Real-time observation of flexible domain movements in CRISPR-Cas9

Secondary Conformational Checkpoint in CRISPR-Cas9

A specific checkpoint between target DNA binding and cleavage primarily governs the precision of Cas9 gene editing. Although various CRISPR-Cas9 variants have been developed to improve DNA cleavage accuracy, we still lack a comprehensive understanding of how they work at the molecular level. Herein, we have focused on studying the late-stage conformational transitions of Cas9 and an evolved Cas9 mutant (evoCas9) that start from the precleavage state. Our submilliseconds of dynamic simulations reveal that the presence of base mismatches leads the HNH nuclease domain of Cas9 to alter its principal functional modes of motion, thereby impairing its conformational activation. This observation suggests the existence of a secondary conformational checkpoint that fine-tunes the final DNA cleavage activation. Remarkably, evoCas9 is prone to deviating from the normal activation pathway with base mismatches. This is characterized by a noticeable shift in the positioning of the HNH domain and a significantly perturbed allosteric communication network within the enzyme. Therefore, the mutations evolved in evoCas9 also reinforce the secondary checkpoint in addition to the previously identified primary checkpoint, collectively ensuring this variant's high gene-editing accuracy. This mechanism should also apply to other Cas9-guide RNA variants with enhanced fidelity.

Binding to the conserved and stably folded guide RNA pseudoknot induces Cas12a conformational changes during ribonucleoprotein assembly

Ribonucleoproteins (RNPs) comprise one or more RNA and protein molecules that interact to form a stable complex, which commonly involves conformational changes in the more flexible RNA components. Here, we propose that Cas12a RNP assembly with its cognate CRISPR RNA (crRNA) guide instead proceeds primarily through Cas12a conformational changes during binding to more stable, prefolded crRNA 5' pseudoknot handles. Phylogenetic reconstructions and sequence and structure alignments revealed that the Cas12a proteins are divergent in sequence and structure while the crRNA 5' repeat region, which folds into a pseudoknot and anchors binding to Cas12a, is highly conserved. Molecular dynamics simulations of three Cas12a proteins and their cognate guides revealed substantial flexibility for unbound apo-Cas12a. In contrast, crRNA 5' pseudoknots were predicted to be stable and independently folded. Limited trypsin hydrolysis, differential scanning fluorimetry, thermal denaturation, and CD analyses supported conformational changes of Cas12a during RNP assembly and an independently folded crRNA 5' pseudoknot. This RNP assembly mechanism may be rationalized by evolutionary pressure to conserve CRISPR loci repeat sequence, and therefore guide RNA structure, to maintain function across all phases of the CRISPR defense mechanism.

Dynamics of Target DNA Binding and Cleavage by Staphylococcus aureus Cas9 as Revealed by High-Speed Atomic Force Microscopy

Programmable DNA binding and cleavage by CRISPR-Cas9 has revolutionized the life sciences. However, the off-target cleavage observed in DNA sequences with some homology to the target still represents a major limitation for a more widespread use of Cas9 in biology and medicine. For this reason, complete understanding of the dynamics of DNA binding, interrogation and cleavage by Cas9 is crucial to improve the efficiency of genome editing. Here, we use high-speed atomic force microscopy (HS-AFM) to investigate Staphylococcus aureus Cas9 (SaCas9) and its dynamics of DNA binding and cleavage. Upon binding to single-guide RNA (sgRNA), SaCas9 forms a close bilobed structure that transiently and flexibly adopts also an open configuration. The SaCas9-mediated DNA cleavage is characterized by release of cleaved DNA and immediate dissociation, confirming that SaCas9 operates as a multiple turnover endonuclease. According to present knowledge, the process of searching for target DNA is mainly governed by three-dimensional diffusion. Independent HS-AFM experiments show a potential long-range attractive interaction between SaCas9-sgRNA and its target DNA. The interaction precedes the formation of the stable ternary complex and is observed exclusively in the vicinity of the protospacer-adjacent motif (PAM), up to distances of several nanometers. The direct visualization of the process by sequential topographic images suggests that SaCas9-sgRNA binds to the target sequence first, while the following binding of the PAM is accompanied by local DNA bending and formation of the stable complex. Collectively, our HS-AFM data reveal a potential and unexpected behavior of SaCas9 during the search for DNA targets.

