The structure of the cohesin ATPase elucidates the mechanism of SMC-kleisin ring opening

Cohesin ring gates are specialized for meiotic cell division

Cohesin is a ring complex closed with structural maintenance of chromosome 1 (SMC-1), SMC-3, and a kleisin subunit, mediating sister chromatid cohesion in mitosis and meiosis. Kleisin N- and C-terminal domains interact with SMC-3 and SMC-1, forming two distinct cohesin gates. Whether these gates are specialized for mitosis and meiosis remains elusive. Here, we create Caenorhabditis elegans mutants that express chimeric proteins swapping N- and C-terminal domains between different kleisins to investigate how these gates are specialized for different cell division programs. Replacing the meiotic REC-8 N-terminus with that of a cell division-unrelated kleisin COH-1 or the mitotic kleisin sister chromatid cohesion protein 1 (SCC-1) disrupts inter-sister chromatid cohesion and causes severe meiotic defects. Swapping the REC-8 C-terminus with that of COH-1 or SCC-1 largely retains the meiotic functions of REC-8 but causes age-related chromosome abnormalities. A specialized C-terminus is also required for the functions of SCC-1. Furthermore, point mutations in the REC-8 C-terminus cause severe meiotic defects without impairing the SMC-1-kleisin interaction, suggesting an integrated SMC-1-kleisin gate. These findings suggest the requirements for specialized cohesin gates in different biological processes.

Mechanism of DNA capture by the MukBEF SMC complex and its inhibition by a viral DNA mimic

Ring-like structural maintenance of chromosome (SMC) complexes are crucial for genome organization and operate through mechanisms of DNA entrapment and loop extrusion. Here, we explore the DNA loading process of the bacterial SMC complex MukBEF. Using cryoelectron microscopy (cryo-EM), we demonstrate that ATP binding opens one of MukBEF's three potential DNA entry gates, exposing a DNA capture site that positions DNA at the open neck gate. We discover that the gp5.9 protein of bacteriophage T7 blocks this capture site by DNA mimicry, thereby preventing DNA loading and inactivating MukBEF. We propose a comprehensive and unidirectional loading mechanism in which DNA is first captured at the complex's periphery and then ingested through the DNA entry gate, powered by a single cycle of ATP hydrolysis. These findings illuminate a fundamental aspect of how ubiquitous DNA organizers are primed for genome maintenance and demonstrate how this process can be disrupted by viruses.

Sister chromatid cohesion through the lens of biochemical experiments

Faithful chromosome segregation in eukaryotes relies on physical cohesion between newly duplicated sister chromatids. Cohesin is a ring-shaped ATPase assembly that mediates sister chromatid cohesion through its ability to topologically entrap DNA. Cohesin, assisted by several regulatory proteins, binds to DNA prior to DNA replication and then holds two sister DNAs together when it encounters the replication machinery. Cohesion establishment further requires cohesin acetylation, which confers near eternal stability on chromatin-bound cohesin until the onset of chromosome segregation. In addition to a wealth of experimental evidence from cellular studies, recent advances in reconstitution approaches are now beginning to unravel the biochemical properties of cohesin that underlie its function in sister chromatid cohesion. This review summarizes recent insights into the mechanism of cohesion establishment.

A unified model for cohesin function in sisterchromatid cohesion and chromatin loop formation

The ring-shaped cohesin complex topologically entraps two DNAs to establish sister chromatid cohesion. Cohesin also shapes the interphase chromatin landscape by forming DNA loops, which it is thought to achieve using an in vitro-observed loop extrusion mechanism. However, recent studies revealed that loop-extrusion-deficient cohesin retains its ability to form chromatin loops, suggesting a divergence of in vitro and in vivo loop formation. Instead of loop extrusion, we examine whether cohesin forms chromatin loops by a mechanism akin to sister chromatid cohesion establishment: sequential topological capture of two DNAs. We explore similarities and differences between the "loop capture" and the "loop extrusion" model, how they compare at explaining experimental observations, and how future approaches can delineate their possible respective contributions. We extend our DNA-DNA capture model for cohesin function to related structural maintenance of chromosomes (SMC) family members, condensin, the Smc5-Smc6 complex, and bacterial SMC complexes.

Early onset of septal FtsK localization allows for efficient DNA segregation in SMC-deleted Corynebacterium glutamicum strains

