residue S607, which may serve a catalytic role similar to an Arg-finger motif in many ATPases and GTPases in stabilizing the phosphoryl-transfer transition state.

Structure of the large terminase from a hyperthermophilic virus reveals a unique mechanism for oligomerization and ATP hydrolysis

The crystal structure of the large terminase from the Geobacillus stearothermophilus bacteriophage D6E shows a unique relative orientation of the N-terminal adenosine triphosphatase (ATPase) and C-terminal nuclease domains. This monomeric ‘initiation’ state with the two domains ‘locked’ together is stabilized via a conserved C-terminal arm, which may interact with the portal protein during motor assembly, as predicted for several bacteriophages. Further work supports the formation of an active oligomeric state: (i) AUC data demonstrate the presence of oligomers; (ii) mutational analysis reveals a trans-arginine finger, R158, indispensable for ATP hydrolysis; (iii) the location of this arginine is conserved with the HerA/FtsK ATPase superfamily; (iv) a molecular docking model of the pentamer is compatible with the location of the identified arginine finger. However, this pentameric model is structurally incompatible with the monomeric ‘initiation’ state and is supported by the observed increase in k_cat of ATP hydrolysis, from 7.8 ± 0.1 min⁻¹ to 457.7 ± 9.2 min⁻¹ upon removal of the C-terminal nuclease domain. Taken together, these structural, biophysical and biochemical data suggest a model where transition from the ‘initiation’ state into a catalytically competent pentameric state, is accompanied by substantial domain rearrangements, triggered by the removal of the C-terminal arm from the ATPase active site.

Allosteric effects of ATP binding on the nucleotide-binding domain of a heterodimeric ATP-binding cassette transporter

ATP-binding cassette (ABC) exporters mediate vital transport of a variety of molecules across the lipid bilayer in all organisms. To explore the allosteric effect of ATP binding at the asymmetric ATPase sites, molecular dynamics simulations were performed on the nucleotide-binding domains (NBDs) of a heterodimeric exporter TM287/288 in 4 different ATP-bound states. The results showed that ATP bound at the degenerate site can maintain a semi-open conformation of NBD1-NBD2, which may be defective in ATP hydrolysis. By contrast, when bound at the consensus site, ATP can induce an intra-domain rotation of the α-helical subdomain towards the RecA-like subdomain of NBD2 at the degenerate site. The rotation of the α-helical subdomain rearranged the hydrogen bond networks at the NBD1-NBD2 interface, induced a significant conformational change in the D-loop at the degenerate site and inter- and intra-domain communications at both sites, and eventually elicited dimerization of NBD1-NBD2. These findings indicate that the asymmetric ATPase sites of the heterodimeric exporter are structurally and functionally distinct.

Probing native metal ion association sites through quenching of fluorophores in the nucleotide-binding domains of the ABC transporter MsbA

ATP-binding cassette (ABC) transporters are ubiquitously present in prokaryotic and eukaryotic cells. Binding of ATP to the nucleotide-binding domains (NBDs) elicits major conformational changes of the transporters resulting in the transport of the substrate across the membrane. The availability of a crystal structure of the NBDs enabled us to elucidate the local structure and small-scale dynamics in the NBDs. Here, we labeled the ABC transporter MsbA, a homodimeric flippase from Escherichia coli, with a fluorescent probe, Alexa532, within the NBDs. ATP application elicited collisional quenching, whereas no quenching was observed after the addition of ATP analogs or ATP hydrolysis inhibitors. The Alexa532-conjugated MsbA variants exhibited transition metal ion Förster resonance energy transfer (tmFRET) after the addition of Ni²⁺, and ATP decreased this Ni²⁺-mediated FRET of the NBDs. Structure modeling developed from crystallographic data and examination of tmFRET measurements of MsbA variants in the absence of ATP revealed the presence of metal ion-associated pockets (MiAPs) in the NBDs. Three histidines were predicted to participate in chelating Ni²⁺ in the two possible MiAPs. Performing histidine-substitution experiments with the NBDs showed that the dissociation constant for Ni²⁺ of MiAP2 was smaller than that of MiAP1. The structural allocation of the MiAPs was further supported by showing that the addition of Cu²⁺ resulted in higher quenching than Ni²⁺ Taken together, the present study showed that the NBDs contain two native binding sites for metal ions and ATP addition affects the Ni²⁺-binding activity of the MiAPs.

The natural history of molecular functions inferred from an extensive phylogenomic analysis of gene ontology data

The origin and natural history of molecular functions hold the key to the emergence of cellular organization and modern biochemistry. Here we use a genomic census of Gene Ontology (GO) terms to reconstruct phylogenies at the three highest (1, 2 and 3) and the lowest (terminal) levels of the hierarchy of molecular functions, which reflect the broadest and the most specific GO definitions, respectively. These phylogenies define evolutionary timelines of functional innovation. We analyzed 249 free-living organisms comprising the three superkingdoms of life, Archaea, Bacteria, and Eukarya. Phylogenies indicate catalytic, binding and transport functions were the oldest, suggesting a 'metabolism-first' origin scenario for biochemistry. Metabolism made use of increasingly complicated organic chemistry. Primordial features of ancient molecular functions and functional recruitments were further distilled by studying the oldest child terms of the oldest level 1 GO definitions. Network analyses showed the existence of an hourglass pattern of enzyme recruitment in the molecular functions of the directed acyclic graph of molecular functions. Older high-level molecular functions were thoroughly recruited at younger lower levels, while very young high-level functions were used throughout the timeline. This pattern repeated in every one of the three mappings, which gave a criss-cross pattern. The timelines and their mappings were remarkable. They revealed the progressive evolutionary development of functional toolkits, starting with the early rise of metabolic activities, followed chronologically by the rise of macromolecular biosynthesis, the establishment of controlled interactions with the environment and self, adaptation to oxygen, and enzyme coordinated regulation, and ending with the rise of structural and cellular complexity. This historical account holds important clues for dissection of the emergence of biomcomplexity and life.

