ATPase site architecture and helicase mechanism of an archaeal MCM

The hexameric helicase DnaB adopts a nonplanar conformation during translocation

DNA polymerases can only synthesize nascent DNA from single-stranded DNA (ssDNA) templates. In bacteria, the unwinding of parental duplex DNA is carried out by the replicative DNA helicase (DnaB) that couples NTP hydrolysis to 5' to 3' translocation. The crystal structure of the DnaB hexamer in complex with GDP-AlF(4) and ssDNA reported here reveals that DnaB adopts a closed spiral staircase quaternary structure around an A-form ssDNA with each C-terminal domain coordinating two nucleotides of ssDNA. The structure not only provides structural insights into the translocation mechanism of superfamily IV helicases but also suggests that members of this superfamily employ a translocation mechanism that is distinct from other helicase superfamilies. We propose a hand-over-hand mechanism in which sequential hydrolysis of NTP causes a sequential 5' to 3' movement of the subunits along the helical axis of the staircase, resulting in the unwinding of two nucleotides per subunit.

ATP-dependent conformational dynamics underlie the functional asymmetry of the replicative helicase from a minimalist eukaryote

The heterohexameric minichromosome maintenance (MCM2-7) complex is an ATPase that serves as the central replicative helicase in eukaryotes. During initiation, the ring-shaped MCM2-7 particle is thought to open to facilitate loading onto DNA. The conformational state accessed during ring opening, the interplay between ATP binding and MCM2-7 architecture, and the use of these events in the regulation of DNA unwinding are poorly understood. To address these issues in isolation from the regulatory complexity of existing eukaryotic model systems, we investigated the structure/function relationships of a naturally minimized MCM2-7 complex from the microsporidian parasite Encephalitozoon cuniculi. Electron microscopy and small-angle X-ray scattering studies show that, in the absence of ATP, MCM2-7 spontaneously adopts a left-handed, open-ring structure. Nucleotide binding does not promote ring closure but does cause the particle to constrict in a two-step process that correlates with the filling of high- and low-affinity ATPase sites. Our findings support the idea that an open ring forms the default conformational state of the isolated MCM2-7 complex, and they provide a structural framework for understanding the multiphasic ATPase kinetics observed in different MCM2-7 systems.

ATPase site architecture is required for self-assembly and remodeling activity of a hexameric AAA+ transcriptional activator

AAA+ proteins (ATPases associated with various cellular activities) are oligomeric ATPases that use ATP hydrolysis to remodel their substrates. By similarity with GTPases, a dynamic organization of the nucleotide-binding pockets between ATPase protomers is proposed to regulate functionality. Using the transcription activator PspF as an AAA+ model, we investigated contributions of conserved residues for roles in ATP hydrolysis and intersubunit communication. We determined the R-finger residue and revealed that it resides in a conserved “R-hand” motif (R_xD_xxxR) needed for its “trans-acting” activity. Further, a divergent Walker A glutamic acid residue acts synergistically with a tyrosine residue to function in ADP-dependent subunit-subunit coordination, forming the “ADP-switch” motif. Another glutamic acid controls hexamer formation in the presence of nucleotides. Together, these results lead to a “residue-nucleotide” interaction map upon which to base AAA+ core regulation.

MCM structure and mechanics: what we have learned from archaeal MCM

Minichromosome maintenance (MCM) complexes have been identified as the primary replicative helicases responsible for unwinding DNA for genome replication. The focus of this chapter is to discuss the current structural and functional understanding of MCMs and their role at origins of replication, which are based mostly on the studies of MCM proteins and MCM complexes from archaeal genomes.

Mitochondrial AAA proteases--towards a molecular understanding of membrane-bound proteolytic machines

Mitochondrial AAA proteases play an important role in the maintenance of mitochondrial proteostasis. They regulate and promote biogenesis of mitochondrial proteins by acting as processing enzymes and ensuring the selective turnover of misfolded proteins. Impairment of AAA proteases causes pleiotropic defects in various organisms including neurodegeneration in humans. AAA proteases comprise ring-like hexameric complexes in the mitochondrial inner membrane and are functionally conserved from yeast to man, but variations are evident in the subunit composition of orthologous enzymes. Recent structural and biochemical studies revealed how AAA proteases degrade their substrates in an ATP dependent manner. Intersubunit coordination of the ATP hydrolysis leads to an ordered ATP hydrolysis within the AAA ring, which ensures efficient substrate dislocation from the membrane and translocation to the proteolytic chamber. In this review, we summarize recent findings on the molecular mechanisms underlying the versatile functions of mitochondrial AAA proteases and their relevance to those of the other AAA+ machines.

Molecular machines in archaeal DNA replication

The archaeal DNA replication apparatus is a simplified version of that of eukaryotes and has attracted attention as a tractable model system for the orthologous, but significantly more complex eukaryal machinery. A variety of archaeal model organisms have been investigated with strong emphasis on structural and biochemical analyses of replication-associated proteins. In this review we will describe recent advances in understanding the properties of the replicative helicase, the MCM complex, and the role of the sliding clamp, PCNA, in mediating a range of protein-DNA transactions. Although both complexes form ring shaped assemblies, they play very distinct roles at the leading and trailing edges of the replication fork machinery respectively.