Structural maintenance of chromosomes (SMC) are ubiquitously distributed proteins involved in chromosome organization. Deletion of smc causes severe growth phenotypes in many organisms. Surprisingly, smc can be deleted in Corynebacterium glutamicum, a member of the Actinomycetota phylum, without any apparent growth phenotype. SMC in C. glutamicum is loaded in a ParB-dependent fashion to the chromosome and functions in replichore cohesion. The unexpected absence of a growth phenotype in the smc mutant prompted us to screen for synthetic interactions within C. glutamicum. We generated a high-density Tn5 library from wild-type and smc-deleted C. glutamicum strains. Transposon sequencing data revealed that the DNA translocase FtsK is essential in an smc-deletion strain. In wild-type cells, FtsK localized to the septa and cell poles, showing polar enrichment during the earlier stages of the life cycle and relocating to the septum in the later stages. However, deletion of smc resulted in an earlier onset of pole-to-septum FtsK relocation, suggesting that prolonged FtsK complex activity is both required and sufficient to compensate for the absence of SMC, thus achieving efficient chromosome segregation in C. glutamicum. Deletion of ParB increases SMC and FtsK mobility. While the change in SMC dynamics aligns with previous data showing ParB's role in SMC loading on DNA, the change in FtsK mobility suggests defects in chromosome segregation. Based on our data, we propose an efficient mechanism for reliable DNA segregation in the absence of replichore arm cohesion in smc mutant cells.IMPORTANCEFaithful DNA segregation is of fundamental importance for life. Bacteria have developed efficient systems to coordinate chromosome compaction, DNA segregation, and cell division. A key factor in DNA compaction is the SMC complex that is found to be essential in many bacteria. In members of the Actinomycetota, smc is dispensable, but the reason for the lack of an smc phenotype in these bacteria remained unclear. We show here that the divisome-associated DNA pump FtsK can compensate for SMC loss and the subsequent loss in correct chromosome organization. In cells with distorted chromosomes, FtsK is recruited and stabilized earlier to the septum, allowing for DNA segregation for a larger part of the cell cycle, until chromosomes are segregated.

Molecular mechanism of condensin I activation by KIF4A

During mitosis, the condensin I and II complexes compact chromatin into chromosomes. Loss of the chromokinesin, KIF4A, results in reduced condensin I association with chromosomes, but the molecular mechanism behind this phenotype is unknown. In this study, we reveal that KIF4A binds directly to the human condensin I HAWK subunit, NCAPG, via a conserved disordered short linear motif (SLiM) located in its C-terminal tail. KIF4A competes for NCAPG binding to an overlapping site with SLiMs at the N-terminus of NCAPH and the C-terminus of NCAPD2, which mediate two auto-inhibitory interactions within condensin I. Consistently, the KIF4A SLiM peptide alone is sufficient to stimulate ATPase and DNA loop extrusion activities of condensin I. We identify similar SLiMs in the known yeast condensin interactors, Sgo1 and Lrs4, which bind yeast condensin subunit, Ycg1, the equivalent HAWK to NCAPG. Our findings, together with previous work on condensin II and cohesin, demonstrate that SLiM binding to the NCAPG-equivalent HAWK subunit is a conserved mechanism of regulation in SMC complexes.

Nucleic Acids and Electrical Signals

Nucleic acids are highly charged, and electrical forces are involved heavily in how our DNA is compacted and packaged into such a small space, how chromosomes are formed, and how DNA damage is repaired. In addition, electrical forces are crucial to the formation of non-canonical DNA structures called G-Quadruplexes which play multiple biological roles.

Cohesin mutations in acute myeloid leukemia

The cohesin complex, encoded by SMC3, SMC1A, RAD21, and STAG2, is a critical regulator of DNA-looping and gene expression. Over a decade has passed since recurrent mutations affecting cohesin subunits were first identified in myeloid malignancies such as Acute Myeloid Leukemia (AML). Since that time there has been tremendous progress in our understanding of chromatin structure and cohesin biology, but critical questions remain because of the multiple critical functions the cohesin complex is responsible for. Recent findings have been particularly noteworthy with the identification of crosstalk between DNA-looping and chromatin domains, a deeper understanding of how cohesin establishes sister chromatid cohesion, a renewed interest in cohesin's role for DNA damage response, and work demonstrating cohesin's importance for Polycomb repression. Despite these exciting findings, the role of cohesin in normal hematopoiesis, and the precise mechanisms by which cohesin mutations promote cancer, remain poorly understood. This review discusses what is known about the role of cohesin in normal hematopoiesis, and how recent findings could shed light on the mechanisms through which cohesin mutations promote leukemic transformation. Important unanswered questions in the field, such as whether cohesin plays a role in HSC heterogeneity, and the mechanisms by which it regulates gene expression at a molecular level, will also be discussed. Particular attention will be given to the potential therapeutic vulnerabilities of leukemic cells with cohesin subunit mutations.

Requirement of Nek2a and cyclin A2 for Wapl-dependent removal of cohesin from prophase chromatin

Sister chromatid cohesion is mediated by the cohesin complex. In mitotic prophase cohesin is removed from chromosome arms in a Wapl- and phosphorylation-dependent manner. Sgo1-PP2A protects pericentromeric cohesion by dephosphorylation of cohesin and its associated Wapl antagonist sororin. However, Sgo1-PP2A relocates to inner kinetochores well before sister chromatids are separated by separase, leaving pericentromeric regions unprotected. Why deprotected cohesin is not removed by Wapl remains enigmatic. By reconstituting Wapl-dependent cohesin removal from chromatin in vitro, we discovered a requirement for Nek2a and Cdk1/2-cyclin A2. These kinases phosphorylate cohesin-bound Pds5b, thereby converting it from a sororin- to a Wapl-interactor. Replacement of endogenous Pds5b by a phosphorylation mimetic variant causes premature sister chromatid separation (PCS). Conversely, phosphorylation-resistant Pds5b impairs chromosome arm separation in prometaphase-arrested cells and suppresses PCS in the absence of Sgo1. Early mitotic degradation of Nek2a and cyclin A2 may therefore explain why only separase, but not Wapl, can trigger anaphase.