Insights into the Mechanism of CRISPR/Cas9-Based Genome Editing from Molecular Dynamics Simulations

The CRISPR/Cas9 system is a popular genome-editing tool with immense therapeutic potential. It is a simple two-component system (Cas9 protein and RNA) that recognizes the DNA sequence on the basis of RNA:DNA complementarity, and the Cas9 protein catalyzes the double-stranded break in the DNA. In the past decade, near-atomic resolution structures at various stages of the CRISPR/Cas9 DNA editing pathway have been reported along with numerous experimental and computational studies. Such studies have boosted knowledge of the genome-editing mechanism. Despite such advancements, the application of CRISPR/Cas9 in therapeutics is still limited, primarily due to off-target effects. Several studies aim at engineering high-fidelity Cas9 to minimize the off-target effects. Molecular Dynamics (MD) simulations have been an excellent complement to the experimental studies for investigating the mechanism of CRISPR/Cas9 editing in terms of structure, thermodynamics, and kinetics. MD-based studies have uncovered several important molecular aspects of Cas9, such as nucleotide binding, catalytic mechanism, and off-target effects. In this Review, the contribution of MD simulation to understand the CRISPR/Cas9 mechanism has been discussed, preceded by an overview of the history, mechanism, and structural aspects of the CRISPR/Cas9 system. These studies are important for the rational design of highly specific Cas9 and will also be extremely promising for achieving more accurate genome editing in the future.

Decrypting the mechanistic basis of CRISPR/Cas9 protein

CRISPR/Cas system, a newly but extensively investigated genome-editing method, harbors practical solutions for various genetic problems. It relies on short guide RNAs (gRNAs) to recruit the Cas9 protein, a DNA cleaving enzyme, to its genomic target DNAs. The Cas9 enzyme exhibits some unique properties, like the ability to differentiate self vs. non-self - DNA strands using the base-pairing potential of crRNA, i.e., only CRISPR DNA is entirely complementary to the CRISPR repeat sequences at the crRNA whereas the presence of mismatches in the upstream region of the spacer permit CRISPR interference which is inhibited in case of CRISPR-DNA, allosteric regulation in its domains, and domain reorientation on sgRNA binding. Several groups have contributed their efforts in understanding the functioning of the CRISPR/Cas system, but even then, there is a lot more to explore in this area. The structural and sequence-based understanding of the whole CRISPR-associated bacterial ortholog family landscape is still ambiguous. A better understanding of the underlying energetics of the CRISPR/Cas9 system should reveal critical parameters to design better CRISPR/Cas9s.

Engineering and Design of Programmable Genome Editors

Programmable genome editors are enzymes that can be targeted to a specific location in the genome for making site-specific alterations or deletions. The engineering, design, and development of sequence-specific editors has resulted in a dramatic increase in the precision of editing for nucleotide sequences. These editors can target specific locations in a genome, in vivo. The genome editors are being deployed for the development of genetically modified organisms for agriculture and industry, and for gene therapy of inherited human genetic disorders, cancer, and immunotherapy. Experimental and computational studies of structure, binding, activity, dynamics, and folding, reviewed here, have provided valuable insights that have the potential for increasing the functional efficiency of these gene/genome editors. Biochemical and biophysical studies of the specificities of natural and engineered genome editors reveal that increased binding affinity can be detrimental because of the increase of off-target effects and that the engineering and design of genome editors with higher specificity may require modulation and control of the conformational dynamics.

