Structural insight into multistage inhibition of CRISPR-Cas12a by AcrVA4

Engineering stimuli-responsive CRISPR-Cas systems for versatile biosensing

The precise target recognition and nuclease-mediated effective signal amplification capacities of CRISPR-Cas systems have attracted considerable research interest within the biosensing field. Guided by insights into their structural and biochemical mechanisms, researchers have endeavored to engineer the key biocomponents of CRISPR-Cas systems with stimulus-responsive functionalities. By the incorporation of protein/nucleic acid engineering techniques, a variety of conditional CRISPR-Cas systems whose activities depend on the presence of target triggers have been established for the efficient detection of diverse types of non-nucleic acid analytes. In this review, we summarized recent research progress in engineering Cas proteins, guide RNA, and substrate nucleic acids to possess target analyte-responsive abilities for diverse biosensing applications. Furthermore, we also discussed the challenges and future possibilities of the stimulus-responsive CRISPR-Cas systems in versatile biosensing.

Versatile plant genome engineering using anti-CRISPR-Cas12a systems

CRISPR-Cas12a genome engineering systems have been widely used in plant research and crop breeding. To date, the performance and use of anti-CRISPR-Cas12a systems have not been fully established in plants. Here, we conduct in silico analysis to identify putative anti-CRISPR systems for Cas12a. These putative anti-CRISPR proteins, along with known anti-CRISPR proteins, are assessed for their ability to inhibit Cas12a cleavage activity in vivo and in planta. Among all anti-CRISPR proteins tested, AcrVA1 shows robust inhibition of Mb2Cas12a and LbCas12a in E. coli. Further tests show that AcrVA1 inhibits LbCas12a mediated genome editing in rice protoplasts and stable transgenic lines. Impressively, co-expression of AcrVA1 mitigates off-target effects by CRISPR-LbCas12a, as revealed by whole genome sequencing. In addition, transgenic plants expressing AcrVA1 exhibit different levels of inhibition to LbCas12a mediated genome editing, representing a novel way of fine-tuning genome editing efficiency. By controlling temporal and spatial expression of AcrVA1, we show that inducible and tissue specific genome editing can be achieved in plants. Furthermore, we demonstrate that AcrVA1 also inhibits LbCas12a-based CRISPR activation (CRISPRa) and based on this principle we build logic gates to turn on and off target genes in plant cells. Together, we have established an efficient anti-CRISPR-Cas12a system in plants and demonstrate its versatile applications in mitigating off-target effects, fine-tuning genome editing efficiency, achieving spatial-temporal control of genome editing, and generating synthetic logic gates for controlling target gene expression in plant cells.

Current Updates of CRISPR/Cas System and Anti-CRISPR Proteins: Innovative Applications to Improve the Genome Editing Strategies

The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated sequence (CRISPR/Cas) system is a cutting-edge genome-editing tool employed to explore the functions of normal and disease-related genes. The CRISPR/Cas system has a remarkable diversity in the composition and architecture of genomic loci and Cas protein sequences. Owing to its excellent efficiency and specificity, this system adds an outstanding dimension to biomedical research on genetic manipulation of eukaryotic cells. However, safe, efficient, and specific delivery of this system to target cells and tissues and their off-target effects are considered critical bottlenecks for the therapeutic applications. Recently discovered anti-CRISPR proteins (Acr) play a significant role in limiting the effects of this system. Acrs are relatively small proteins that are highly specific to CRISPR variants and exhibit remarkable structural diversity. The in silico approaches, crystallography, and cryo-electron microscopy play significant roles in elucidating the mechanisms of action of Acrs. Acrs block the CRISPR/Cas system mainly by employing four mechanisms: CRISPR/Cas complex assembly interruption, target-binding interference, target cleavage prevention, and degradation of cyclic oligonucleotide signaling molecules. Engineered CRISPR/Cas systems are frequently used in gene therapy, diagnostics, and functional genomics. Understanding the molecular mechanisms underlying Acr action may help in the safe and effective use of CRISPR/Cas tools for genetic modification, particularly in the context of medicine. Thus, attempts to regulate prokaryotic CRISPR/Cas surveillance complexes will advance the development of antimicrobial drugs and treatment of human diseases. In this review, recent updates on CRISPR/Cas systems, especially CRISPR/Cas9 and Acrs, and their novel mechanistic insights are elaborated. In addition, the role of Acrs in the novel applications of CRISPP/Cas biotechnology for precise genome editing and other applications is discussed.