Cohesin organizes 3D DNA contacts surrounding active enhancers in C. elegans

In mammals, cohesin and CTCF organize the 3D genome into topologically associated domains (TADs) to regulate communication between cis-regulatory elements. Many organisms, including S. cerevisiae, C. elegans, and A. thaliana contain cohesin but lack CTCF. Here, we used C. elegans to investigate the function of cohesin in 3D genome organization in the absence of CTCF. Using Hi-C data, we observe cohesin-dependent features called "fountains", which are also reported in zebrafish and mice. These are population average reflections of DNA loops originating from distinct genomic regions and are ~20-40 kb in C. elegans. Hi-C analysis upon cohesin and WAPL depletion support the idea that cohesin is preferentially loaded at NIPBL occupied sites and loop extrudes in an effectively two-sided manner. ChIP-seq analyses show that cohesin translocation along the fountain trajectory depends on a fully intact complex and is extended upon WAPL-1 depletion. Hi-C contact patterns at individual fountains suggest that cohesin processivity is unequal on each side, possibly due to collision with cohesin loaded from surrounding sites. The putative cohesin loading sites are closest to active enhancers and fountain strength is associated with transcription. Compared to mammals, average processivity of C. elegans cohesin is ~10-fold shorter and NIPBL binding does not depend on cohesin. We propose that preferential loading and loop extrusion by cohesin is an evolutionarily conserved mechanism that regulates the 3D interactions of enhancers in animal genomes.

The cohesin ATPase cycle is mediated by specific conformational dynamics and interface plasticity of SMC1A and SMC3 ATPase domains

Cohesin is key to eukaryotic genome organization and acts throughout the cell cycle in an ATP-dependent manner. The mechanisms underlying cohesin ATPase activity are poorly understood. Here, we characterize distinct steps of the human cohesin ATPase cycle and show that the SMC1A and SMC3 ATPase domains undergo specific but concerted structural rearrangements along this cycle. Specifically, whereas the proximal coiled coil of the SMC1A ATPase domain remains conformationally stable, that of the SMC3 displays an intrinsic flexibility. The ATP-dependent formation of the heterodimeric SMC1A/SMC3 ATPase module (engaged state) favors this flexibility, which is counteracted by NIPBL and DNA binding (clamped state). Opening of the SMC3/RAD21 interface (open-engaged state) stiffens the SMC3 proximal coiled coil, thus constricting together with that of SMC1A the ATPase module DNA-binding chamber. The plasticity of the ATP-dependent interface between the SMC1A and SMC3 ATPase domains enables these structural rearrangements while keeping the ATP gate shut. VIDEO ABSTRACT.

Molecular mechanism and functional significance of Wapl interaction with the Cohesin complex

The ring-shaped Cohesin complex, consisting of core subunits Smc1, Smc3, Scc1, and SA2 (or its paralog SA1), topologically entraps two duplicated sister DNA molecules to establish sister chromatid cohesion in S-phase. It remains largely elusive how the Cohesin release factor Wapl binds the Cohesin complex, thereby inducing Cohesin disassociation from mitotic chromosomes to allow proper resolution and separation of sister chromatids. Here, we show that Wapl uses two structural modules containing the FGF motif and the YNARHWN motif, respectively, to simultaneously bind distinct pockets in the extensive composite interface between Scc1 and SA2. Strikingly, only when both docking modules are mutated, Wapl completely loses the ability to bind the Scc1-SA2 interface and release Cohesin, leading to erroneous chromosome segregation in mitosis. Surprisingly, Sororin, which contains a conserved FGF motif and functions as a master antagonist of Wapl in S-phase and G2-phase, does not bind the Scc1-SA2 interface. Moreover, Sgo1, the major protector of Cohesin at mitotic centromeres, can only compete with the FGF motif but not the YNARHWN motif of Wapl for binding Scc1-SA2 interface. Our data uncover the molecular mechanism by which Wapl binds Cohesin to ensure precise chromosome segregation.

Cohesin-Dependent Loop Extrusion: Molecular Mechanics and Role in Cell Physiology

The most prominent representatives of multisubunit SMC complexes, cohesin and condensin, are best known as structural components of mitotic chromosomes. It turned out that these complexes, as well as their bacterial homologues, are molecular motors, the ATP-dependent movement of these complexes along DNA threads leads to the formation of DNA loops. In recent years, we have witnessed an avalanche-like accumulation of data on the process of SMC dependent DNA looping, also known as loop extrusion. This review briefly summarizes the current understanding of the place and role of cohesin-dependent extrusion in cell physiology and presents a number of models describing the potential molecular mechanism of extrusion in a most compelling way. We conclude the review with a discussion of how the capacity of cohesin to extrude DNA loops may be mechanistically linked to its involvement in sister chromatid cohesion.

