The archaeal CASCADE complex from Sulfolobus solfataricus features a crescent-shaped Csa2 structure that forms a stable complex with Cas5a, sufficient to bind crRNA and complementary ssDNA.
The study elucidates the structural architecture and functional RNA/DNA binding capabilities of the archaeal CASCADE complex, providing insights into CRISPR-mediated antiviral defense mechanisms.
In response to viral infection, many prokaryotes incorporate fragments of virus-derived DNA into loci called clustered regularly interspaced short palindromic repeats (CRISPRs). The loci are then transcribed, and the processed CRISPR transcripts are used to target invading viral DNA and RNA. The Escherichia coli "CRISPR-associated complex for antiviral defense" (CASCADE) is central in targeting invading DNA. Here we report the structural and functional characterization of an archaeal CASCADE (aCASCADE) from Sulfolobus solfataricus. Tagged Csa2 (Cas7) expressed in S. solfataricus co-purifies with Cas5a-, Cas6-, Csa5-, and Cas6-processed CRISPR-RNA (crRNA). Csa2, the dominant protein in aCASCADE, forms a stable complex with Cas5a. Transmission electron microscopy reveals a helical complex of variable length, perhaps due to substoichiometric amounts of other CASCADE components. A recombinant Csa2-Cas5a complex is sufficient to bind crRNA and complementary ssDNA. The structure of Csa2 reveals a crescent-shaped structure unexpectedly composed of a modified RNA-recognition motif and two additional domains present as insertions in the RNA-recognition motif. Conserved residues indicate potential crRNA- and target DNA-binding sites, and the H160A variant shows significantly reduced affinity for crRNA. We propose a general subunit architecture for CASCADE in other bacteria and Archaea.
Lintner et al. (Thu,) reported a other. The archaeal CASCADE complex from Sulfolobus solfataricus features a crescent-shaped Csa2 structure that forms a stable complex with Cas5a, sufficient to bind crRNA and complementary ssDNA.