CRISPR-Cas9 bends and twists DNA to read its sequence

In bacterial defense and genome editing applications, the CRISPR-associated protein Cas9 searches millions of DNA base pairs to locate a 20-nucleotide, guide RNA-complementary target sequence that abuts a protospacer-adjacent motif (PAM). Target capture requires Cas9 to unwind DNA at candidate sequences using an unknown ATP-independent mechanism. Here we show that Cas9 sharply bends and undertwists DNA on PAM binding, thereby flipping DNA nucleotides out of the duplex and toward the guide RNA for sequence interrogation. Cryogenic-electron microscopy (cryo-EM) structures of Cas9-RNA-DNA complexes trapped at different states of the interrogation pathway, together with solution conformational probing, reveal that global protein rearrangement accompanies formation of an unstacked DNA hinge. Bend-induced base flipping explains how Cas9 'reads' snippets of DNA to locate target sites within a vast excess of nontarget DNA, a process crucial to both bacterial antiviral immunity and genome editing. This mechanism establishes a physical solution to the problem of complementarity-guided DNA search and shows how interrogation speed and local DNA geometry may influence genome editing efficiency.

Probing the Dynamics of Streptococcus pyogenes Cas9 Endonuclease Bound to the sgRNA Complex Using Hydrogen-Deuterium Exchange Mass Spectrometry

The Cas9 endonuclease is an essential component of the CRISPR-Cas-based genome editing tools. The attainment of high specificity and efficiency of Cas9 during targetted DNA cleavage is the main problem that limits the clinical application of the CRISPR-Cas9 system. A deep understanding of the Cas9 mechanism and its structural-functional relationships is required to develop strategies for precise gene editing. Here, we present the first attempt to describe the solution structure of Cas9 from S. pyogenes using hydrogen-deuterium exchange mass spectrometry (HDX-MS) coupled to molecular dynamics simulations. HDX data revealed multiple protein regions with deuterium uptake levels varying from low to high. By analysing the difference in relative deuterium uptake by apoCas9 and its complex with sgRNA, we identified peptides involved in the complex formation and possible changes in the protein conformation. The REC3 domain was shown to undergo the most prominent conformational change upon enzyme-RNA interactions. Detection of the HDX in two forms of the enzyme provided detailed information about changes in the Cas9 structure induced by sgRNA binding and quantified the extent of the changes. The study demonstrates the practical utility of HDX-MS for the elucidation of mechanistic aspects of Cas9 functioning.

CRISPR/Cas9 On- and Off-Target Activity Using Correlative Force and Fluorescence Single-Molecule Microscopy

The discovery of CRISPR/Cas9 as an easily programmable endonuclease heralds a new era of genetic manipulation. With this comes the prospect of novel gene therapy approaches, and the potential to cure previously untreatable genetic diseases. However, reports of spurious off-target editing by CRISPR/Cas9 pose a significant hurdle to realizing this potential. A deeper understanding of the factors that affect Cas9 specificity is vital for development of safe and efficient therapeutics. Here, we describe methods for the use of optical tweezers combined with confocal fluorescence microscopy and microfluidics for the analysis of on- and off-target activity of Cas9 activity.

Heterogeneous dissociation process of truncated RNAs by oligomerized Vasa helicase

RNA helicases are enzymes that generally unwind double-stranded RNA using ATP hydrolysis energy, mainly involved in RNA metabolism, transcription, translation, and mRNA splicing. While the helicase core is crucial for RNA unwinding activity, N- and C-terminal extensions of specific helicases may contain an intrinsically disordered region for electrostatic interaction, resulting in the formation of droplets in the cytoplasm. However, how the disordered region of the RNA helicase contributes to RNA unwinding and dissociation remains unclear. Here, we focused on Bombyx mori Vasa, which unwinds truncated target transposon RNAs from the piRNA-induced silencing complex piRISC. In this study, we used single-molecule techniques to visualise how Vasa dynamically interacts with piRISC and investigate how Vasa oligomerization is involved in the process of piRNA amplification, named the ping-pong pathway. We found that Vasa’s oligomerization is required during these processes in vitro and in vivo, and that Vasa triggers the dissociation of truncated RNA in heterogeneous pathways. Our single-molecule results suggest that oligomerized Vasa guides the timing of the process regulating overall dissociation efficiency.