Cohesin Complex: Structure and Principles of Interaction with DNA

Accurate duplication and separation of long linear genomic DNA molecules is associated with a number of purely mechanical problems. SMC complexes are key components of the cellular machinery that ensures decatenation of sister chromosomes and compaction of genomic DNA during division. Cohesin, one of the essential eukaryotic SMC complexes, has a typical ring structure with intersubunit pore through which DNA molecules can be threaded. Capacity of cohesin for such topological entrapment of DNA is crucial for the phenomenon of post-replicative association of sister chromatids better known as cohesion. Recently, it became apparent that cohesin and other SMC complexes are, in fact, motor proteins with a very peculiar movement pattern leading to formation of DNA loops. This specific process has been called loop extrusion. Extrusion underlies multiple functions of cohesin beyond cohesion, but molecular mechanism of the process remains a mystery. In this review, we summarized the data on molecular architecture of cohesin, effect of ATP hydrolysis cycle on this architecture, and known modes of cohesin-DNA interactions. Many of the seemingly disparate facts presented here will probably be incorporated in a unified mechanistic model of loop extrusion in the not-so-distant future.

Establishment of dsDNA-dsDNA interactions by the condensin complex

Condensin is a structural maintenance of chromosomes (SMC) complex family member thought to build mitotic chromosomes by DNA loop extrusion. However, condensin variants unable to extrude loops, yet proficient in chromosome formation, were recently described. Here, we explore how condensin might alternatively build chromosomes. Using bulk biochemical and single-molecule experiments with purified fission yeast condensin, we observe that individual condensins sequentially and topologically entrap two double-stranded DNAs (dsDNAs). Condensin loading transitions through a state requiring DNA bending, as proposed for the related cohesin complex. While cohesin then favors the capture of a second single-stranded DNA (ssDNA), second dsDNA capture emerges as a defining feature of condensin. We provide complementary in vivo evidence for DNA-DNA capture in the form of condensin-dependent chromatin contacts within, as well as between, chromosomes. Our results support a "diffusion capture" model in which condensin acts in mitotic chromosome formation by sequential dsDNA-dsDNA capture.

Developing a peptide to disrupt cohesin head domain interactions

Cohesin mediates the 3-D structure of chromatin and is involved in maintaining genome stability and function. The cohesin core comprises Smc1 and Smc3, elongated-shaped proteins that dimerize through globular domains at their edges, called head and hinge. ATP binding to the Smc heads induces their dimerization and the formation of two active sites, while ATP hydrolysis results in head disengagement. This ATPase cycle is essential for driving cohesin activity. We report on the development of the first cohesin-inhibiting peptide (CIP). The CIP binds Smc3 in vitro and inhibits the ATPase activity of the holocomplex. Treating yeast cells with the CIP prevents cohesin's tethering activity and, interestingly, leads to the accumulation of cohesin on chromatin. CIP3 also affects cohesin activity in human cells. Altogether, we demonstrate the power of peptides to inhibit cohesin in cells and discuss the potential application of CIPs as a therapeutic approach.

Structural biology of SMC complexes across the tree of life

Structural maintenance of chromosomes (SMC) complexes guard and organize the three-dimensional structure of chromosomal DNA across the tree of life. Many SMC functions can be explained by an inherent motor activity that extrudes large DNA loops while the complexes move along their substrate. Here, we review recent structural insights into the architecture and conservation of these molecular machines, their interaction with DNA, and the conformational changes that are linked to their ATP hydrolysis cycle.

DNA segment capture by Smc5/6 holocomplexes

Three distinct structural maintenance of chromosomes (SMC) complexes facilitate chromosome folding and segregation in eukaryotes, presumably by DNA loop extrusion. How SMCs interact with DNA to extrude loops is not well understood. Among the SMC complexes, Smc5/6 has dedicated roles in DNA repair and preventing a buildup of aberrant DNA junctions. In the present study, we describe the reconstitution of ATP-dependent DNA loading by yeast Smc5/6 rings. Loading strictly requires the Nse5/6 subcomplex which opens the kleisin neck gate. We show that plasmid molecules are topologically entrapped in the kleisin and two SMC subcompartments, but not in the full SMC compartment. This is explained by the SMC compartment holding a looped DNA segment and by kleisin locking it in place when passing between the two flanks of the loop for neck-gate closure. Related segment capture events may provide the power stroke in subsequent DNA extrusion steps, possibly also in other SMC complexes, thus providing a unifying principle for DNA loading and extrusion.

DNA passes through cohesin's hinge as well as its Smc3-kleisin interface

The ring model proposes that sister chromatid cohesion is mediated by co-entrapment of sister DNAs inside a single tripartite cohesin ring. The model explains how Scc1 cleavage triggers anaphase but has hitherto only been rigorously tested using small circular mini-chromosomes in yeast, where covalently circularizing the ring by crosslinking its three interfaces induces catenation of individual and sister DNAs. If the model applies to real chromatids, then the ring must have a DNA entry gate essential for mitosis. Whether this is situated at the Smc3/Scc1 or Smc1/Smc3 hinge interface is an open question. We have previously demonstrated DNA entrapment by cohesin in vitro (Collier et al., 2020). Here we show that cohesin in fact possesses two DNA gates, one at the Smc3/Scc1 interface and a second at the Smc1/3 hinge. Unlike the Smc3/Scc1 interface, passage of DNAs through SMC hinges depends on both Scc2 and Scc3, a pair of regulatory subunits necessary for entrapment in vivo. This property together with the lethality caused by locking this interface but not that between Smc3 and Scc1 in vivo suggests that passage of DNAs through the hinge is essential for building sister chromatid cohesion. Passage of DNAs through the Smc3/Scc1 interface is necessary for cohesin’s separase-independent release from chromosomes and may therefore largely serve as an exit gate.

