Anti-CRISPRs: The natural inhibitors for CRISPR-Cas systems

Abstract

CRISPR (clustered regularly interspaced short palindromic repeats)-Cas (CRISPR associated protein) systems serve as the adaptive immune system by which prokaryotes defend themselves against phages. It has also been developed into a series of powerful gene-editing tools. As the natural inhibitors of CRISPR-Cas systems, anti-CRISPRs (Acrs) can be used as the "off-switch" for CRISPR-Cas systems to limit the off-target effects caused by Cas9. Since the discovery of CRISPR-Cas systems, much research has focused on the identification, mechanisms and applications of Acrs. In light of the rapid development and scientific significance of this field, this review summarizes the history and research status of Acrs, and considers future applications.

Keywords: CRISPR‐Cas system; Cas9; anti‐CRISPRs.

Figure 1

Schematics of anti‐CRISPR protein interfere with different stages of type I, type II and type V CRISPR‐Cas systems. A, In type I CRISPR‐Cas systems, the nine subunits of the Cascade come together with the CRISPR RNA (crRNA) to form the surveillance complex, which uses the spacer sequence (cyan) to search for target DNA (red). Then the Cas3 nuclease is recruited to the complex and cleaves the target DNA. AcrIF1, AcrIF2, and AcrIF10 can block the target DNA binding. AcrIE1 and AcrIF3 prevent Cas3 recruitment and thereby prevent DNA cleavage. B, In type II and type V CRISPR‐Cas systems, the crRNA is loaded onto Cas protein (Cas9 or Cas12a) to form a ribonucleoprotein complex, which binds to target DNA and then cleaves it. AcrIIC2 is shown to interfere with crRNA‐ and DNA‐loading. AcrIIA2, AcrIIA4, AcrIIC3, AcrIIC4, AcrIIC5, AcrVA1, AcrVA4 and AcrVA5 are known to block the target DNA binding. AcrIIC1 prevents the target DNA cleavage