Aug 2026· iScience· Vol 29· 0 citations· 38 references
Medicine
TL;DR
The crystal structure of AcrIIA17 is presented and its mechanism of Staphylococcus aureus Cas9 (SauCas9) inhibition is elucidated, identifying AcrIIA17 as an Acr protein that targets the Cas9 BH domain and reveal the BH domain as a regulatory checkpoint in Cas9 activation.
Abstract
Summary Anti-CRISPR (Acr) proteins have evolved in bacteriophages and mobile genetic elements to counteract CRISPR-Cas immune systems through diverse inhibitory mechanisms. Here, we present the crystal structure of AcrIIA17 and elucidate its mechanism of Staphylococcus aureus Cas9 (SauCas9) inhibition. AcrIIA17 adopts a previously uncharacterized protein fold and exists as a monomer in solution. Biochemical analyses reveal that AcrIIA17 inhibits SauCas9 activity in a strictly order-dependent manner, effectively suppressing DNA cleavage only when it engages Cas9 prior to single guide RNA (sgRNA) loading, whereas pre-assembled Cas9-sgRNA ribonucleoprotein (RNP) complexes are resistant to inhibition. Domain-mapping experiments demonstrate that AcrIIA17 directly binds to the bridge helix (BH) domain of SauCas9, and structure-guided mutagenesis confirms that this interaction is essential for its inhibitory function. Together, our findings identify AcrIIA17 as an Acr protein that targets the Cas9 BH domain and reveal the BH domain as a regulatory checkpoint in Cas9 activation.
The mechanisms of nuclease activation are explored by solving seven ternary cryo-electron mi-croscopy structures of wild-type Cas13d in complex with matched and mismatched targets and an active site loop in the HEPN domains that regulates substrate accessibility is identified.
Chia-Wei Chou, Selma Sinan, Hung-Che Kuo et al.· bioRxiv· 0 citations
Many bacteriophages encode anti-CRISPR (Acr) proteins that inhibit the CRISPR-Cas immune systems. Rapid acr gene expression upon phage entry enables CRISPR-Cas neutralisation, but can impact phage fitness if unregulated. Therefore, Acr production is often controlled by distinct families of co-encoded anti-CRISPR-associated (Aca) proteins, which are usually helix-turn-helix (HTH) regulators that bind DNA within acr–aca operon promoters. Previously, we demonstrated that the Aca2 family additionally represses Acr production translationally by binding structured RNA motifs within the 5′ untranslated regions (UTRs) of the acr–aca mRNA. Here, through systematic bioinformatic analyses, we provide evidence of structured RNA motifs in the 5′ UTRs of operons encoding members of other Aca families, and that Aca1 also specifically binds its cognate RNA motif. Additionally, many Aca proteins are predicted to regulate not only their own but also adjacent operons with potential anti-defence genes. Indeed, we show that Aca14, newly identified in this study, represses two predicted anti-defence operons. Aca14 is a ribbon-helix-helix domain protein, revealing regulatory diversity beyond the canonical HTH Aca family members. Collectively, our findings expand the understanding of acr regulation in mobile genetic elements and reveal novel mechanisms by which phages fine-tune anti-defence gene expression. Graphical abstract
Maximilian Feussner, N. Birkholz, So Yeon Lee et al.· bioRxiv· 0 citations
Abstract CRISPR–Cas13a is mainly known as an RNA-guided RNA endonuclease. Recent studies show that Leptotrichia buccalis Cas13a (LbuCas13a) can interact with DNA substrates too, without PAM or PFS constraints, but current understanding of DNA-mediated activation is largely based on continuous target strands. Here, we define a structural accessibility principle for LbuCas13a activation by noncontiguous DNA. We show that activation occurs only when overhang positioning creates an accessible protein–DNA interface. Outer overhangs near the crRNA repeat-adjacent side restore strong trans-cleavage activity by stabilizing key LbuCas13a–DNA contacts, whereas distal outer overhangs support only weak activation. In contrast, inner overhangs cause steric mismatch, destabilize the complex, and block formation of an active conformation. Molecular modeling and molecular dynamics simulations support this structure-dependent rule. Noncontiguous DNA also broadens the single-nucleotide discrimination window of LbuCas13a and enables accurate IDH1 R132H detection in glioma tissues. We further develop a one-step APE1-activated CRISPR–LbuCas13a reaction (ACROSS) for sensitive APE1 detection. Because activated LbuCas13a cleaves RNA reporters but not DNA-triggering products, ACROSS preserves the activating structure and supports stable signaling in vitro, in live cells, and in breast cancer serum samples.
Weitao Wang, Yuhan Chen, Ziyun Li et al.· Nucleic Acids Research· 0 citations
It is shown that short RNAs can directly occupy the canonical crRNA-binding channel and trigger a catalytically competent trans cleavage state in the absence of PAM recognition or canonical R-loop formation.
I. Iwe, S. Singh, K. Guan et al.· medRxiv· 0 citations
Nairoviruses are emerging tick-borne pathogens for which effective antiviral therapies are currently unavailable. Although nucleoproteins (NPs) are essential for viral genome encapsulation and have been extensively characterized at the structural level, whether they perform additional functions during viral replication remains unclear. Here, we investigated the NP of the representative nairovirus Tacheng tick virus 1 (TcTV1). We found that the TcTV1 NP binds to nucleic acids in a sequence-independent manner and assembles into tetramer-based ribonucleoprotein complexes upon nucleic acid binding. This assembly process is accompanied by a pronounced conformational rearrangement that facilitates NP polymerization. In addition to its role in RNA encapsulation, TcTV1 NP exhibits intrinsic endonuclease activity that does not require metal ions and preferentially cleaves unstructured single-stranded RNA, while structured RNA substrates are largely resistant to cleavage. Functional analysis indicates that the stalk domain of NP plays a central role in coordinating RNA binding, oligomerization, and access to the nuclease-active site, thereby influencing whether an RNA molecule is protected or degraded. Finally, we identified a small-molecule compound that interferes with both RNA binding and nuclease activity by targeting a conserved functional region of nairovirus NP. Together, these results reveal an expanded functional repertoire of nairovirus NPs and suggest that NP-mediated RNA discrimination may contribute to viral replication. Our findings also support the feasibility of targeting NP for the development of antiviral drugs against emerging nairoviruses.
Zan Li, Shan Du, Feng Gao et al.· Proceedings of the National...· 0 citations
Type I CRISPR-Cas systems constitute the most prevalent prokaryotic adaptive immune pathways and are classified into seven subtypes (I-A to I-G). These antiviral systems typically exploit a Cascade complex for RNA-guided DNA recognition and a Cas3 helicase-nuclease effector for DNA degradation, yet their diverse activation mechanisms remain not fully understood. In this study, we isolate the I-A Cascade from Saccharolobus islandicus, revealing a minimal form of Cascade lacking both Cas3 and the CRISPR-RNA maturase Cas6. Cas3 is recruited to the R-loop structure formed after Cascade binding to target DNA, which activates the effector for both cis- and trans-DNA cleavage. Strikingly, ATP not only enables the processive target degradation by Cas3 but also suppresses the trans-cleavage of the same enzyme. Together, the Sa. islandicus I-A system operates via target-dependent Cas3 recruitment-a mechanism distinct from other characterized I-A systems, thus underscoring the mechanistic diversity within type I CRISPR-Cas immunity.
Suping Jiang, Xuhui Tian, Fang Wang et al.· Cell Reports· 0 citations
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