Nucleic acid binding surface and dimer interface revealed by CRISPR-associated CasB protein structures

The CRISPR system is an adaptive RNA-based microbial immune system against invasive genetic elements. CasB is an essential protein component in Type I-E Cascade. Here, we characterize CasB proteins from three different organisms as non-specific nucleic acid binding proteins. The Thermobifida fusca CasB crystal structure reveals conserved positive surface charges, which we show are important for its nucleic acid binding function. EM docking reveals that CasB dimerization aligns individual nucleic acid binding surfaces into a curved, elongated binding surface inside Type I-E Cascade, consistent with the putative functions of CasB in ds-DNA recruitment and crRNA–DNA duplex formation steps.Structured summary of protein interactionsTthCasB and TthCasBbind by x-ray crystallography (View interaction)TfuCasB1 and TfuCasB1bind by molecular sieving (View Interaction: 1, 2)

► CRISPR-associated CasB is characterized as nucleic acid binding protein. ► Crystal structure of T. fusca CasB is determined at 1.9 Å. ► Nucleic acid binding sites are identified by mutagenesis. ► Docking of CasB into EM density of Cascade provides the functional formation. ► Potential molecular function of CasB in Cascade is proposed.