Mechanisms of Chromosome Folding and Nuclear Organization: Their Interplay and Open Questions

Microscopy and genomic approaches provide detailed descriptions of the three-dimensional folding of chromosomes and nuclear organization. The fundamental question is how activity of molecules at the nanometer scale can lead to complex and orchestrated spatial organization at the scale of chromosomes and the whole nucleus. At least three key mechanisms can bridge across scales: (1) tethering of specific loci to nuclear landmarks leads to massive reorganization of the nucleus; (2) spatial compartmentalization of chromatin, which is driven by molecular affinities, results in spatial isolation of active and inactive chromatin; and (3) loop extrusion activity of SMC (structural maintenance of chromosome) complexes can explain many features of interphase chromatin folding and underlies key phenomena during mitosis. Interestingly, many features of chromosome organization ultimately result from collective action and the interplay between these mechanisms, and are further modulated by transcription and topological constraints. Finally, we highlight some outstanding questions that are critical for our understanding of nuclear organization and function. We believe many of these questions can be answered in the coming years.

DNA tension-modulated translocation and loop extrusion by SMC complexes revealed by molecular dynamics simulations

Structural Maintenance of Chromosomes (SMC) complexes play essential roles in genome organization across all domains of life. To determine how the activities of these large (≈50 nm) complexes are controlled by ATP binding and hydrolysis, we developed a molecular dynamics model that accounts for conformational motions of the SMC and DNA. The model combines DNA loop capture with an ATP-induced 'power stroke' to translocate the SMC complex along DNA. This process is sensitive to DNA tension: at low tension (0.1 pN), the model makes loop-capture steps of average 60 nm and up to 200 nm along DNA (larger than the complex itself), while at higher tension, a distinct inchworm-like translocation mode appears. By tethering DNA to an experimentally-observed additional binding site ('safety belt'), the model SMC complex can perform loop extrusion (LE). The dependence of LE on DNA tension is distinct for fixed DNA tension vs. fixed DNA end points: LE reversal occurs above 0.5 pN for fixed tension, while LE stalling without reversal occurs at about 2 pN for fixed end points. Our model matches recent experimental results for condensin and cohesin, and makes testable predictions for how specific structural variations affect SMC function.

A walk through the SMC cycle: From catching DNAs to shaping the genome

SMC protein complexes are molecular machines that provide structure to chromosomes. These complexes bridge DNA elements and by doing so build DNA loops in cis and hold together the sister chromatids in trans. We discuss how drastic conformational changes allow SMC complexes to build such intricate DNA structures. The tight regulation of these complexes controls fundamental chromosomal processes such as transcription, recombination, repair, and mitosis.

Clamping of DNA shuts the condensin neck gate

SignificanceDNA needs to be compacted to fit into nuclei and during cell division, when dense chromatids are formed for their mechanical segregation, a process that depends on the protein complex condensin. It forms and enlarges loops in DNA through loop extrusion. Our work resolves the atomic structure of a DNA-bound state of condensin in which ATP has not been hydrolyzed. The DNA is clamped within a compartment that has been reported previously in other structural maintenance of chromosomes (SMC) complexes, including Rad50, cohesin, and MukBEF. With the caveat of important differences, it means that all SMC complexes cycle through at least some similar states and undergo similar conformational changes in their head modules, while hydrolyzing ATP and translocating DNA.

Comprehensive Analysis of SMC Gene Family Prognostic Value and Immune Infiltration in Patients With Pancreatic Adenocarcinoma

Pancreatic adenocarcinoma (PAAD) is a malignant tumor with high morbidity and mortality worldwide. Members from the structural maintenance of chromosomes (SMC) gene family function as oncogenes in various tumor types, but their roles in PAAD have not been elucidated. In this study, we aimed to explore the role of the SMC family in tumor progression and cancer immune infiltration in PAAD using integrative bioinformatic analyses. The results showed that the SMC 1A, 2, 3, 4, and 6 were overexpressed in PAAD tissues; of these, SMC 1A, 4, 5, and 6 could be potential prognostic biomarkers for PAAD. The expression of SMC genes was found to be strongly associated with immune cell infiltration. According to the infiltrative status of various immune cells, the mRNA expression of SMC genes in PAAD was associated with the overall and recurrence-free survival of patients. In conclusion, the SMC gene family is associated with PAAD and may be involved in tumorigenesis and cancer-immune interactions; thus, members from this gene family may serve as promising prognostic and therapeutic biomarkers of PAAD.