Steric exclusion and wrapping of the excluded DNA strand occurs along discrete external binding paths during MCM helicase unwinding

The minichromosome maintenance (MCM) helicase complex is essential for the initiation and elongation of DNA replication in both the eukaryotic and archaeal domains. The archaeal homohexameric MCM helicase from Sulfolobus solfataricus serves as a model for understanding mechanisms of DNA unwinding. In this report, the displaced 5'-tail is shown to provide stability to the MCM complex on DNA and contribute to unwinding. Mutations in a positively charged patch on the exterior surface of the MCM hexamer destabilize this interaction, alter the path of the displaced 5'-tail DNA and reduce unwinding. DNA footprinting and single-molecule fluorescence experiments support a previously unrecognized wrapping of the 5'-tail. This mode of hexameric helicase DNA unwinding is termed the steric exclusion and wrapping (SEW) model, where the 3'-tail is encircled by the helicase while the displaced 5'-tail wraps around defined paths on the exterior of the helicase. The novel wrapping mechanism stabilizes the MCM complex in a positive unwinding mode, protects the displaced single-stranded DNA tail and prevents reannealing.

Structure and function of the AAA+ nucleotide binding pocket

Members of the diverse superfamily of AAA+ proteins are molecular machines responsible for a wide range of essential cellular processes. In this review we summarise structural and functional data surrounding the nucleotide binding pocket of these versatile complexes. Protein Data Bank (PDB) structures of closely related AAA+ ATPase are overlaid and biologically relevant motifs are displayed. Interactions between protomers are illustrated on the basis of oligomeric structures of each AAA+ subgroup. The possible role of conserved motifs in the nucleotide binding pocket is assessed with regard to ATP binding and hydrolysis, oligomerisation and inter-subunit communication. Our comparison indicates that in particular the roles of the arginine finger and sensor 2 residues differ subtly between AAA+ subgroups, potentially providing a means for functional diversification.

The interplay of DNA binding, ATP hydrolysis and helicase activities of the archaeal MCM helicase

The MCM (minichromosome maintenance) proteins of archaea are widely believed to be the replicative DNA helicase of these organisms. Most archaea possess a single MCM orthologue that forms homo-multimeric assemblies with a single hexamer believed to be the active form. In the present study we characterize the roles of highly conserved residues in the ATPase domain of the MCM of the hyperthermophilic archaeon Sulfolobus solfataricus. Our results identify a potential conduit for communicating DNA-binding information to the ATPase active site.

Requirements for the catalytic cycle of the N-ethylmaleimide-Sensitive Factor (NSF)

The N-ethylmaleimide-Sensitive Factor (NSF) was one of the initial members of the ATPases Associated with various cellular Activities Plus (AAA(+)) family. In this review, we discuss what is known about the mechanism of NSF action and how that relates to the mechanisms of other AAA(+) proteins. Like other family members, NSF binds to a protein complex (i.e., SNAP-SNARE complex) and utilizes ATP hydrolysis to affect the conformations of that complex. SNAP-SNARE complex disassembly is essential for SNARE recycling and sustained membrane trafficking. NSF is a homo-hexamer; each protomer is composed of an N-terminal domain, NSF-N, and two adjacent AAA-domains, NSF-D1 and NSF-D2. Mutagenesis analysis has established specific roles for many of the structural elements of NSF-D1, the catalytic ATPase domain, and NSF-N, the SNAP-SNARE binding domain. Hydrodynamic analysis of NSF, labeled with (Ni(2+)-NTA)(2)-Cy3, detected conformational differences in NSF, in which the ATP-bound conformation appears more compact than the ADP-bound form. This indicates that NSF undergoes significant conformational changes as it progresses through its ATP-hydrolysis cycle. Incorporating these data, we propose a sequential mechanism by which NSF uses NSF-N and NSF-D1 to disassemble SNAP-SNARE complexes. We also illustrate how analytical centrifugation might be used to study other AAA(+) proteins.

FtsK DNA translocase: the fast motor that knows where it's going

FtsK is a double-stranded DNA translocase, a motor that converts the chemical energy of binding and hydrolysing ATP into movement of a DNA substrate. It moves DNA at an amazing rate->5000 bp per second-and is powerful enough to remove other proteins from the DNA. In bacteria it is localised to the site of cell division, the septum, where it functions as a DNA pump at the late stages of the cell cycle, to expedite cytokinesis and chromosome segregation. The N terminus of the protein is involved in the cell-cycle-specific localisation and assembly of the cell-division machinery, whereas the C terminus forms the motor. The motor portion of FtsK has been studied by a combination of biochemistry, genetics, X-ray crystallography and single-molecule mechanical assays, and these will be the focus here. The motor can be divided into three subdomains: α, β and γ. The α and β domains multimerise to produce a hexameric ring with a central channel for dsDNA, and contain a RecA-like nucleotide-binding/hydrolysis fold. The motor is given directionality by the regulatory γ domain, which binds to polarised chromosomal sequences-5'-GGGNAGGG-3', known as KOPS-to ensure that the motor is loaded onto DNA in a specific orientation such that subsequent translocation is always towards the region of the chromosome where replication usually terminates (the terminus), and specifically to the 28 bp dif site, located in this region. Once the FtsK translocase has located the dif site it then interacts with the XerCD site-specific recombinases to activate recombination.