How Intrinsic Dynamics Mediates the Allosteric Mechanism in the ABC Transporter Nucleotide Binding Domain Dimer

A protein's architecture facilitates specific motions-intrinsic dynamic modes-that are employed to effect function. Here we used molecular dynamics (MD) simulations to investigate the dynamics of the MJ0796 ABC transporter nucleotide-binding domain (NBD). ABC transporter NBDs form a rotationally symmetric dimer whereby two equivalent active sites are formed at their interface; in complex with a dimer of transmembrane domains they hydrolyze ATP to energize translocation of substrates across cellular membranes. Our data suggest the ABC NBD's ensemble of functional states can be understood predominately in terms of conformational changes between its major subdomains, occurring along two orthogonal dynamic modes. The data show that ligands and oligomeric interactions modulate the equilibrium conformation of the NBD with respect to these motions, suggesting that allostery is achieved by affecting the energetic profile along these two modes. The observed dynamics and allostery integrate consonantly and logically within a mechanistic framework for the ABC NBD dimer, which is supported by a large body of experimental and theoretical data, providing a higher resolution view of the enzyme's dynamic cycle. Our study shows how valuable mechanistic inferences can be derived from accessible short-time scale MD simulations of an enzyme's substructures.

Molecular architecture of the yeast Elongator complex reveals an unexpected asymmetric subunit arrangement

Elongator is a ~850 kDa protein complex involved in multiple processes from transcription to tRNA modification. Conserved from yeast to humans, Elongator is assembled from two copies of six unique subunits (Elp1 to Elp6). Despite the wealth of structural data on the individual subunits, the overall architecture and subunit organization of the full Elongator and the molecular mechanisms of how it exerts its multiple activities remain unclear. Using single-particle electron microscopy (EM), we revealed that yeast Elongator adopts a bilobal architecture and an unexpected asymmetric subunit arrangement resulting from the hexameric Elp456 subassembly anchored to one of the two Elp123 lobes that form the structural scaffold. By integrating the EM data with available subunit crystal structures and restraints generated from cross-linking coupled to mass spectrometry, we constructed a multiscale molecular model that showed the two Elp3, the main catalytic subunit, are located in two distinct environments. This work provides the first structural insights into Elongator and a framework to understand the molecular basis of its multifunctionality.

Crystal Structure of a Type IV Pilus Assembly ATPase: Insights into the Molecular Mechanism of PilB from Thermus thermophilus

Type IV pili (T4P) mediate bacterial motility and virulence. The PilB/GspE family ATPases power the assembly of T4P and type 2 secretion systems. We determined the structure of the ATPase region of PilB (PilB_ATP) in complex with ATPγS to provide a model of a T4P assembly ATPase and a view of a PilB/GspE family hexamer at better than 3-Å resolution. Spatial positioning and conformations of the protomers suggest a mechanism of force generation. All six PilB_ATP protomers contain bound ATPγS. Two protomers form a closed conformation poised for ATP hydrolysis. The other four molecules assume an open conformation but separate into two pairs with distinct active-site accessibilities. We propose that one pair represents the post-hydrolysis phase while the other pair appears poised for ADP/ATP exchange. Collectively, the data suggest that T4P assembly is powered by coordinating concurrent substrate binding with ATP hydrolysis across the PilB hexamer.

Structure of the frequency-interacting RNA helicase: a protein interaction hub for the circadian clock

In the Neurospora crassa circadian clock, a protein complex of frequency (FRQ), casein kinase 1a (CK1a), and the FRQ-interacting RNA Helicase (FRH) rhythmically represses gene expression by the white-collar complex (WCC). FRH crystal structures in several conformations and bound to ADP/RNA reveal differences between FRH and the yeast homolog Mtr4 that clarify the distinct role of FRH in the clock. The FRQ-interacting region at the FRH N-terminus has variable structure in the absence of FRQ A known mutation that disrupts circadian rhythms (R806H) resides in a positively charged surface of the KOW domain, far removed from the helicase core. We show that changes to other similarly located residues modulate interactions with the WCC and FRQ A V142G substitution near the N-terminus also alters FRQ and WCC binding to FRH, but produces an unusual short clock period. These data support the assertion that FRH helicase activity does not play an essential role in the clock, but rather FRH acts to mediate contacts among FRQ, CK1a and the WCC through interactions involving its N-terminus and KOW module.

Crystal structure of the flagellar accessory protein FlaH of Methanocaldococcus jannaschii suggests a regulatory role in archaeal flagellum assembly

Archaeal flagella are unique structures that share functional similarity with bacterial flagella, but are structurally related to bacterial type IV pili. The flagellar accessory protein FlaH is one of the conserved components of the archaeal motility system. However, its function is not clearly understood. Here, we present the 2.2 Å resolution crystal structure of FlaH from the hyperthermophilic archaeon, Methanocaldococcus jannaschii. The protein has a characteristic RecA-like fold, which has been found previously both in archaea and bacteria. We show that FlaH binds to immobilized ATP-however, it lacks ATPase activity. Surface plasmon resonance analysis demonstrates that ATP affects the interaction between FlaH and the archaeal motor protein FlaI. In the presence of ATP, the FlaH-FlaI interaction becomes significantly weaker. A database search revealed similarity between FlaH and several DNA-binding proteins of the RecA superfamily. The closest structural homologs of FlaH are KaiC-like proteins, which are archaeal homologs of the circadian clock protein KaiC from cyanobacteria. We propose that one of the functions of FlaH may be the regulation of archaeal motor complex assembly.