SMC complexes: Lifting the lid on loop extrusion

Loop extrusion has emerged as a prominent hypothesis for how SMC complexes shape chromosomes - single molecule in vitro observations have yielded fascinating images of this process. When not extruding loops, SMC complexes are known to topologically entrap one or more DNAs. Here, we review how structural insight into the SMC complex cohesin has led to a molecular framework for both activities: a Brownian ratchet motion, associated with topological DNA entry, might repeat itself to elicit loop extrusion. After contrasting alternative loop extrusion models, we explore whether topological loading or loop extrusion is more adept at explaining in vivo SMC complex function. SMC variants that experimentally separate topological loading from loop extrusion will in the future probe their respective contributions to chromosome biology.

Cryo-EM structure of MukBEF reveals DNA loop entrapment at chromosomal unloading sites

The ring-like structural maintenance of chromosomes (SMC) complex MukBEF folds the genome of Escherichia coli and related bacteria into large loops, presumably by active DNA loop extrusion. MukBEF activity within the replication terminus macrodomain is suppressed by the sequence-specific unloader MatP. Here, we present the complete atomic structure of MukBEF in complex with MatP and DNA as determined by electron cryomicroscopy (cryo-EM). The complex binds two distinct DNA double helices corresponding to the arms of a plectonemic loop. MatP-bound DNA threads through the MukBEF ring, while the second DNA is clamped by the kleisin MukF, MukE, and the MukB ATPase heads. Combinatorial cysteine cross-linking confirms this topology of DNA loop entrapment in vivo. Our findings illuminate how a class of near-ubiquitous DNA organizers with important roles in genome maintenance interacts with the bacterial chromosome.

Theory and simulations of condensin mediated loop extrusion in DNA

Condensation of hundreds of mega-base-pair-long human chromosomes in a small nuclear volume is a spectacular biological phenomenon. This process is driven by the formation of chromosome loops. The ATP consuming motor, condensin, interacts with chromatin segments to actively extrude loops. Motivated by real-time imaging of loop extrusion (LE), we created an analytically solvable model, predicting the LE velocity and step size distribution as a function of external load. The theory fits the available experimental data quantitatively, and suggests that condensin must undergo a large conformational change, induced by ATP binding, bringing distant parts of the motor to proximity. Simulations using a simple model confirm that the motor transitions between an open and a closed state in order to extrude loops by a scrunching mechanism, similar to that proposed in DNA bubble formation during bacterial transcription. Changes in the orientation of the motor domains are transmitted over ~50 nm, connecting the motor head and the hinge, thus providing an allosteric basis for LE.

A Brownian ratchet model for DNA loop extrusion by the cohesin complex

The cohesin complex topologically encircles DNA to promote sister chromatid cohesion. Alternatively, cohesin extrudes DNA loops, thought to reflect chromatin domain formation. Here, we propose a structure-based model explaining both activities. ATP and DNA binding promote cohesin conformational changes that guide DNA through a kleisin N-gate into a DNA gripping state. Two HEAT-repeat DNA binding modules, associated with cohesin's heads and hinge, are now juxtaposed. Gripping state disassembly, following ATP hydrolysis, triggers unidirectional hinge module movement, which completes topological DNA entry by directing DNA through the ATPase head gate. If head gate passage fails, hinge module motion creates a Brownian ratchet that, instead, drives loop extrusion. Molecular-mechanical simulations of gripping state formation and resolution cycles recapitulate experimentally observed DNA loop extrusion characteristics. Our model extends to asymmetric and symmetric loop extrusion, as well as z-loop formation. Loop extrusion by biased Brownian motion has important implications for chromosomal cohesin function.

Folding of cohesin's coiled coil is important for Scc2/4-induced association with chromosomes

Cohesin's association with and translocation along chromosomal DNAs depend on an ATP hydrolysis cycle driving the association and subsequent release of DNA. This involves DNA being 'clamped' by Scc2 and ATP-dependent engagement of cohesin's Smc1 and Smc3 head domains. Scc2's replacement by Pds5 abrogates cohesin's ATPase and has an important role in halting DNA loop extrusion. The ATPase domains of all SMC proteins are separated from their hinge dimerisation domains by 50-nm-long coiled coils, which have been observed to zip up along their entire length and fold around an elbow, thereby greatly shortening the distance between hinges and ATPase heads. Whether folding exists in vivo or has any physiological importance is not known. We present here a cryo-EM structure of the apo form of cohesin that reveals the structure of folded and zipped-up coils in unprecedented detail and shows that Scc2 can associate with Smc1's ATPase head even when it is fully disengaged from that of Smc3. Using cysteine-specific crosslinking, we show that cohesin's coiled coils are frequently folded in vivo, including when cohesin holds sister chromatids together. Moreover, we describe a mutation (SMC1D588Y) within Smc1's hinge that alters how Scc2 and Pds5 interact with Smc1's hinge and that enables Scc2 to support loading in the absence of its normal partner Scc4. The mutant phenotype of loading without Scc4 is only explicable if loading depends on an association between Scc2/4 and cohesin's hinge, which in turn requires coiled coil folding.