Single chain forms of the enhancer binding protein PspF provide insights into geometric requirements for gene activation

Genetic information in the DNA is accessed by the molecular machine RNA polymerase following a highly conserved process, invariably involving the transition between double-stranded and single-stranded DNA states. In the case of the bacterial enhancer-dependent RNA polymerase (which is essential for adaptive responses and bacterial pathogenesis), the DNA melting event depends on specialized hexameric AAA+ ATPase activators. Involvement of such factors in transcription was demonstrated 25 years ago, but why these activators need to be hexameric, whether all the subunits operate identically, what is the contribution of each of the six subunits within the hexamer (structural, functional, or both), and how many active subunits are required for transcription activation remain open questions. Using engineered single-chain polypeptides covalently linking two or three subunits of the activator (allowing the subunit distribution within a hexamer to be fixed), we now show that (i) individual subunits have differential contributions to the activities of the oligomer and (ii) only a fraction of the subunits within the hexameric ATPase is directly required for gene activation. We establish that nucleotide-dependent coordination across three subunits of the hexameric bacterial enhancer binding proteins (bEBPs) is necessary for engagement and remodeling of the closed complex (RPc). Outcomes revealed features of bEBP, distinguishing their mode of action from fully processive AAA+ proteins or from simple bimodal switches. We now propose that the hexamer functions with asymmetric organization, potentially involving a split planar (open ring) or spiral character.

The nuts and bolts of ring-translocase structure and mechanism

Ring-shaped, oligomeric translocases are multisubunit enzymes that couple the hydrolysis of Nucleoside TriPhosphates (NTPs) to directed movement along extended biopolymer substrates. These motors help unwind nucleic acid duplexes, unfold protein chains, and shepherd nucleic acids between cellular and/or viral compartments. Substrates are translocated through a central pore formed by a circular array of catalytic subunits. Cycles of nucleotide binding, hydrolysis, and product release help reposition translocation loops in the pore to direct movement. How NTP turnover allosterically induces these conformational changes, and the extent of mechanistic divergence between motor families, remain outstanding problems. This review examines the current models for ring-translocase function and highlights the fundamental gaps remaining in our understanding of these molecular machines.

Pre-replication complex proteins assemble at regions of low nucleosome occupancy within the Chinese hamster dihydrofolate reductase initiation zone

Genome-scale mapping of pre-replication complex proteins has not been reported in mammalian cells. Poor enrichment of these proteins at specific sites may be due to dispersed binding, poor epitope availability or cell cycle stage-specific binding. Here, we have mapped sites of biotin-tagged ORC and MCM protein binding in G1-synchronized populations of Chinese hamster cells harboring amplified copies of the dihydrofolate reductase (DHFR) locus, using avidin-affinity purification of biotinylated chromatin followed by high-density microarray analysis across the DHFR locus. We have identified several sites of significant enrichment for both complexes distributed throughout the previously identified initiation zone. Analysis of the frequency of initiations across stretched DNA fibers from the DHFR locus confirmed a broad zone of de-localized initiation activity surrounding the sites of ORC and MCM enrichment. Mapping positions of mononucleosomal DNA empirically and computing nucleosome-positioning information in silico revealed that ORC and MCM map to regions of low measured and predicted nucleosome occupancy. Our results demonstrate that specific sites of ORC and MCM enrichment can be detected within a mammalian intitiation zone, and suggest that initiation zones may be regions of generally low nucleosome occupancy where flexible nucleosome positioning permits flexible pre-RC assembly sites.

Origin DNA melting and unwinding in DNA replication

Genomic DNA replication is a necessary step in the life cycles of all organisms. To initiate DNA replication, the double-stranded DNA (dsDNA) at the origin of replication must be separated or melted; this melted region is propagated and a mature replication fork is formed. To accomplish origin recognition, initial DNA melting, and the eventual formation of a replication fork, coordinated activity of initiators, helicases, and other cellular factors are required. In this review, we focus on recent advances in the structural and biochemical studies of the initiators and the replicative helicases in multiple replication systems, with emphasis on the systems in archaeal and eukaryotic cells. These studies have yielded insights into the plausible mechanisms of the early stages of DNA replication.

Mutational analysis of an archaeal minichromosome maintenance protein exterior hairpin reveals critical residues for helicase activity and DNA binding

Background

The mini-chromosome maintenance protein (MCM) complex is an essential replicative helicase for DNA replication in Archaea and Eukaryotes. While the eukaryotic complex consists of six homologous proteins (MCM2-7), the archaeon Sulfolobus solfataricus has only one MCM protein (ssoMCM), six subunits of which form a homohexamer. We have recently reported a 4.35Å crystal structure of the near full-length ssoMCM. The structure reveals a total of four β-hairpins per subunit, three of which are located within the main channel or side channels of the ssoMCM hexamer model generated based on the symmetry of the N-terminal Methanothermobacter thermautotrophicus (mtMCM) structure. The fourth β-hairpin, however, is located on the exterior of the hexamer, near the exit of the putative side channels and next to the ATP binding pocket.

Results

In order to better understand this hairpin's role in DNA binding and helicase activity, we performed a detailed mutational and biochemical analysis of nine residues on this exterior β-hairpin (EXT-hp). We examined the activities of the mutants related to their helicase function, including hexamerization, ATPase, DNA binding and helicase activities. The assays showed that some of the residues on this EXT-hp play a role for DNA binding as well as for helicase activity.

Conclusions

These results implicate several current theories regarding helicase activity by this critical hexameric enzyme. As the data suggest that EXT-hp is involved in DNA binding, the results reported here imply that the EXT-hp located near the exterior exit of the side channels may play a role in contacting DNA substrate in a manner that affects DNA unwinding.