Kinoshita et al. use single molecule imaging to show that the N-terminal extension of BmVasa, a germ-specific DEAD box RNA helicase, is necessary for protein oligomerization and the dissociation of target RNA from the Siwi-piRISC complex. The authors conclude that oligomerized Vasa guides the timing of the regulation of overall dissociation efficiency.

Probing the stability of the SpCas9-DNA complex after cleavage

CRISPR-Cas9 is a ribonucleoprotein complex that sequence-specifically binds and cleaves double-stranded DNA. Wildtype Cas9 and its nickase and cleavage-incompetent mutants have been used in various biological techniques due to their versatility and programmable specificity. Cas9 has been shown to bind very stably to DNA even after cleavage of the individual DNA strands, inhibiting further turnovers and considerably slowing down in-vivo repair processes. This poses an obstacle in genome editing applications. Here, we employed single-molecule magnetic tweezers to investigate the binding stability of different Streptococcus pyogenes Cas9 variants after cleavage by challenging them with supercoiling. We find that different release mechanisms occur depending on which DNA strand is cleaved. After initial target strand cleavage, supercoils are only removed after the collapse of the R-loop. We identified several states with different stabilities of the R-loop. Most importantly, we find that the post-cleavage state of Cas9 exhibits a higher stability than the pre-cleavage state. After non-target strand cleavage, supercoils are immediately but slowly released by swiveling of the non-target strand around Cas9 bound to the target strand. Consequently, Cas9 and its non-target strand nicking mutant stay stably bound to the DNA for many hours even at elevated torsional stress.

Efficient DNA interrogation of SpCas9 governed by its electrostatic interaction with DNA beyond the PAM and protospacer

Streptococcus pyogenes Cas9 (SpCas9), a programmable RNA-guided DNA endonuclease, has been widely repurposed for biological and medical applications. Critical interactions between SpCas9 and DNA confer the high specificity of the enzyme in genome engineering. Here, we unveil that an essential SpCas9–DNA interaction located beyond the protospacer adjacent motif (PAM) is realized through electrostatic forces between four positively charged lysines among SpCas9 residues 1151–1156 and the negatively charged DNA backbone. Modulating this interaction by substituting lysines with amino acids that have distinct charges revealed a strong dependence of DNA target binding and cleavage activities of SpCas9 on the charge. Moreover, the SpCas9 mutants show markedly distinguishable DNA interaction sites beyond the PAM compared with wild-type SpCas9. Functionally, this interaction governs DNA sampling and participates in protospacer DNA unwinding during DNA interrogation. Overall, a mechanistic and functional understanding of this vital interaction explains how SpCas9 carries out efficient DNA interrogation.

Engineering of the genome editing protein Cas9 to slide along DNA

The genome editing protein Cas9 faces engineering challenges in improving off-target DNA cleavage and low editing efficiency. In this study, we aimed to engineer Cas9 to be able to slide along DNA, which might facilitate genome editing and reduce off-target cleavage. We used two approaches to achieve this: reducing the sliding friction along DNA by removing the interactions of Cas9 residues with DNA and facilitating sliding by introducing the sliding-promoting tail of Nhp6A. Seven engineered mutants of Cas9 were prepared, and their performance was tested using single-molecule fluorescence microscopy. Comparison of the mutations enabled the identification of key residues of Cas9 to enhance the sliding along DNA in the presence and absence of single guide RNA (sgRNA). The attachment of the tail to Cas9 mutants enhanced sliding along DNA, particularly in the presence of sgRNA. Together, using the proposed approaches, the sliding ability of Cas9 was improved up to eightfold in the presence of sgRNA. A sliding model of Cas9 and its engineering action are discussed herein.

