Structural basis for assembly of non-canonical small subunits into type I-C Cascade

• Published in: Nature communications Vol. 11; no. 1; pp. 5931 - 6
• Main Authors: O’Brien, Roisin E., Santos, Inês C., Wrapp, Daniel, Bravo, Jack P. K., Schwartz, Evan A., Brodbelt, Jennifer S., Taylor, David W.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 23.11.2020; Nature Publishing Group; Nature Portfolio
• Subjects: 101/28; 631/337/4041; 631/45/173; 631/45/500; 631/45/535/1258/1259; Adaptive immunity; Adaptive systems; Antiviral drugs; Archaea; Assembly; Bacterial Proteins - chemistry; Bacterial Proteins - metabolism; Binding Sites; Clustered Regularly Interspaced Short Palindromic Repeats - genetics; CRISPR; CRISPR-Associated Proteins - chemistry; CRISPR-Associated Proteins - metabolism; CRISPR-Cas Systems; Cryoelectron Microscopy; Desulfovibrio vulgaris; Desulfovibrio vulgaris - chemistry; Desulfovibrio vulgaris - genetics; DNA - chemistry; DNA - metabolism; Genome editing; Genomes; Humanities and Social Sciences; Models, Molecular; Molecular modelling; multidisciplinary; Multiprotein Complexes - chemistry; Multiprotein Complexes - metabolism; Nucleic acids; Nucleotide Motifs; Protein Conformation; Protein Subunits - chemistry; Protein Subunits - metabolism; Ribonucleic acid; RNA; RNA, Bacterial - chemistry; RNA, Bacterial - metabolism; Science; Science (multidisciplinary)
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-020-19785-8

Bacteria and archaea employ CRISPR (clustered, regularly, interspaced, short palindromic repeats)-Cas (CRISPR-associated) systems as a type of adaptive immunity to target and degrade foreign nucleic acids. While a myriad of CRISPR-Cas systems have been identified to date, type I-C is one of the most commonly found subtypes in nature. Interestingly, the type I-C system employs a minimal Cascade effector complex, which encodes only three unique subunits in its operon. Here, we present a 3.1 Å resolution cryo-EM structure of the Desulfovibrio vulgaris type I-C Cascade, revealing the molecular mechanisms that underlie RNA-directed complex assembly. We demonstrate how this minimal Cascade utilizes previously overlooked, non-canonical small subunits to stabilize R-loop formation. Furthermore, we describe putative PAM and Cas3 binding sites. These findings provide the structural basis for harnessing the type I-C Cascade as a genome-engineering tool. Type I-C Cascade (the CRISPR-associated complex for antiviral defense) is a minimal system, comprising only three unique Cas proteins. Cryo-EM structure of the Desulfovibrio vulgaris type I-C Cascade reveals the molecular mechanisms that underlie RNA-directed complex assembly.