The evolutionary history of archaeal MCM helicases: a case study of vertical evolution combined with hitchhiking of mobile genetic elements

Genes encoding DNA replication proteins have been frequently exchanged between cells and mobile elements, such as viruses or plasmids. This raises potential problems to reconstruct their history. Here, we combine phylogenetic and genomic context analyses to study the evolution of the replicative minichromosome maintenance (MCM) helicases in Archaea. Several archaeal genomes encode more than one copy of the mcm gene. Genome context analysis reveals that most of these additional copies are encoded within mobile elements. Exhaustive analysis of these elements reveals diverse groups of integrated archaeal plasmids or viruses, including several head-and-tail proviruses. Some MCMs encoded by mobile elements are structurally distinct from their cellular counterparts, with one case of novel domain organization. Both genome context and phylogenetic analysis indicate that MCM encoded by mobile elements were recruited from cellular genomes. An accelerated evolution and a dramatic expansion of methanococcal MCMs suggest a host-to-virus-to-host transfer loop, possibly triggered by the loss of the archaeal initiator protein Cdc6 in Methanococcales. Surprisingly, despite extensive transfer of mcm genes between viruses, plasmids, and cells, the topology of the MCM tree is strikingly congruent with the consensus archaeal phylogeny, indicating that mobile elements encoding mcm have coevolved with their hosts and that DNA replication proteins can be also useful to reconstruct the history of the archaeal domain.

Insights into the MCM functional mechanism: lessons learned from the archaeal MCM complex

The helicase function of the minichromosome maintenance protein (MCM) is essential for genomic DNA replication in archaea and eukaryotes. There has been rapid progress in studies of the structure and function of MCM proteins from different organisms, leading to better understanding of the MCM helicase mechanism. Because there are a number of excellent reviews on this topic, we will use this review to summarize some of the recent progress, with particular focus on the structural aspects of MCM and their implications for helicase function. Given the hexameric and double hexameric architecture observed by X-ray crystallography and electron microscopy of MCMs from archaeal and eukaryotic cells, we summarize and discuss possible unwinding modes by either a hexameric or a double hexameric helicase. Additionally, our recent crystal structure of a full length archaeal MCM has provided structural information on an intact, multi-domain MCM protein, which includes the salient features of four unusual beta-hairpins from each monomer, and the side channels of a hexamer/double hexamer. These new structural data enable a closer examination of the structural basis of the unwinding mechanisms by MCM.

The Saccharomyces cerevisiae Mcm6/2 and Mcm5/3 ATPase active sites contribute to the function of the putative Mcm2-7 'gate'

The Mcm2-7 complex is the eukaryotic replicative helicase, a toroidal AAA⁺ molecular motor that uses adenosine triphosphate (ATP) binding and hydrolysis to separate duplex DNA strands during replication. This heterohexameric helicase contains six different and essential subunits (Mcm2 through Mcm7), with the corresponding dimer interfaces forming ATPase active sites from conserved motifs of adjacent subunits. As all other known hexameric helicases are formed from six identical subunits, the function of the unique heterohexameric organization of Mcm2-7 is of particular interest. Indeed, prior work using mutations in the conserved Walker A box ATPase structural motif strongly suggests that individual ATPase active sites contribute differentially to Mcm2-7 activity. Although only a specific subset of active sites is required for helicase activity, another ATPase active site (Mcm2/5) may serve as a reversible ATP-dependent discontinuity (‘gate’) within the hexameric ring structure. This study analyzes the contribution that two other structural motifs, the Walker B box and arginine finger, make to each Mcm2-7 ATPase active site. Mutational analysis of these motifs not only confirms that Mcm ATPase active sites contribute unequally to activity but implicates the involvement of at least two additional active sites (Mcm5/3 and 6/2) in modulating the activity of the putative Mcm2/5 gate.

Engineered interfaces of an AAA+ ATPase reveal a new nucleotide-dependent coordination mechanism

Homohexameric ring AAA(+) ATPases are found in all kingdoms of life and are involved in all cellular processes. To accommodate the large spectrum of substrates, the conserved AAA(+) core has become specialized through the insertion of specific substrate-binding motifs. Given their critical roles in cellular function, understanding the nucleotide-driven mechanisms of action is of wide importance. For one type of member AAA(+) protein (phage shock protein F, PspF), we identified and established the functional significance of strategically placed arginine and glutamate residues that form interacting pairs in response to nucleotide binding. We show that these interactions are critical for "cis" and "trans" subunit communication, which support coordination between subunits for nucleotide-dependent substrate remodeling. Using an allele-specific suppression approach for ATPase and substrate remodeling, we demonstrate that the targeted residues directly interact and are unlikely to make any other pairwise critical interactions. We then propose a mechanistic rationale by which the nucleotide-bound state of adjacent subunits can be sensed without direct involvement of R-finger residues. As the structural AAA(+) core is conserved, we propose that the functional networks established here could serve as a template to identify similar residue pairs in other AAA(+) proteins.

Separating speed and ability to displace roadblocks during DNA translocation by FtsK

FtsK translocates dsDNA directionally at >5 kb/s, even under strong forces. In vivo, the action of FtsK at the bacterial division septum is required to complete the final stages of chromosome unlinking and segregation. Despite the availability of translocase structures, the mechanism by which ATP hydrolysis is coupled to DNA translocation is not understood. Here, we use covalently linked translocase subunits to gain insight into the DNA translocation mechanism. Covalent trimers of wild-type subunits dimerized efficiently to form hexamers with high translocation activity and an ability to activate XerCD-dif chromosome unlinking. Covalent trimers with a catalytic mutation in the central subunit formed hexamers with two mutated subunits that had robust ATPase activity. They showed wild-type translocation velocity in single-molecule experiments, activated translocation-dependent chromosome unlinking, but had an impaired ability to displace either a triplex oligonucleotide, or streptavidin linked to biotin-DNA, during translocation along DNA. This separation of translocation velocity and ability to displace roadblocks is more consistent with a sequential escort mechanism than stochastic, hand-off, or concerted mechanisms.

