Cryo-EM Structures Reveal Mechanism and Inhibition of DNA Targeting by a CRISPR-Cas Surveillance Complex

Cell. 2017 Oct 5;171(2):414-426.e12. doi: 10.1016/j.cell.2017.09.006.

Abstract

Prokaryotic cells possess CRISPR-mediated adaptive immune systems that protect them from foreign genetic elements, such as invading viruses. A central element of this immune system is an RNA-guided surveillance complex capable of targeting non-self DNA or RNA for degradation in a sequence- and site-specific manner analogous to RNA interference. Although the complexes display considerable diversity in their composition and architecture, many basic mechanisms underlying target recognition and cleavage are highly conserved. Using cryoelectron microscopy (cryo-EM), we show that the binding of target double-stranded DNA (dsDNA) to a type I-F CRISPR system yersinia (Csy) surveillance complex leads to large quaternary and tertiary structural changes in the complex that are likely necessary in the pathway leading to target dsDNA degradation by a trans-acting helicase-nuclease. Comparison of the structure of the surveillance complex before and after dsDNA binding, or in complex with three virally encoded anti-CRISPR suppressors that inhibit dsDNA binding, reveals mechanistic details underlying target recognition and inhibition.

Keywords: CRISPR-Cas Csy complexes; CRISPR-Cas systems; DNA-protein interactions; PAM recognition; adaptive immunity; anti-CRISPR inhibitors; conformational transitions; cryoelectron microscopy; gene editing.

MeSH terms

  • Bacterial Proteins / chemistry*
  • Bacteriophages / genetics
  • Bacteriophages / immunology
  • CRISPR-Associated Proteins / chemistry*
  • CRISPR-Associated Proteins / immunology
  • CRISPR-Associated Proteins / ultrastructure
  • CRISPR-Cas Systems*
  • Cryoelectron Microscopy*
  • DNA, Viral / chemistry
  • Models, Chemical
  • Models, Molecular
  • Multiprotein Complexes / chemistry
  • Pseudomonas aeruginosa / chemistry*
  • Pseudomonas aeruginosa / immunology*
  • Pseudomonas aeruginosa / metabolism
  • Pseudomonas aeruginosa / ultrastructure

Substances

  • Bacterial Proteins
  • CRISPR-Associated Proteins
  • DNA, Viral
  • Multiprotein Complexes