A novel method for synthesizing authentic SARS-CoV-2 main protease

The SARS-CoV-2 main protease (Mpro) plays a crucial role in virus amplification and is an ideal target for antiviral drugs. Currently, authentic Mpro is prepared through two rounds of proteolytic cleavage. In this method, Mpro carries a self-cleavage site at the N-terminus and a protease cleavage site followed by an affinity tag at the C-terminus. This article proposes a novel method for producing authentic Mpro through single digestion. Mpro was constructed by fusing a His tag containing TEV protease cleavage sites at the N-terminus. The expressed recombinant protein was digested by TEV protease, and the generated protein had a decreased molecular weight and significantly increased activity, which was consistent with that of authentic Mpro generated by the previous method. These findings indicated that authentic Mpro was successfully obtained. Moreover, the substrate specificity of Mpro was investigated. Mpro had a strong preference for Phe at position the P2, which suggested that the S2 subsite was an outstanding target for designing inhibitors. This article also provides a reference for the preparation of Mpro for sudden coronavirus infection in the future.

Inhibition mechanisms of CRISPR-Cas9 by AcrIIA25.1 and AcrIIA32

In the ongoing arms race between bacteria and bacteriophages, bacteriophages have evolved anti-CRISPR proteins to counteract bacterial CRISPR-Cas systems. Recently, AcrIIA25.1 and AcrIIA32 have been found to effectively inhibit the activity of SpyCas9 both in bacterial and human cells. However, their molecular mechanisms remain elusive. Here, we report the cryo-electron microscopy structures of ternary complexes formed by AcrIIA25.1 and AcrIIA32 bound to SpyCas9-sgRNA. Using structural analysis and biochemical experiments, we revealed that AcrIIA25.1 and AcrIIA32 recognize a novel, previously-unidentified anti-CRISPR binding site on SpyCas9. We found that both AcrIIA25.1 and AcrIIA32 directly interact with the WED domain, where they spatially obstruct conformational changes of the WED and PI domains, thereby inhibiting SpyCas9 from recognizing protospacer adjacent motif (PAM) and unwinding double-stranded DNA. In addition, they may inhibit nuclease activity by blocking the dynamic conformational changes of the SpyCas9 surveillance complex. In summary, our data elucidate the inhibition mechanisms of two new anti-CRISPR proteins, provide new strategies for the modulation of SpyCas9 activity, and expand our understanding of the diversity of anti-CRISPR protein inhibition mechanisms.

Engineering Anti-CRISPR Proteins to Create CRISPR-Cas Protein Switches for Activatable Genome Editing and Viral Protease Detection

Proteins capable of switching between distinct active states in response to biochemical cues are ideal for sensing and controlling biological processes. Activatable CRISPR-Cas systems are significant in precise genetic manipulation and sensitive molecular diagnostics, yet directly controlling Cas protein function remains challenging. Herein, we explore anti-CRISPR (Acr) proteins as modules to create synthetic Cas protein switches (CasPSs) based on computational chemistry-directed rational protein interface engineering. Guided by molecular fingerprint analysis, electrostatic potential mapping, and binding free energy calculations, we rationally engineer the molecular interaction interface between Cas12a and its cognate Acr proteins (AcrVA4 and AcrVA5) to generate a series of orthogonal protease-responsive CasPSs. These CasPSs enable the conversion of specific proteolytic events into activation of Cas12a function with high switching ratios (up to 34.3-fold). These advancements enable specific proteolysis-inducible genome editing in mammalian cells and sensitive detection of viral protease activities during virus infection. This work provides a promising strategy for developing CRISPR-Cas tools for controllable gene manipulation and regulation and clinical diagnostics.