The Mcm complex: unwinding the mechanism of a replicative helicase

The Mcm2-7 complex serves as the eukaryotic replicative helicase, the molecular motor that both unwinds duplex DNA and powers fork progression during DNA replication. Consistent with its central role in this process, much prior work has illustrated that Mcm2-7 loading and activation are landmark events in the regulation of DNA replication. Unlike any other hexameric helicase, Mcm2-7 is composed of six unique and essential subunits. Although the unusual oligomeric nature of this complex has long hampered biochemical investigations, recent advances with both the eukaryotic as well as the simpler archaeal Mcm complexes provide mechanistic insight into their function. In contrast to better-studied homohexameric helicases, evidence suggests that the six Mcm2-7 complex ATPase active sites are functionally distinct and are likely specialized to accommodate the regulatory constraints of the eukaryotic process.

Novel essential residues of Hda for interaction with DnaA in the regulatory inactivation of DnaA: unique roles for Hda AAA Box VI and VII motifs

Escherichia coli ATP-DnaA initiates chromosomal replication. For preventing extra-initiations, a complex of ADP-Hda and the DNA-loaded replicase clamp promotes DnaA-ATP hydrolysis, yielding inactive ADP-DnaA. However, the Hda-DnaA interaction mode remains unclear except that the Hda Box VII Arg finger (Arg-153) and DnaA sensor II Arg-334 within each AAA(+) domain are crucial for the DnaA-ATP hydrolysis. Here, we demonstrate that direct and functional interaction of ADP-Hda with DnaA requires the Hda residues Ser-152, Phe-118 and Asn-122 as well as Hda Arg-153 and DnaA Arg-334. Structural analyses suggest intermolecular interactions between Hda Ser-152 and DnaA Arg-334 and between Hda Phe-118 and the DnaA Walker B motif region, in addition to an intramolecular interaction between Hda Asn-122 and Arg-153. These interactions likely sustain a specific association of ADP-Hda and DnaA, promoting DnaA-ATP hydrolysis. Consistently, ATP-DnaA and ADP-DnaA interact with the ADP-Hda-DNA-clamp complex with similar affinities. Hda Phe-118 and Asn-122 are contained in the Box VI region, and their hydrophobic and electrostatic features are basically conserved in the corresponding residues of other AAA(+) proteins, suggesting a conserved role for Box VI. These findings indicate novel interaction mechanisms for Hda-DnaA as well as a potentially fundamental mechanism in AAA(+) protein interactions.

Coupling ATP utilization to protein remodeling by ClpB, a hexameric AAA+ protein

ClpB and Hsp104 are members of the AAA+ (ATPases associated with various cellular activities) family of proteins and are molecular machines involved in thermotolerance. They are hexameric proteins containing 12 ATP binding sites with two sites per protomer. ClpB and Hsp104 possess some innate protein remodeling activities; however, they require the collaboration of the DnaK/Hsp70 chaperone system to disaggregate and reactivate insoluble aggregated proteins. We investigated the mechanism by which ClpB couples ATP utilization to protein remodeling with and without the DnaK system. When wild-type ClpB, which is unable to remodel proteins alone in the presence of ATP, was mixed with a ClpB mutant that is unable to hydrolyze ATP, the heterohexamers surprisingly gained protein remodeling activity. Optimal protein remodeling by the heterohexamers in the absence of the DnaK system required approximately three active and three inactive protomers. In addition, the location of the active and inactive ATP binding sites in the hexamer was not important. The results suggest that in the absence of the DnaK system, ClpB acts by a probabilistic mechanism. However, when we measured protein disaggregation by ClpB heterohexamers in conjunction with the DnaK system, incorporation of a single inactive ClpB subunit blocked activity, supporting a sequential mechanism of ATP utilization. Taken together, the results suggest that the mechanism of ATP utilization by ClpB is adaptable and can vary depending on the specific substrate and the presence of the DnaK system.

Dissecting the N-ethylmaleimide-sensitive factor: required elements of the N and D1 domains

N-Ethylmaleimide-sensitive factor (NSF) is a homo-hexameric member of the AAA(+) (ATPases associated with various cellular activities plus) family. It plays an essential role in most intracellular membrane trafficking through its binding to and disassembly of soluble NSF attachment protein (SNAP) receptor (SNARE) complexes. Each NSF protomer contains an N-terminal domain (NSF-N) and two AAA domains, a catalytic NSF-D1 and a structural NSF-D2. This study presents detailed mutagenesis analyses of NSF-N and NSF-D1, dissecting their roles in ATP hydrolysis, SNAP.SNARE binding, and complex disassembly. Our results show that a positively charged surface on NSF-N, bounded by Arg(67) and Lys(105), and the conserved residues in the central pore of NSF-D1 (Tyr(296) and Gly(298)) are involved in SNAP.SNARE binding but not basal ATP hydrolysis. Mutagenesis of Sensor 1 (Thr(373)-Arg(375)), Sensor 2 (Glu(440)-Glu(442)), and Arginine Fingers (Arg(385) and Arg(388)) in NSF-D1 shows that each region plays a discrete role. Sensor 1 is important for basal ATPase activity and nucleotide binding. Sensor 2 plays a role in ATP- and SNAP-dependent SNARE complex binding and disassembly but does so in cis and not through inter-protomer interactions. Arginine Fingers are important for SNAP.SNARE complex-stimulated ATPase activity and complex disassembly. Mutants at these residues have a dominant-negative phenotype in cells, suggesting that Arginine Fingers function in trans via inter-protomer interactions. Taken together, these data establish functional roles for many of the structural elements of the N domain and of the D1 ATP-binding site of NSF.