Complex dependence of CRISPR-Cas9 binding strength on guide RNA spacer lengths

It is established that for CRISPR-Cas9 applications guide RNAs with 17-20 bp long spacer sequences are optimal for accurate target binding and cleavage. In this work we perform cell-free CRISPRa (CRISPR activation) and CRISPRi (CRISPR inhibition) experiments to demonstrate the existence of a complex dependence of CRISPR-Cas9 binding as a function of the spacer length and complementarity. Our results show that significantly truncated or mismatched spacer sequences can form stronger guide-target bonds than the conventional 17-20 bp long spacers. To explain this phenomenon, we take into consideration previous structural and single-molecule CRISPR-Cas9 experiments and develop a novel thermodynamic model of CRISPR-Cas9 target recognition.

Real-time observation of Cas9 postcatalytic domain motions

CRISPR-Cas9 from Streptococcus pyogenes is an RNA-guided DNA endonuclease, which has become the most popular genome editing tool. Coordinated domain motions of Cas9 prior to DNA cleavage have been extensively characterized but our understanding of Cas9 conformations postcatalysis is limited. Because Cas9 can remain stably bound to the cleaved DNA for hours, its postcatalytic conformation may influence genome editing mechanisms. Here, we use single-molecule fluorescence resonance energy transfer to characterize the HNH domain motions of Cas9 that are coupled with cleavage activity of the target strand (TS) or nontarget strand (NTS) of DNA substrate. We reveal an NTS-cleavage-competent conformation following the HNH domain conformational activation. The 3' flap generated by NTS cleavage can be rapidly digested by a 3' to 5' single-stranded DNA-specific exonuclease, indicating Cas9 exposes the 3' flap for potential interaction with the DNA repair machinery. We find evidence that the HNH domain is highly flexible post-TS cleavage, explaining a recent observation that the HNH domain was not visible in a postcatalytic cryo-EM structure. Our results illuminate previously unappreciated regulatory roles of DNA cleavage activity on Cas9's conformation and suggest possible biotechnological applications.

DeepFRET, a software for rapid and automated single-molecule FRET data classification using deep learning

Single-molecule Förster Resonance energy transfer (smFRET) is an adaptable method for studying the structure and dynamics of biomolecules. The development of high throughput methodologies and the growth of commercial instrumentation have outpaced the development of rapid, standardized, and automated methodologies to objectively analyze the wealth of produced data. Here we present DeepFRET, an automated, open-source standalone solution based on deep learning, where the only crucial human intervention in transiting from raw microscope images to histograms of biomolecule behavior, is a user-adjustable quality threshold. Integrating standard features of smFRET analysis, DeepFRET consequently outputs the common kinetic information metrics. Its classification accuracy on ground truth data reached >95% outperforming human operators and commonly used threshold, only requiring ~1% of the time. Its precise and rapid operation on real data demonstrates DeepFRET's capacity to objectively quantify biomolecular dynamics and the potential to contribute to benchmarking smFRET for dynamic structural biology.

Allosteric regulation of CRISPR-Cas9 for DNA-targeting and cleavage

The CRISPR-Cas9 system from Streptococcus pyogenes has been exploited as a programmable RNA-guided DNA-targeting and DNA-editing platform. This evolutionary tool enables diverse genetic manipulations with unprecedented precision and ease. Cas9 is an allosteric enzyme, which is allosterically regulated in conformational activation, target recognition, and DNA cleavage. Here, we outline the underlying allosteric control over the Cas9 complex assembly and targeting specificity. We further review the strategies for mitigating intrinsic Cas9 off-target effects through allosteric modulations and the advances in engineering controllable Cas9 systems that are responsive to external allosteric signals. Future development of highly specific, tunable CRISPR-Cas9 systems through allosteric modulations would greatly benefit applications that require both conditional control and high precision.