Current Advances in Covalent Stabilization of Macromolecular Complexes for Structural Biology

Structural characterization of macromolecular assemblies is often limited by the transient nature of the interactions. The development of specific chemical tools to covalently tether interacting proteins to each other has played a major role in various fundamental discoveries in recent years. To this end, protein engineering techniques such as mutagenesis, incorporation of unnatural amino acids, and methods using synthetic substrate/cosubstrate derivatives were employed. In this review, we give an overview of both commonly used and recently developed biochemical methodologies for covalent stabilization of macromolecular complexes enabling structural investigation via crystallography, nuclear magnetic resonance, and cryo-electron microscopy. We divided the strategies into nonenzymatic- and enzymatic-driven cross-linking and further categorized them in either naturally occurring or engineered covalent linkage. This review offers a compilation of recent advances in diverse scientific fields where the structural characterization of macromolecular complexes was achieved by the aid of intermolecular covalent linkage.

Expanding the versatility of natural and de novo designed coiled coils and helical bundles

Natural helical bundles (HBs) constitute a ubiquitous class of protein folds built of two or more longitudinally arranged α-helices. They adopt topologies that include symmetric, highly regular assemblies all the way to asymmetric, loosely packed domains. The diverse functional spectrum of HBs ranges from structural scaffolds to complex and dynamic effectors as molecular motors, signaling and sensing molecules, enzymes, and molecular switches. Symmetric HBs, particularly coiled coils, offer simple model systems providing an ideal entry point for protein folding and design studies. Herein, we review recent progress unveiling new structural features and functional mechanisms in natural HBs and cover staggering advances in the de novo design of HBs, giving rise to exotic structures and the creation of novel functions.

Live-cell single-molecule tracking highlights requirements for stable Smc5/6 chromatin association in vivo

The essential Smc5/6 complex is required in response to replication stress and is best known for ensuring the fidelity of homologous recombination. Using single-molecule tracking in live fission yeast to investigate Smc5/6 chromatin association, we show that Smc5/6 is chromatin associated in unchallenged cells and this depends on the non-SMC protein Nse6. We define a minimum of two Nse6-dependent sub-pathways, one of which requires the BRCT-domain protein Brc1. Using defined mutants in genes encoding the core Smc5/6 complex subunits, we show that the Nse3 double-stranded DNA binding activity and the arginine fingers of the two Smc5/6 ATPase binding sites are critical for chromatin association. Interestingly, disrupting the single-stranded DNA (ssDNA) binding activity at the hinge region does not prevent chromatin association but leads to elevated levels of gross chromosomal rearrangements during replication restart. This is consistent with a downstream function for ssDNA binding in regulating homologous recombination.

Gradual opening of Smc arms in prokaryotic condensin

Multi-subunit SMC ATPases control chromosome superstructure apparently by catalyzing a DNA-loop-extrusion reaction. SMC proteins harbor an ABC-type ATPase "head" and a "hinge" dimerization domain connected by a coiled coil "arm." Two arms in a SMC dimer can co-align, thereby forming a rod-shaped particle. Upon ATP binding, SMC heads engage, and arms are thought to separate. Here, we study the shape of Bacillus subtilis Smc-ScpAB by electron-spin resonance spectroscopy. Arm separation is readily detected proximal to the heads in the absence of ligands, and separation near the hinge largely depends on ATP and DNA. Artificial blockage of arm opening eliminates DNA stimulation of ATP hydrolysis but does not prevent basal ATPase activity. We report an arm contact as being important for controlling the transformations. Point mutations at this arm interface eliminated Smc function. We propose that partially open, intermediary conformations provide directionality to SMC DNA translocation by (un)binding suitable DNA substrates.

Hit the brakes - a new perspective on the loop extrusion mechanism of cohesin and other SMC complexes

The three-dimensional structure of chromatin is determined by the action of protein complexes of the structural maintenance of chromosome (SMC) family. Eukaryotic cells contain three SMC complexes, cohesin, condensin, and a complex of Smc5 and Smc6. Initially, cohesin was linked to sister chromatid cohesion, the process that ensures the fidelity of chromosome segregation in mitosis. In recent years, a second function in the organization of interphase chromatin into topologically associated domains has been determined, and loop extrusion has emerged as the leading mechanism of this process. Interestingly, fundamental mechanistic differences exist between mitotic tethering and loop extrusion. As distinct molecular switches that aim to suppress loop extrusion in different biological contexts have been identified, we hypothesize here that loop extrusion is the default biochemical activity of cohesin and that its suppression shifts cohesin into a tethering mode. With this model, we aim to provide an explanation for how loop extrusion and tethering can coexist in a single cohesin complex and also apply it to the other eukaryotic SMC complexes, describing both similarities and differences between them. Finally, we present model-derived molecular predictions that can be tested experimentally, thus offering a new perspective on the mechanisms by which SMC complexes shape the higher-order structure of chromatin.