Running in reverse: the structural basis for translocation polarity in hexameric helicases

Hexameric helicases couple ATP hydrolysis to processive separation of nucleic acid duplexes, a process critical for gene expression, DNA replication, and repair. All hexameric helicases fall into two families with opposing translocation polarities: the 3'-->5' AAA+ and 5'-->3' RecA-like enzymes. To understand how a RecA-like hexameric helicase engages and translocates along substrate, we determined the structure of the E. coli Rho transcription termination factor bound to RNA and nucleotide. Interior nucleic acid-binding elements spiral around six bases of RNA in a manner unexpectedly reminiscent of an AAA+ helicase, the papillomavirus E1 protein. Four distinct ATP-binding states, representing potential catalytic intermediates, are coupled to RNA positioning through a complex allosteric network. Comparative studies with E1 suggest that RecA and AAA+ hexameric helicases use different portions of their chemomechanical cycle for translocating nucleic acid and track in opposite directions by reversing the firing order of ATPase sites around the hexameric ring. For a video summary of this article, see the PaperFlick file with the Supplemental Data available online.

Structural biology of MCM helicases

The eukaryotic MCM2-7 complex is recruited onto origins of replication during the G1 phase of the cell cycle and acts as the main helicase at the replication fork during the S phase. Over the last few years a number of structural reports on MCM proteins using both electron microscopy and protein crystallography have been published. The crystal structures of two (almost) full-length archaeal homologs provide the first atomic pictures of a MCM helicase. However one of the structures is at low resolution and the other is of an inactive MCM. Moreover, both proteins are monomeric in the crystal, whereas the activity of the complex is critically dependent on oligomerization. Lower resolution structures derived from electron microscopy studies are therefore crucial to complement the crystallographic analysis and to assemble the multimeric complex that is active in the cell. A critical analysis of all the structural results elucidates the potential conformational changes and dynamic behavior of MCM helicase to provide a first insight into the gamut of molecular configurations adopted during the processes of DNA melting and unwinding.

An intersubunit signaling network coordinates ATP hydrolysis by m-AAA proteases

Ring-shaped AAA+ ATPases control a variety of cellular processes by substrate unfolding and remodeling of macromolecular structures. However, how ATP hydrolysis within AAA+ rings is regulated and coupled to mechanical work is poorly understood. Here we demonstrate coordinated ATP hydrolysis within m-AAA protease ring complexes, conserved AAA+ machines in the inner membrane of mitochondria. ATP binding to one AAA subunit inhibits ATP hydrolysis by the neighboring subunit, leading to coordinated rather than stochastic ATP hydrolysis within the AAA ring. Unbiased genetic screens define an intersubunit signaling pathway involving conserved AAA motifs and reveal an intimate coupling of ATPase activities to central AAA pore loops. Coordinated ATP hydrolysis between adjacent subunits is required for membrane dislocation of substrates, but not for substrate processing. These findings provide insight into how AAA+ proteins convert energy derived from ATP hydrolysis into mechanical work.

Motor mechanism for protein threading through Hsp104

The protein-remodeling machine Hsp104 dissolves amorphous aggregates as well as ordered amyloid assemblies such as yeast prions. Force generation originates from a tandem AAA+ (ATPases associated with various cellular activities) cassette, but the mechanism and allostery of this action remain to be established. Our cryoelectron microscopy maps of Hsp104 hexamers reveal substantial domain movements upon ATP binding and hydrolysis in the first nucleotide-binding domain (NBD1). Fitting atomic models of Hsp104 domains to the EM density maps plus supporting biochemical measurements show how the domain movements displace sites bearing the substrate-binding tyrosine loops. This provides the structural basis for N- to C-terminal substrate threading through the central cavity, enabling a clockwise handover of substrate in the NBD1 ring and coordinated substrate binding between NBD1 and NBD2. Asymmetric reconstructions of Hsp104 in the presence of ATPγS or ATP support sequential rather than concerted ATP hydrolysis in the NBD1 ring.

ATP binding and hydrolysis by Mcm2 regulate DNA binding by Mcm complexes

The essential minichromosome maintenance (Mcm) proteins Mcm2 through Mcm7 likely comprise the replicative helicase in eukaryotes. In addition to Mcm2-7, other subcomplexes, including one comprising Mcm4, Mcm6, and Mcm7, unwind DNA. Using Mcm4/6/7 as a tool, we reveal a role for nucleotide binding by Saccharomyces cerevisiae Mcm2 in modulating DNA binding by Mcm complexes. Previous studies have shown that Mcm2 inhibits DNA unwinding by Mcm4/6/7. Here, we show that interaction of Mcm2 and Mcm4/6/7 is not sufficient for inhibition; rather, Mcm2 requires nucleotides for its regulatory role. An Mcm2 mutant that is defective for ATP hydrolysis (K549A), as well as ATP analogues, was used to show that ADP binding by Mcm2 is required to inhibit DNA binding and unwinding by Mcm4/6/7. This Mcm2-mediated regulation of Mcm4/6/7 is independent of Mcm3/5. Furthermore, the importance of ATP hydrolysis by Mcm2 to the regulation of the native complex was apparent from the altered DNA binding properties of Mcm2(KA)-7. Moreover, together with the finding that Mcm2(K549A) does not support yeast viability, these results indicate that the nucleotide-bound state of Mcm2 is critical in regulating the activities of Mcm4/6/7 and Mcm2-7 complexes.