Protein-Mutation-Induced Conformational Changes of the DNA and Nuclease Domain in CRISPR/Cas9 Systems by Molecular Dynamics Simulations

Class 2 CRISPR (clustered regularly interspaced short palindromic repeats) systems offer a unique protocol for genome editing in eukaryotic cells. The nuclease activity of Cas9 has been harnessed to perform precise genome editing by creating double-strand breaks. However, the nuclease activity of Cas9 can be triggered when there is imperfect complementarity between the RNA guide sequence and an off-target genomic site, which is a major limitation of the CRISPR technique for practical applications. Hence, understanding the binding mechanisms in CRISPR/Cas9 for predicting ways to increase cleavage specificity is a timely research target. One way to understand and tune the binding strength is to study wild-type and mutant Cas9, in complex with a guide RNA and a target DNA. We have performed classical all-atom MD simulations over a cumulative time scale of 13.5 μs of CRISPR/Cas9 ternary complexes with the wild-type Cas9 from Streptococcus pyogenes and three of its mutants: K855A, H982A, and the combination K855A+H982A, selected from the outcome of experimental work. Our results reveal significant structural impact of the mutations, with implications for specificity. We find that the "unwound" part of the nontarget DNA strand exhibits enhanced flexibility in complexes with Cas9 mutants and tries to move away from the HNH/RuvC interface, where it is otherwise stabilized by electrostatic couplings in the wild-type complex. Our findings refine an electrostatic model by which cleavage specificity can be optimized through protein mutations.

Allosteric Motions of the CRISPR-Cas9 HNH Nuclease Probed by NMR and Molecular Dynamics

CRISPR-Cas9 is a widely employed genome-editing tool with functionality reliant on the ability of the Cas9 endonuclease to introduce site-specific breaks in double-stranded DNA. In this system, an intriguing allosteric communication has been suggested to control its DNA cleavage activity through flexibility of the catalytic HNH domain. Here, solution NMR experiments and a novel Gaussian-accelerated molecular dynamics (GaMD) simulation method are used to capture the structural and dynamic determinants of allosteric signaling within the HNH domain. We reveal the existence of a millisecond time scale dynamic pathway that spans HNH from the region interfacing the adjacent RuvC nuclease and propagates up to the DNA recognition lobe in full-length CRISPR-Cas9. These findings reveal a potential route of signal transduction within the CRISPR-Cas9 HNH nuclease, advancing our understanding of the allosteric pathway of activation. Further, considering the role of allosteric signaling in the specificity of CRISPR-Cas9, this work poses the mechanistic basis for novel engineering efforts aimed at improving its genome-editing capability.

Coevolutionary Couplings Unravel PAM-Proximal Constraints of CRISPR-SpCas9

The clustered regularly interspaced short palindromic repeats (CRISPR) system, an immune system analog found in prokaryotes, allows a single-guide RNA to direct a CRISPR-associated protein (Cas) with combined helicase and nuclease activity to DNA. The presence of a specific protospacer adjacent motif (PAM) next to the DNA target site plays a crucial role in determining both efficacy and specificity of gene editing. Herein, we introduce a coevolutionary framework to computationally unveil nonobvious molecular interactions in CRISPR systems and experimentally probe their functional role. Specifically, we use direct coupling analysis, a statistical inference framework used to infer direct coevolutionary couplings, in the context of protein/nucleic acid interactions. Applied to Streptococcus pyogenes Cas9, a Hamiltonian metric obtained from coevolutionary relationships reveals, to our knowledge, novel PAM-proximal nucleotide preferences at the seventh position of S. pyogenes Cas9 PAM (5'-NGRNNNT-3'), which was experimentally confirmed by in vitro and functional assays in human cells. We show that coevolved and conserved interactions point to specific clues toward rationally engineering new generations of Cas9 systems and may eventually help decipher the diversity of this family of proteins.