Shewanella phage encoding a putative anti-CRISPR-like gene represents a novel potential viral family

Shewanella is a prevalent bacterial genus in deep-sea environments including marine sediments, exhibiting diverse metabolic capabilities that indicate its significant contributions to the marine biogeochemical cycles. However, only a few Shewanella phages were isolated and deposited in the NCBI database. In this study, we report the isolation and characterization of a novel Shewanella phage, vB_SbaS_Y11, that infects Shewanella KR11 and was isolated from the sewage in Qingdao, China. Transmission electron microscopy revealed that vB_SbaS_Y11 has an icosahedral head and a long tail. The genome of vB_SbaS_Y11 is a linear, double-stranded DNA with a length of 62,799 bp and a G+C content of 46.9%, encoding 71 putative open reading frames. No tRNA genes or integrase-related feature genes were identified. An uncharacterized anti-CRISPR AcrVA2 gene was detected in its genome. Phylogenetic analysis based on the amino acid sequences of whole genomes and comparative genomic analyses indicate that vB_SbaS_Y11 has a novel genomic architecture and shares low similarity to Pseudomonas virus H66 and Pseudomonas phage F116. vB_SbaS_Y11 represents a potential new family-level virus cluster with eight metagenomic assembled viral genomes named Ranviridae.IMPORTANCEThe Gram-negative Shewanella bacterial genus currently includes about 80 species of mostly aquatic Gammaproteobacteria, which were isolated around the globe in a multitude of environments, such as freshwater, seawater, coastal sediments, and the deepest trenches. Here, we present a Shewanella phage vB_SbaS_Y11 that contains an uncharacterized anti-CRISPR AcrVA2 gene and belongs to a potential virus family, Ranviridae. This study will enhance the knowledge about the genome, diversity, taxonomic classification, and global distribution of Shewanella phage populations.

Purification and functional validation of LtCas12a protein

Here, we present a protocol for generating LtCas12a protein recognizing distinct TTNA (N represented A, T, C, G) protospacer adjacent motif sequence. We describe steps for transforming and harvesting bacterial cells and protein purification including nickel affinity chromatography and dialysis. We then detail procedures for verification of LtCas12a with cis- and trans-cleavage activities. For complete details on the use and execution of this protocol, please refer to Chen et al. (2023).1.

Inhibitory mechanism of CRISPR-Cas9 by AcrIIC4

CRISPR-Cas systems act as the adaptive immune systems of bacteria and archaea, targeting and destroying invading foreign mobile genetic elements (MGEs) such as phages. MGEs have also evolved anti-CRISPR (Acr) proteins to inactivate the CRISPR-Cas systems. Recently, AcrIIC4, identified from Haemophilus parainfluenzae phage, has been reported to inhibit the endonuclease activity of Cas9 from Neisseria meningitidis (NmeCas9), but the inhibition mechanism is not clear. Here, we biochemically and structurally investigated the anti-CRISPR activity of AcrIIC4. AcrIIC4 folds into a helix bundle composed of three helices, which associates with the REC lobe of NmeCas9 and sgRNA. The REC2 domain of NmeCas9 is locked by AcrIIC4, perturbing the conformational dynamics required for the target DNA binding and cleavage. Furthermore, mutation of the key residues in the AcrIIC4-NmeCas9 and AcrIIC4-sgRNA interfaces largely abolishes the inhibitory effects of AcrIIC4. Our study offers new insights into the mechanism of AcrIIC4-mediated suppression of NmeCas9 and provides guidelines for the design of regulatory tools for Cas9-based gene editing applications.

Phage Against the Machine: Discovery and Mechanism of Type V Anti-CRISPRs

The discovery of diverse bacterial CRISPR-Cas systems has reignited interest in understanding bacterial defense pathways while yielding exciting new tools for genome editing. CRISPR-Cas systems are widely distributed in prokaryotes, found in 40% of bacteria and 90% of archaea, where they function as adaptive immune systems against bacterial viruses (phage) and other mobile genetic elements. In turn, phage have evolved inhibitors, called anti-CRISPR proteins, to prevent targeting. Type V CRISPR-Cas12 systems have emerged as a particularly exciting arena in this co-evolutionary arms race. Type V anti-CRISPRs have highly diverse and novel mechanisms of action, some of which appear to be unusually potent or widespread. In this review, we discuss the discovery and mechanism of these anti-CRISPRs as well as future areas for exploration.