Replisome structure and conformational dynamics underlie fork progression past obstacles

Replisomes are multiprotein complexes that unzip the parental helix and duplicate the separated strands during genome replication. The antiparallel structure of DNA poses unique geometric constraints to the process, and the replisome has evolved unique dynamic features that solve this problem. Interestingly, the solution to duplex DNA replication has been co-opted to solve many other important problems that replisomes must contend with during the duplication of long chromosomes. For example, along its path the replisome will encounter lesions and DNA-bound proteins. Recent studies show that the replisome can circumvent lesions on either strand, using the strategy normally applied to the lagging strand synthesis. Circumventing lesions can also be assisted by other proteins that transiently become a part of the replisome. The replisome must also contend with DNA-binding proteins and recent studies reveal a fascinating process that enables it to bypass RNA polymerase without stopping.

Promiscuous usage of nucleotides by the DNA helicase of bacteriophage T7: determinants of nucleotide specificity

The multifunctional protein encoded by gene 4 of bacteriophage T7 (gp4) provides both helicase and primase activity at the replication fork. T7 DNA helicase preferentially utilizes dTTP to unwind duplex DNA in vitro but also hydrolyzes other nucleotides, some of which do not support helicase activity. Very little is known regarding the architecture of the nucleotide binding site in determining nucleotide specificity. Crystal structures of the T7 helicase domain with bound dATP or dTTP identified Arg-363 and Arg-504 as potential determinants of the specificity for dATP and dTTP. Arg-363 is in close proximity to the sugar of the bound dATP, whereas Arg-504 makes a hydrogen bridge with the base of bound dTTP. T7 helicase has a serine at position 319, whereas bacterial helicases that use rATP have a threonine in the comparable position. Therefore, in the present study we have examined the role of these residues (Arg-363, Arg-504, and Ser-319) in determining nucleotide specificity. Our results show that Arg-363 is responsible for dATP, dCTP, and dGTP hydrolysis, whereas Arg-504 and Ser-319 confer dTTP specificity. Helicase-R504A hydrolyzes dCTP far better than wild-type helicase, and the hydrolysis of dCTP fuels unwinding of DNA. Substitution of threonine for serine 319 reduces the rate of hydrolysis of dTTP without affecting the rate of dATP hydrolysis. We propose that different nucleotides bind to the nucleotide binding site of T7 helicase by an induced fit mechanism. We also present evidence that T7 helicase uses the energy derived from the hydrolysis of dATP in addition to dTTP for mediating DNA unwinding.

Intersubunit coordination in a homomeric ring ATPase

Homomeric ring ATPases perform many vital and varied tasks in the cell, ranging from chromosome segregation to protein degradation. Here we report the direct observation of the intersubunit coordination and step size of such a ring ATPase, the double-stranded-DNA packaging motor in the bacteriophage phi29. Using high-resolution optical tweezers, we find that packaging occurs in increments of 10 base pairs (bp). Statistical analysis of the preceding dwell times reveals that multiple ATPs bind during each dwell, and application of high force reveals that these 10-bp increments are composed of four 2.5-bp steps. These results indicate that the hydrolysis cycles of the individual subunits are highly coordinated by means of a mechanism novel for ring ATPases. Furthermore, a step size that is a non-integer number of base pairs demands new models for motor-DNA interactions.

Intersubunit allosteric communication mediated by a conserved loop in the MCM helicase

The minichromosome maintenance (MCM) helicase is the presumptive replicative helicase in archaea and eukaryotes. The archaeal homomultimeric MCM has a two-tier structure. One tier contains the AAA+ motor domains of the proteins, and these are the minimal functional helicase domains. The second tier is formed by the N-terminal domains. These domains are not essential for MCM helicase activity but act to enhance the processivity of the helicase. We reveal that a conserved loop facilitates communication between processivity and motor tiers. Interestingly, this allostery seems to be mediated by interactions between, rather than within, individual protomers in the MCM ring.

Crystal structure of a near-full-length archaeal MCM: functional insights for an AAA+ hexameric helicase

The minichromosome maintenance protein (MCM) complex is an essential replicative helicase for DNA replication in Archaea and Eukaryotes. Whereas the eukaryotic complex consists of 6 homologous proteins (MCM2-7), the archaeon Sulfolobus solfataricus has only 1 MCM protein (ssoMCM), 6 subunits of which form a homohexamer. Here, we report a 4.35-A crystal structure of the near-full-length ssoMCM. The structure shows an elongated fold, with 5 subdomains that are organized into 2 large N- and C-terminal domains. A near-full-length ssoMCM hexamer generated based on the 6-fold symmetry of the N-terminal Methanothermobacter thermautotrophicus (mtMCM) hexamer shows intersubunit distances suitable for bonding contacts, including the interface around the ATP pocket. Four unusual beta-hairpins of each subunit are located inside the central channel or around the side channels in the hexamer. Additionally, the hexamer fits well into the double-hexamer EM map of mtMCM. Our mutational analysis of residues at the intersubunit interfaces and around the side channels demonstrates their critical roles for hexamerization and helicase function. These structural and biochemical results provide a basis for future study of the helicase mechanisms of the archaeal and eukaryotic MCM complexes in DNA replication.