The post-PAM interaction of RNA-guided spCas9 with DNA dictates its target binding and dissociation

Cas9 is an RNA-guided endonuclease that targets complementary DNA for cleavage and has been repurposed for many biological usages. Cas9 activities are governed by its direct interactions with DNA. However, information about this interplay and the mechanism involved in its direction of Cas9 activity remain obscure. Using a single-molecule approach, we probed Cas9/sgRNA/DNA interactions along the DNA sequence and found two stable interactions flanking the protospacer adjacent motif (PAM). Unexpectedly, one of them is located approximately 14 base pairs downstream of the PAM (post-PAM interaction), which is beyond the apparent footprint of Cas9 on DNA. Loss or occupation of this interaction site on DNA impairs Cas9 binding and cleavage. Consistently, a downstream helicase could readily displace DNA-bound Cas9 by disrupting this relatively weak post-PAM interaction. Our work identifies a critical interaction of Cas9 with DNA that dictates its binding and dissociation, which may suggest distinct strategies to modulate Cas9 activity.

Francisella novicida Cas9 interrogates genomic DNA with very high specificity and can be used for mammalian genome editing

Genome editing using the CRISPR/Cas9 system has been used to make precise heritable changes in the DNA of organisms. Although the widely used Streptococcus pyogenes Cas9 (SpCas9) and its engineered variants have been efficiently harnessed for numerous gene-editing applications across different platforms, concerns remain regarding their putative off-targeting at multiple loci across the genome. Here we report that Francisella novicida Cas9 (FnCas9) shows a very high specificity of binding to its intended targets and negligible binding to off-target loci. The specificity is determined by its minimal binding affinity with DNA when mismatches to the target single-guide RNA (sgRNA) are present in the sgRNA:DNA heteroduplex. FnCas9 produces staggered cleavage, higher homology-directed repair rates, and very low nonspecific genome editing compared to SpCas9. We demonstrate FnCas9-mediated correction of the sickle cell mutation in patient-derived induced pluripotent stem cells and propose that it can be used for precise therapeutic genome editing for a wide variety of genetic disorders.

Structural and functional insights into the bona fide catalytic state of Streptococcus pyogenes Cas9 HNH nuclease domain

The CRISPR-associated endonuclease Cas9 from Streptococcus pyogenes (SpyCas9), along with a programmable single-guide RNA (sgRNA), has been exploited as a significant genome-editing tool. Despite the recent advances in determining the SpyCas9 structures and DNA cleavage mechanism, the cleavage-competent conformation of the catalytic HNH nuclease domain of SpyCas9 remains largely elusive and debatable. By integrating computational and experimental approaches, we unveiled and validated the activated Cas9-sgRNA-DNA ternary complex in which the HNH domain is neatly poised for cleaving the target DNA strand. In this catalysis model, the HNH employs the catalytic triad of D839-H840-N863 for cleavage catalysis, rather than previously implicated D839-H840-D861, D837-D839-H840, or D839-H840-D861-N863. Our study contributes critical information to defining the catalytic conformation of the HNH domain and advances the knowledge about the conformational activation underlying Cas9-mediated DNA cleavage.

CRISPR RNA-guided autonomous delivery of Cas9

Cas9 is an endonuclease that can be programed to autonomously deliver diverse effectors to specified genetic addresses. High-resolution structures of this protein and its associated CRISPR RNA guide explain the molecular mechanisms of CRISPR-RNA-guided DNA recognition and provide a molecular blueprint that has facilitated structure-guided functional remodeling. Here we retrace events that led from early efforts to understand the central role of Cas9 in CRISPR-mediated adaptive immunity to contemporary efforts aimed at developing and deploying this enzyme for programmable genetic editing.