Optimization of the expression of the main protease from SARS-CoV-2

The main protease (M^pro) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) plays a vital role in viral replication. To study the function of M^pro and screen inhibitors targeting M^pro, it is necessary to prepare high-purity and high-activity M^pro. In this study, four types of SARS-CoV-2 M^pros containing different termini were prepared, and their activities were determined successfully. The results showed that the activity of wild-type (WT) M^pro was the highest, and the additional residues at the N-terminus but not at the C-terminus had a major effect on the enzyme activity. To explain this, the alignment of structures of different forms of M^pro was determined, and the additional residues at the N-terminus were found to interfere with the formation of the substrate binding pocket. This study confirms the importance of the natural N-terminus to the activity of M^pro and suggests that WT-GPH₆ (M^pro with eight additional residues at the C-terminus) can be used as a substitute for authentic M^pro to screen inhibitors. In short, this study provides a reference for the expression and purification of new coronaviruses confronted in the future.

CRISPR-based nucleic acid diagnostics for pathogens

Pathogenic infection remains the primary threat to human health, such as the global COVID-19 pandemic. It is important to develop rapid, sensitive and multiplexed tools for detecting pathogens and their mutated variants, particularly the tailor-made strategies for point-of-care diagnosis allowing for use in resource-constrained settings. The rapidly evolving CRISPR/Cas systems have provided a powerful toolbox for pathogenic diagnostics via nucleic acid tests. In this review, we firstly describe the resultant promising class 2 (single, multidomain effector) and recently explored class 1 (multisubunit effector complexes) CRISPR tools. We present diverse engineering nucleic acid diagnostics based on CRISPR/Cas systems for pathogenic viruses, bacteria and fungi, and highlight the application for detecting viral variants and drug-resistant bacteria enabled by CRISPR-based mutation profiling. Finally, we discuss the challenges involved in on-site diagnostic assays and present emerging CRISPR systems and CRISPR cascade that potentially enable multiplexed and preamplification-free pathogenic diagnostics.

Structures of apo Cas12a and its complex with crRNA and DNA reveal the dynamics of ternary complex formation and target DNA cleavage

Cas12a is a programmable nuclease for adaptive immunity against invading nucleic acids in CRISPR-Cas systems. Here, we report the crystal structures of apo Cas12a from Lachnospiraceae bacterium MA2020 (Lb2) and the Lb2Cas12a+crRNA complex, as well as the cryo-EM structure and functional studies of the Lb2Cas12a+crRNA+DNA complex. We demonstrate that apo Lb2Cas12a assumes a unique, elongated conformation, whereas the Lb2Cas12a+crRNA binary complex exhibits a compact conformation that subsequently rearranges to a semi-open conformation in the Lb2Cas12a+crRNA+DNA ternary complex. Notably, in solution, apo Lb2Cas12a is dynamic and can exist in both elongated and compact forms. Residues from Met493 to Leu523 of the WED domain undergo major conformational changes to facilitate the required structural rearrangements. The REC lobe of Lb2Cas12a rotates 103° concomitant with rearrangement of the hinge region close to the WED and RuvC II domains to position the RNA-DNA duplex near the catalytic site. Our findings provide insight into crRNA recognition and the mechanism of target DNA cleavage.

Insights into the inhibition of type I-F CRISPR-Cas system by a multifunctional anti-CRISPR protein AcrIF24

CRISPR-Cas systems are prokaryotic adaptive immune systems and phages use anti-CRISPR proteins (Acrs) to counteract these systems. Here, we report the structures of AcrIF24 and its complex with the crRNA-guided surveillance (Csy) complex. The HTH motif of AcrIF24 can bind the Acr promoter region and repress its transcription, suggesting its role as an Aca gene in self-regulation. AcrIF24 forms a homodimer and further induces dimerization of the Csy complex. Apart from blocking the hybridization of target DNA to the crRNA, AcrIF24 also induces the binding of non-sequence-specific dsDNA to the Csy complex, similar to AcrIF9, although this binding seems to play a minor role in AcrIF24 inhibitory capacity. Further structural and biochemical studies of the Csy-AcrIF24-dsDNA complexes and of AcrIF24 mutants reveal that the HTH motif of AcrIF24 and the PAM recognition loop of the Csy complex are structural elements essential for this non-specific dsDNA binding. Moreover, AcrIF24 and AcrIF9 display distinct characteristics in inducing non-specific DNA binding. Together, our findings highlight a multifunctional Acr and suggest potential wide distribution of Acr-induced non-specific DNA binding.

