April 19, 2012Molecular Cell248 citationsOpen Access
Mechanism of Foreign DNA Selection in a Bacterial Adaptive Immune System
Key Points
- This research aims to understand how bacteria recognize and bind to foreign DNA using the CasA protein.
- Combined a 2.3 Å crystal structure of CasA with cryo-EM structures of Cascade.
- Identified critical loop regions and a conserved three base pair motif needed for target selection.
- CasA is essential for nonself target recognition and binding.
- The CasA loop directly interacts with a specific three base pair motif crucial for foreign DNA detection.
Abstract
In bacterial and archaeal CRISPR immune pathways, DNA sequences from invading bacteriophage or plasmids are integrated into CRISPR loci within the host genome, conferring immunity against subsequent infections. The ribonucleoprotein complex Cascade utilizes RNAs generated from these loci to target complementary "nonself" DNA sequences for destruction, while avoiding binding to "self" sequences within the CRISPR locus. Here we show that CasA, the largest protein subunit of Cascade, is required for nonself target recognition and binding. Combining a 2.3 Å crystal structure of CasA with cryo-EM structures of Cascade, we have identified a loop that is required for viral defense. This loop contacts a conserved three base pair motif that is required for nonself target selection. Our data suggest a model in which the CasA loop scans DNA for this short motif prior to target destabilization and binding, maximizing the efficiency of DNA surveillance by Cascade.