Structures of a CRISPR-Cas9 R-loop complex primed for DNA cleavage.
ABSTRACT: Bacterial adaptive immunity and genome engineering involving the CRISPR (clustered regularly interspaced short palindromic repeats)-associated (Cas) protein Cas9 begin with RNA-guided DNA unwinding to form an RNA-DNA hybrid and a displaced DNA strand inside the protein. The role of this R-loop structure in positioning each DNA strand for cleavage by the two Cas9 nuclease domains is unknown. We determine molecular structures of the catalytically active Streptococcus pyogenes Cas9 R-loop that show the displaced DNA strand located near the RuvC nuclease domain active site. These protein-DNA interactions, in turn, position the HNH nuclease domain adjacent to the target DNA strand cleavage site in a conformation essential for concerted DNA cutting. Cas9 bends the DNA helix by 30°, providing the structural distortion needed for R-loop formation.
Project description:CRISPR-Cas9 technology has been widely used for genome engineering. Its RNA-guided endonuclease Cas9 binds specifically to target DNA and then cleaves the two DNA strands with HNH and RuvC nuclease domains. However, structural information regarding the DNA cleavage-activating state of two nuclease domains remains sparse. Here, we report a 5.2?Å cryo-EM structure of Cas9 in complex with sgRNA and target DNA. This structure reveals a conformational state of Cas9 in which the HNH domain is closest to the DNA cleavage site. Compared with two known HNH states, our structure shows that the HNH active site moves toward the cleavage site by about 25 and 13?Å, respectively. In combination with EM-based molecular dynamics simulations, we show that residues of the nuclease domains in our structure could form cleavage-compatible conformations with the target DNA. Together, these results strongly suggest that our cryo-EM structure resembles a DNA cleavage-activating architecture of Cas9.
Project description:High-resolution Cas9 structures have yet to reveal catalytic conformations due to HNH nuclease domain positioning away from the cleavage site. Nme1Cas9 and Nme2Cas9 are compact nucleases for in vivo genome editing. Here, we report structures of meningococcal Cas9 homologs in complex with sgRNA, dsDNA, or the AcrIIC3 anti-CRISPR protein. DNA-bound structures represent an early step of target recognition, a later HNH pre-catalytic state, the HNH catalytic state, and a cleaved-target-DNA-bound state. In the HNH catalytic state of Nme1Cas9, the active site is seen poised at the scissile phosphodiester linkage of the target strand, providing a high-resolution view of the active conformation. The HNH active conformation activates the RuvC domain. Our structures explain how Nme1Cas9 and Nme2Cas9 read distinct PAM sequences and how AcrIIC3 inhibits Nme1Cas9 activity. These structures provide insights into Cas9 domain rearrangements, guide-target engagement, cleavage mechanism, and anti-CRISPR inhibition, facilitating the optimization of these genome-editing platforms.
Project description:Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) systems provide bacteria and archaea with adaptive immunity against viruses and plasmids by using CRISPR RNAs (crRNAs) to guide the silencing of invading nucleic acids. We show here that in a subset of these systems, the mature crRNA that is base-paired to trans-activating crRNA (tracrRNA) forms a two-RNA structure that directs the CRISPR-associated protein Cas9 to introduce double-stranded (ds) breaks in target DNA. At sites complementary to the crRNA-guide sequence, the Cas9 HNH nuclease domain cleaves the complementary strand, whereas the Cas9 RuvC-like domain cleaves the noncomplementary strand. The dual-tracrRNA:crRNA, when engineered as a single RNA chimera, also directs sequence-specific Cas9 dsDNA cleavage. Our study reveals a family of endonucleases that use dual-RNAs for site-specific DNA cleavage and highlights the potential to exploit the system for RNA-programmable genome editing.
Project description:CRISPR-Cas9 is a powerful tool for target genome editing in living cells. Significant advances have been made to understand how this system cleaves target DNA. HNH is a nuclease domain, which shares structural similarity with the HNH endonuclease characterzied by a beta-beta-alpha-metal fold. Therefore, based on one- and two-metal-ion mechanisms, homology modeling and molecular dynamics (MD) simulation are suitable tools for building an atomic model of Cas9 in the DNA cleavage state. Here, by modeling and MD, we presented an atomic model of SpCas9-sgRNA-DNA complex with the cleavage state. This model shows that the HNH and RuvC conformations resemble their DNA cleavage state where the active-sites in the complex coordinate with DNA, Mg<sup>2+</sup> ions, and water. Among them, residues D10, E762, H983, and D986 locate at the first shell of the RuvC active-site and interact with the ions directly, residues H982 or/and H985 are general (Lewis) bases, and the coordinated water is located at the positions for nucleophilic attack of the scissile phosphate. Meanwhile, this catalytic model led us to engineer a new SpCas9 variant (SpCas9-H982A + H983D) with reduced off-target effects. Thus, our study provided new mechanistic insights into the CRISPR-Cas9 system in the DNA cleavage state and offered useful guidance for engineering new CRISPR-Cas9 editing systems with improved specificity.