Phages use anti-CRISPR proteins (Acrs) to counteract the bacterial CRISPR-Cas systems. Here, the authors characterize AcrIF24, which functions as an Aca (Acr-associated) to repress and regulate its own transcription, dimerizes the Csy complex, blocks the hybridization of target DNA, and tethers non-sequence-specific DNA to the Csy complex.

Type II anti-CRISPR proteins as a new tool for synthetic biology

The clustered regularly interspaced short palindromic repeats (CRISPR)-Cas (CRISPR-associated proteins) system represents, in prokaryotes, an adaptive and inheritable immune response against invading DNA. The discovery of anti-CRISPR proteins (Acrs), which are inhibitors of CRISPR-Cas, mainly encoded by phages and prophages, showed a co-evolution history between prokaryotes and phages. In the past decade, the CRISPR-Cas systems together with the corresponding Acrs have been turned into a genetic-engineering tool. Among the six types of CRISPR-Cas characterized so far, type II CRISPR-Cas system is the most popular in biotechnology. Here, we discuss about the discovery, the reported inhibitory mechanisms, and the applications in both gene editing and gene transcriptional regulation of type II Acrs. Moreover, we provide insights into future potential research and feasible applications.

Structure-based functional mechanisms and biotechnology applications of anti-CRISPR proteins

CRISPR loci and Cas proteins provide adaptive immunity in prokaryotes against invading bacteriophages and plasmids. In response, bacteriophages have evolved a broad spectrum of anti-CRISPR proteins (anti-CRISPRs) to counteract and overcome this immunity pathway. Numerous anti-CRISPRs have been identified to date, which suppress single-subunit Cas effectors (in CRISPR class 2, type II, V and VI systems) and multisubunit Cascade effectors (in CRISPR class 1, type I and III systems). Crystallography and cryo-electron microscopy structural studies of anti-CRISPRs bound to effector complexes, complemented by functional experiments in vitro and in vivo, have identified four major CRISPR-Cas suppression mechanisms: inhibition of CRISPR-Cas complex assembly, blocking of target binding, prevention of target cleavage, and degradation of cyclic oligonucleotide signalling molecules. In this Review, we discuss novel mechanistic insights into anti-CRISPR function that have emerged from X-ray crystallography and cryo-electron microscopy studies, and how these structures in combination with function studies provide valuable tools for the ever-growing CRISPR-Cas biotechnology toolbox, to be used for precise and robust genome editing and other applications.

Decoupling the bridge helix of Cas12a results in a reduced trimming activity, increased mismatch sensitivity and impaired conformational transitions

The widespread and versatile prokaryotic CRISPR-Cas systems (clustered regularly interspaced short palindromic repeats and associated Cas proteins) constitute powerful weapons against foreign nucleic acids. Recently, the single-effector nuclease Cas12a that belongs to the type V CRISPR-Cas system was added to the Cas enzymes repertoire employed for gene editing purposes. Cas12a is a bilobal enzyme composed of the REC and Nuc lobe connected by the wedge, REC1 domain and bridge helix (BH). We generated BH variants and integrated biochemical and single-molecule FRET (smFRET) studies to elucidate the role of the BH for the enzymatic activity and conformational flexibility of Francisella novicida Cas12a. We demonstrate that the BH impacts the trimming activity and mismatch sensitivity of Cas12a resulting in Cas12a variants with improved cleavage accuracy. smFRET measurements reveal the hitherto unknown open and closed state of apo Cas12a. BH variants preferentially adopt the open state. Transition to the closed state of the Cas12a-crRNA complex is inefficient in BH variants but the semi-closed state of the ternary complex can be adopted even if the BH is deleted in its entirety. Taken together, these insights reveal that the BH is a structural element that influences the catalytic activity and impacts conformational transitions of FnCas12a.