Comprehensive classification of ABC ATPases and their functional radiation in nucleoprotein dynamics and biological conflict systems

ABC ATPases form one of the largest clades of P-loop NTPase fold enzymes that catalyze ATP-hydrolysis and utilize its free energy for a staggering range of functions from transport to nucleoprotein dynamics. Using sensitive sequence and structure analysis with comparative genomics, for the first time we provide a comprehensive classification of the ABC ATPase superfamily. ABC ATPases developed structural hallmarks that unambiguously distinguish them from other P-loop NTPases such as an alternative to arginine-finger-based catalysis. At least five and up to eight distinct clades of ABC ATPases are reconstructed as being present in the last universal common ancestor. They underwent distinct phases of structural innovation with the emergence of inserts constituting conserved binding interfaces for proteins or nucleic acids and the adoption of a unique dimeric toroidal configuration for DNA-threading. Specifically, several clades have also extensively radiated in counter-invader conflict systems where they serve as nodal nucleotide-dependent sensory and energetic components regulating a diversity of effectors (including some previously unrecognized) acting independently or together with restriction-modification systems. We present a unified mechanism for ABC ATPase function across disparate systems like RNA editing, translation, metabolism, DNA repair, and biological conflicts, and some unexpected recruitments, such as MutS ATPases in secondary metabolism.

Condensin complexes: understanding loop extrusion one conformational change at a time

Condensin and cohesin, both members of the structural maintenance of chromosome (SMC) family, contribute to the regulation and structure of chromatin. Recent work has shown both condensin and cohesin extrude DNA loops and most likely work via a conserved mechanism. This review focuses on condensin complexes, highlighting recent in vitro work characterising DNA loop formation and protein structure. We discuss similarities between condensin and cohesin complexes to derive a possible mechanistic model, as well as discuss differences that exist between the different condensin isoforms found in higher eukaryotes.

A Structure-Based Mechanism for DNA Entry into the Cohesin Ring

Despite key roles in sister chromatid cohesion and chromosome organization, the mechanism by which cohesin rings are loaded onto DNA is still unknown. Here we combine biochemical approaches and cryoelectron microscopy (cryo-EM) to visualize a cohesin loading intermediate in which DNA is locked between two gates that lead into the cohesin ring. Building on this structural framework, we design experiments to establish the order of events during cohesin loading. In an initial step, DNA traverses an N-terminal kleisin gate that is first opened upon ATP binding and then closed as the cohesin loader locks the DNA against the ATPase gate. ATP hydrolysis will lead to ATPase gate opening to complete DNA entry. Whether DNA loading is successful or results in loop extrusion might be dictated by a conserved kleisin N-terminal tail that guides the DNA through the kleisin gate. Our results establish the molecular basis for cohesin loading onto DNA.

Transport of DNA within cohesin involves clamping on top of engaged heads by Scc2 and entrapment within the ring by Scc3

In addition to extruding DNA loops, cohesin entraps within its SMC-kleisin ring (S-K) individual DNAs during G1 and sister DNAs during S-phase. All three activities require related hook-shaped proteins called Scc2 and Scc3. Using thiol-specific crosslinking we provide rigorous proof of entrapment activity in vitro. Scc2 alone promotes entrapment of DNAs in the E-S and E-K compartments, between ATP-bound engaged heads and the SMC hinge and associated kleisin, respectively. This does not require ATP hydrolysis nor is it accompanied by entrapment within S-K rings, which is a slower process requiring Scc3. Cryo-EM reveals that DNAs transported into E-S/E-K compartments are 'clamped' in a sub-compartment created by Scc2's association with engaged heads whose coiled coils are folded around their elbow. We suggest that clamping may be a recurrent feature of cohesin complexes active in loop extrusion and that this conformation precedes the S-K entrapment required for sister chromatid cohesion.

Combining high throughput and high quality for cryo-electron microscopy data collection

Cryo-electron microscopy (cryo-EM) can be used to elucidate the 3D structure of macromolecular complexes. Driven by technological breakthroughs in electron-microscope and electron-detector development, coupled with improved image-processing procedures, it is now possible to reach high resolution both in single-particle analysis and in cryo-electron tomography and subtomogram-averaging approaches. As a consequence, the way in which cryo-EM data are collected has changed and new challenges have arisen in terms of microscope alignment, aberration correction and imaging parameters. This review describes how high-end data collection is performed at the EMBL Heidelberg cryo-EM platform, presenting recent microscope implementations that allow an increase in throughput while maintaining aberration-free imaging and the optimization of acquisition parameters to collect high-resolution data.

Cryo-EM structure of the human cohesin-NIPBL-DNA complex

As a ring-shaped adenosine triphosphatase (ATPase) machine, cohesin organizes the eukaryotic genome by extruding DNA loops and mediates sister chromatid cohesion by topologically entrapping DNA. How cohesin executes these fundamental DNA transactions is not understood. Using cryo-electron microscopy (cryo-EM), we determined the structure of human cohesin bound to its loader NIPBL and DNA at medium resolution. Cohesin and NIPBL interact extensively and together form a central tunnel to entrap a 72-base pair DNA. NIPBL and DNA promote the engagement of cohesin's ATPase head domains and ATP binding. The hinge domains of cohesin adopt an "open washer" conformation and dock onto the STAG1 subunit. Our structure explains the synergistic activation of cohesin by NIPBL and DNA and provides insight into DNA entrapment by cohesin.