Project description:The use of paired Cas9 nickases instead of Cas9 nuclease drastically reduces off-target effects. Because both nickases must function for a nickase pair to make a double-strand break, the efficiency of paired nickases can intuitively be expected to be lower than that of either corresponding nuclease alone. Here, we carefully compared the gene-disrupting efficiency of Cas9 paired nickases with that of nucleases. Interestingly, the T7E1 assay and deep sequencing showed that on-target efficiency of paired D10A Cas9 nickases was frequently comparable, but sometimes higher than that of either corresponding nucleases in mammalian cells. As the underlying mechanism, we found that the HNH domain, which is preserved in the D10A Cas9 nickase, has higher activity than the RuvC domain in mammalian cells. In this study, we showed: (i) the in vivo cleavage efficiency of the HNH domain of Cas9 in mammalian cells is higher than that of the RuvC domain, (ii) paired Cas9 nickases are sometimes more efficient than individual nucleases for gene disruption. We envision that our findings which were overlooked in previous reports will serve as a new potential guideline for tool selection for CRISPR-Cas9-mediated gene disruption, facilitating efficient and precise genome editing.
Project description: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.
Project description:The RNA-guided Cas9 endonuclease from Streptococcus pyogenes is a single-turnover enzyme that displays a stable product state after double-stranded-DNA cleavage. Here, we present cryo-EM structures of precatalytic, postcatalytic and product states of the active Cas9-sgRNA-DNA complex in the presence of Mg<sup>2+</sup>. In the precatalytic state, Cas9 adopts the 'checkpoint' conformation with the HNH nuclease domain positioned far away from the DNA. Transition to the postcatalytic state involves a dramatic ~34-Å swing of the HNH domain and disorder of the REC2 recognition domain. The postcatalytic state captures the cleaved substrate bound to the catalytically competent HNH active site. In the product state, the HNH domain is disordered, REC2 returns to the precatalytic conformation, and additional interactions of REC3 and RuvC with nucleic acids are formed. The coupled domain motions and interactions between the enzyme and the RNA-DNA hybrid provide new insights into the mechanism of genome editing by Cas9.
Project description:The CRISPR-associated endonuclease Cas9 can be targeted to specific genomic loci by single guide RNAs (sgRNAs). Here, we report the crystal structure of Streptococcus pyogenes Cas9 in complex with sgRNA and its target DNA at 2.5 Å resolution. The structure revealed a bilobed architecture composed of target recognition and nuclease lobes, accommodating the sgRNA:DNA heteroduplex in a positively charged groove at their interface. Whereas the recognition lobe is essential for binding sgRNA and DNA, the nuclease lobe contains the HNH and RuvC nuclease domains, which are properly positioned for cleavage of the complementary and noncomplementary strands of the target DNA, respectively. The nuclease lobe also contains a carboxyl-terminal domain responsible for the interaction with the protospacer adjacent motif (PAM). This high-resolution structure and accompanying functional analyses have revealed the molecular mechanism of RNA-guided DNA targeting by Cas9, thus paving the way for the rational design of new, versatile genome-editing technologies.
Project description:CRISPR-Cas9 is a genome editing technology with major impact in life sciences. In this system, the endonuclease Cas9 generates double strand breaks in DNA upon RNA-guided recognition of a complementary DNA sequence, which strictly requires the presence of a protospacer adjacent motif (PAM) next to the target site. Although PAM recognition is essential for cleavage, it is unknown whether and how PAM binding activates Cas9 for DNA cleavage at spatially distant sites. Here, we find evidence of a PAM-induced allosteric mechanism revealed by microsecond molecular dynamics simulations. PAM acts as an allosteric effector and triggers the interdependent conformational dynamics of the Cas9 catalytic domains (HNH and RuvC), responsible for concerted cleavage of the two DNA strands. Targeting such an allosteric mechanism should enable control of CRISPR-Cas9 functionality.
Project description:The Cas9 endonuclease is widely used for genome engineering applications by programming its single-guide RNA, and ongoing work is aimed at improving the accuracy and efficiency of DNA targeting. DNA cleavage of Cas9 is controlled by the conformational state of the HNH nuclease domain, but the mechanism that governs HNH activation at on-target DNA while reducing cleavage activity at off-target sites remains poorly understood. Using single-molecule Förster resonance energy transfer, we identified an intermediate state of Streptococcus pyogenes Cas9, representing a conformational checkpoint between DNA binding and cleavage. Upon DNA binding, the HNH domain transitions between multiple conformations before docking into its active state. HNH docking requires divalent cations, but not strand scission, and this docked conformation persists following DNA cleavage. Sequence mismatches between the DNA target and guide RNA prevent transitions from the checkpoint intermediate to the active conformation, providing selective avoidance of DNA cleavage at stably bound off-target sites.