Saccharomyces cerevisiae Synthetic Transcriptional Networks Harnessing dCas12a and Type V-A anti-CRISPR Proteins

Type V-A anti-CRISPR proteins (AcrVAs) represent the response from phages to the CRISPR-Cas12a prokaryotic immune system. CRISPR-Cas12a was repurposed, in high eukaryotes, to carry out gene editing and transcription regulation, the latter via a nuclease-dead Cas12a (dCas12a). Consequently, AcrVAs were adopted to regulate (d)Cas12a activity. However, the usage of both dCas12a-based transcription factors and AcrVAs in the yeast Saccharomyces cerevisiae has not been explored. In this work, we show that, in the baker's yeast, two dCas12a proteins (denAsCas12a and dLbCas12a) work both as activators (upon fusion to a strong activation domain) and repressors, whereas dMbCa12a is nonfunctional. The activation efficiency of dCas12a-ADs manifests a dependence on the number of crRNA binding sites, whereas it is not directly correlated to the amount of crRNA in the cells. Moreover, AcrVA1, AcrVA4, and AcrVA5 are able to inhibit dLbCa12a in yeast, and denAsCas12a is only inhibited by AcrVA1. However, AcrVA1 performs well at high concentration only. Coexpression of two or three AcrVAs does not enhance inhibition of dCas12a(-AD), suggesting a competition between different AcrVAs. Further, AcrVA4 significantly limits gene editing by LbCas12a. Overall, our results indicate that dCas12a:crRNA and AcrVA proteins are highly performant components in S. cerevisiae synthetic transcriptional networks.

Diversity of molecular mechanisms used by anti-CRISPR proteins: the tip of an iceberg?

Bacteriophages (phages) and their preys are engaged in an evolutionary arms race driving the co-adaptation of their attack and defense mechanisms. In this context, phages have evolved diverse anti-CRISPR proteins to evade the bacterial CRISPR-Cas immune system, and propagate. Anti-CRISPR proteins do not share much resemblance with each other and with proteins of known function, which raises intriguing questions particularly relating to their modes of action. In recent years, there have been many structure-function studies shedding light on different CRISPR-Cas inhibition strategies. As the anti-CRISPR field of research is rapidly growing, it is opportune to review the current knowledge on these proteins, with particular emphasis on the molecular strategies deployed to inactivate distinct steps of CRISPR-Cas immunity. Anti-CRISPR proteins can be orthosteric or allosteric inhibitors of CRISPR-Cas machineries, as well as enzymes that irreversibly modify CRISPR-Cas components. This repertoire of CRISPR-Cas inhibition mechanisms will likely expand in the future, providing fundamental knowledge on phage-bacteria interactions and offering great perspectives for the development of biotechnological tools to fine-tune CRISPR-Cas-based gene edition.

Structural basis for inhibition of the type I-F CRISPR-Cas surveillance complex by AcrIF4, AcrIF7 and AcrIF14

CRISPR-Cas systems are adaptive immune systems in bacteria and archaea to defend against mobile genetic elements (MGEs) and have been repurposed as genome editing tools. Anti-CRISPR (Acr) proteins are produced by MGEs to counteract CRISPR-Cas systems and can be used to regulate genome editing by CRISPR techniques. Here, we report the cryo-EM structures of three type I-F Acr proteins, AcrIF4, AcrIF7 and AcrIF14, bound to the type I-F CRISPR-Cas surveillance complex (the Csy complex) from Pseudomonas aeruginosa. AcrIF4 binds to an unprecedented site on the C-terminal helical bundle of Cas8f subunit, precluding conformational changes required for activation of the Csy complex. AcrIF7 mimics the PAM duplex of target DNA and is bound to the N-terminal DNA vise of Cas8f. Two copies of AcrIF14 bind to the thumb domains of Cas7.4f and Cas7.6f, preventing hybridization between target DNA and the crRNA. Our results reveal structural detail of three AcrIF proteins, each binding to a different site on the Csy complex for inhibiting degradation of MGEs.