Conformational Activation Promotes CRISPR-Cas12a Catalysis and Resetting of the Endonuclease Activity

Cas12a, also known as Cpf1, is a type V-A CRISPR-Cas RNA-guided endonuclease that is used for genome editing based on its ability to generate specific dsDNA breaks. Here, we show cryo-EM structures of intermediates of the cleavage reaction, thus visualizing three protein regions that sense the crRNA-DNA hybrid assembly triggering the catalytic activation of Cas12a. Single-molecule FRET provides the thermodynamics and kinetics of the conformational activation leading to phosphodiester bond hydrolysis. These findings illustrate why Cas12a cuts its target DNA and unleashes unspecific cleavage activity, degrading ssDNA molecules after activation. In addition, we show that other crRNAs are able to displace the R-loop inside the protein after target DNA cleavage, terminating indiscriminate ssDNA degradation. We propose a model whereby the conformational activation of the enzyme results in indiscriminate ssDNA cleavage. The displacement of the R-loop by a new crRNA molecule will reset Cas12a specificity, targeting new DNAs.

Dynamics of Cas10 Govern Discrimination between Self and Non-self in Type III CRISPR-Cas Immunity

Adaptive immune systems must accurately distinguish between self and non-self in order to defend against invading pathogens while avoiding autoimmunity. Type III CRISPR-Cas systems employ guide RNA to recognize complementary RNA targets, which triggers the degradation of both the invader's transcripts and their template DNA. These systems can broadly eliminate foreign targets with multiple mutations but circumvent damage to the host genome. To explore the molecular basis for these features, we use single-molecule fluorescence microscopy to study the interaction between a type III-A ribonucleoprotein complex and various RNA substrates. We find that Cas10-the DNase effector of the complex-displays rapid conformational fluctuations on foreign RNA targets, but is locked in a static configuration on self RNA. Target mutations differentially modulate Cas10 dynamics and tune the CRISPR interference activity in vivo. These findings highlight the central role of the internal dynamics of CRISPR-Cas complexes in self versus non-self discrimination and target specificity.

Key role of the REC lobe during CRISPR-Cas9 activation by 'sensing', 'regulating', and 'locking' the catalytic HNH domain

Understanding the conformational dynamics of CRISPR (clustered regularly interspaced short palindromic repeat)-Cas9 is of the utmost importance for improving its genome editing capability. Here, molecular dynamics simulations performed using Anton-2 - a specialized supercomputer capturing micro-to-millisecond biophysical events in real time and at atomic-level resolution - reveal the activation process of the endonuclease Cas9 toward DNA cleavage. Over the unbiased simulation, we observe that the spontaneous approach of the catalytic domain HNH to the DNA cleavage site is accompanied by a remarkable structural remodeling of the recognition (REC) lobe, which exerts a key role for DNA cleavage. Specifically, the significant conformational changes and the collective conformational dynamics of the REC lobe indicate a mechanism by which the REC1-3 regions 'sense' nucleic acids, 'regulate' the HNH conformational transition, and ultimately 'lock' the HNH domain at the cleavage site, contributing to its catalytic competence. By integrating additional independent simulations and existing experimental data, we provide a solid validation of the activated HNH conformation, which had been so far poorly characterized, and we deliver a comprehensive understanding of the role of REC1-3 in the activation process. Considering the importance of the REC lobe in the specificity of Cas9, this study poses the basis for fully understanding how the REC components control the cleavage of off-target sequences, laying the foundation for future engineering efforts toward improved genome editing.

Structure and Dynamics of Cas9 HNH Domain Catalytic State

The bacterial CRISPR-Cas9 immune system has been harnessed as a powerful and versatile genome-editing tool and holds immense promise for future therapeutic applications. Despite recent advances in understanding Cas9 structures and its functional mechanism, little is known about the catalytic state of the Cas9 HNH nuclease domain, and identifying how the divalent metal ions affect the HNH domain conformational transition remains elusive. A deeper understanding of Cas9 activation and its cleavage mechanism can enable further optimization of Cas9-based genome-editing specificity and efficiency. Using two distinct molecular dynamics simulation techniques, we have obtained a cross-validated catalytically active state of Cas9 HNH domain primed for cutting the target DNA strand. Moreover, herein we demonstrate the essential roles of the catalytic Mg²⁺ for the active state formation and stability. Importantly, we suggest that the derived catalytic conformation of the HNH domain can be exploited for rational engineering of Cas9 variants with enhanced specificity.