Biochemical characterization of the minichromosome maintenance protein from the archaeon Thermoplasma acidophilum

Minichromosome maintenance (MCM) proteins are thought to function as the replicative helicases in archaea. Studies have shown that the MCM complex from the thermoacidophilic euryarchaeon Thermoplasma acidophilum (TaMCM) has some properties not reported in other archaeal MCM helicases. Here, the biochemical properties of the TaMCM are studied. The protein binds single-stranded DNA, has DNA-dependent ATPase activity and ATP-dependent 3' --> 5' helicase activity. The optimal helicase conditions with regard to temperature, pH and salinity are similar to the intracellular conditions in T. acidophilum. It is also found that about 1,000 molecules of TaMCM are present per actively growing cell.

Thermoplasma acidophilum Cdc6 protein stimulates MCM helicase activity by regulating its ATPase activity

The minichromosome maintenance (MCM) proteins are thought to function as the replicative helicases in archaea. In most archaeal species studied, the interaction between MCM and the initiator protein, Cdc6, inhibits helicase activity. To date, the only exception is the helicase and Cdc6 proteins from the archaeon Thermoplasma acidophilum. It was previously shown that when the Cdc6 protein interacts with MCM it substantially stimulates helicase activity. It is shown here that the mechanism by which the Cdc6 protein stimulates helicase activity is by stimulating the ATPase activity of MCM. Also, through the use of site-specific substitutions, and truncated and chimeric proteins, it was shown that an intact Cdc6 protein is required for this stimulation. ATP binding and hydrolysis by the Cdc6 protein is not needed for the stimulation. The data suggest that binding of Cdc6 protein to MCM protein changes the structure of the helicase, enhancing the catalytic hydrolysis of ATP and helicase activity.

Mechanism of homotropic control to coordinate hydrolysis in a hexameric AAA+ ring ATPase

AAA(+) proteins are ubiquitous mechanochemical ATPases that use energy from ATP hydrolysis to remodel their versatile substrates. The AAA(+) characteristic hexameric ring assemblies raise important questions about if and how six often identical subunits coordinate hydrolysis and associated motions. The PspF AAA(+) domain, PspF(1-275), remodels the bacterial sigma(54)-RNA polymerase to activate transcription. Analysis of ATP substrate inhibition kinetics on ATP hydrolysis in hexameric PspF(1-275) indicates negative homotropic effects between subunits. Functional determinants required for allosteric control identify: (i) an important link between the ATP bound ribose moiety and the SensorII motif that would allow nucleotide-dependent *-helical */beta subdomain dynamics; and (ii) establishes a novel regulatory role for the SensorII helix in PspF, which may apply to other AAA(+) proteins. Consistent with functional data, homotropic control appears to depend on nucleotide state-dependent subdomain angles imposing dynamic symmetry constraints in the AAA(+) ring. Homotropic coordination is functionally important to remodel the sigma(54) promoter. We propose a structural symmetry-based model for homotropic control in the AAA(+) characteristic ring architecture.

Structural analysis of the Sulfolobus solfataricus MCM protein N-terminal domain

The Mini-Chromosome Maintenance (MCM) proteins are candidates of replicative DNA helicase in eukarya and archaea. Here we report a 2.8 A crystal structure of the N-terminal domain (residues 1-268) of the Sulfolobus solfataricus MCM (Sso MCM) protein. The structure reveals single-hexameric ring-like architecture, at variance from the protein of Methanothermobacter thermoautotrophicus (Mth). Moreover, the central channel in Sso MCM seems significantly narrower than the Mth counterpart, which appears to more favorably accommodate single-stranded DNA than double-stranded DNA, as supported by DNA-binding assays. Structural analysis also highlights the essential role played by the zinc-binding domain in the interaction with nucleic acids and allows us to speculate that the Sso MCM N-ter domain may function as a molecular clamp to grasp the single-stranded DNA passing through the central channel. On this basis possible DNA unwinding mechanisms are discussed.

Coupling and dynamics of subunits in the hexameric AAA+ chaperone ClpB

The bacterial AAA+ protein ClpB and its eukaryotic homologue Hsp104 ensure thermotolerance of their respective organisms by reactivating aggregated proteins in cooperation with the Hsp70/Hsp40 chaperone system. Like many members of the AAA+ superfamily, the ClpB protomers form ringlike homohexameric complexes. The mechanical energy necessary to disentangle protein aggregates is provided by ATP hydrolysis at the two nucleotide-binding domains of each monomer. Previous studies on ClpB and Hsp104 show a complex interplay of domains and subunits resulting in homotypic and heterotypic cooperativity. Using mutations in the Walker A and Walker B nucleotide-binding motifs in combination with mixing experiments we investigated the degree of inter-subunit coupling with respect to different aspects of the ClpB working cycle. We find that subunits are tightly coupled with regard to ATPase and chaperone activity, but no coupling can be observed for ADP binding. Comparison of the data with statistical calculations suggests that for double Walker mutants, approximately two in six subunits are sufficient to abolish chaperone and ATPase activity completely. In further experiments, we determined the dynamics of subunit reshuffling. Our results show that ClpB forms a very dynamic complex, reshuffling subunits on a timescale comparable to steady-state ATP hydrolysis. We propose that this could be a protection mechanism to prevent very stable aggregates from becoming suicide inhibitors for ClpB.