Types I and V Anti-CRISPR Proteins: From Phage Defense to Eukaryotic Synthetic Gene Circuits

Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas (CRISPR-associated proteins), a prokaryotic RNA-mediated adaptive immune system, has been repurposed for gene editing and synthetic gene circuit construction both in bacterial and eukaryotic cells. In the last years, the emergence of the anti-CRISPR proteins (Acrs), which are natural OFF-switches for CRISPR-Cas, has provided a new means to control CRISPR-Cas activity and promoted a further development of CRISPR-Cas-based biotechnological toolkits. In this review, we focus on type I and type V-A anti-CRISPR proteins. We first narrate Acrs discovery and analyze their inhibitory mechanisms from a structural perspective. Then, we describe their applications in gene editing and transcription regulation. Finally, we discuss the potential future usage-and corresponding possible challenges-of these two kinds of anti-CRISPR proteins in eukaryotic synthetic gene circuits.

Structures and Strategies of Anti-CRISPR-Mediated Immune Suppression

More than 50 protein families have been identified that inhibit CRISPR (clustered regularly interspaced short palindromic repeats)-Cas-mediated adaptive immune systems. Here, we analyze the available anti-CRISPR (Acr) structures and describe common themes and unique mechanisms of stoichiometric and enzymatic suppressors of CRISPR-Cas. Stoichiometric inhibitors often function as molecular decoys of protein-binding partners or nucleic acid targets, while enzymatic suppressors covalently modify Cas ribonucleoprotein complexes or degrade immune signaling molecules. We review mechanistic insights that have been revealed by structures of Acrs, discuss some of the trade-offs associated with each of these strategies, and highlight how Acrs are regulated and deployed in the race to overcome adaptive immunity.

Anti-CRISPR proteins targeting the CRISPR-Cas system enrich the toolkit for genetic engineering

Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas adaptive immune defense systems, which are widely distributed in bacteria and Archaea, can provide sequence-specific protection against foreign DNA or RNA in some cases. However, the evolution of defense systems in bacterial hosts did not lead to the elimination of phages, and some phages carry anti-CRISPR genes that encode products that bind to the components mediating the defense mechanism and thus antagonize CRISPR-Cas immune systems of bacteria. Given the extensive application of CRISPR-Cas9 technologies in gene editing, in this review, we focus on the anti-CRISPR proteins (Acrs) that inhibit CRISPR-Cas systems for gene editing. We describe the discovery of Acrs in immune systems involving type I, II, and V CRISPR-Cas immunity, discuss the potential function of Acrs in inactivating type II and V CRISPR-Cas systems for gene editing and gene modulation, and provide an outlook on the development of important biotechnology tools for genetic engineering using Acrs.

Making the cut(s): how Cas12a cleaves target and non-target DNA

CRISPR-Cas12a (previously named Cpf1) is a prokaryotic deoxyribonuclease that can be programmed with an RNA guide to target complementary DNA sequences. Upon binding of the target DNA, Cas12a induces a nick in each of the target DNA strands, yielding a double-stranded DNA break. In addition to inducing cis-cleavage of the targeted DNA, target DNA binding induces trans-cleavage of non-target DNA. As such, Cas12a-RNA guide complexes can provide sequence-specific immunity against invading nucleic acids such as bacteriophages and plasmids. Akin to CRISPR-Cas9, Cas12a has been repurposed as a genetic tool for programmable genome editing and transcriptional control in both prokaryotic and eukaryotic cells. In addition, its trans-cleavage activity has been applied for high-sensitivity nucleic acid detection. Despite the demonstrated value of Cas12a for these applications, the exact molecular mechanisms of both cis- and trans-cleavage of DNA were not completely understood. Recent studies have revealed mechanistic details of Cas12a-mediates DNA cleavage: base pairing of the RNA guide and the target DNA induces major conformational changes in Cas12a. These conformational changes render Cas12a in a catalytically activated state in which it acts as deoxyribonuclease. This deoxyribonuclease activity mediates cis-cleavage of the displaced target DNA strand first, and the RNA guide-bound target DNA strand second. As Cas12a remains in the catalytically activated state after cis-cleavage, it subsequently demonstrates trans-cleavage of non-target DNA. Here, I review the mechanistic details of Cas12a-mediated cis- and trans-cleavage of DNA. In addition, I discuss how bacteriophage-derived anti-CRISPR proteins can inhibit Cas12a activity.