Structural features of Cas2 from Thermococcus onnurineus in CRISPR-cas system type IV.

CRISPR-Cas is RNA-based prokaryotic immune systems that defend against exogenous genetic elements such as plasmids and viruses. Cas1 and Cas2 are highly conserved components that play an essential part in the adaptation stage of all CRISPR-Cas systems. Characterization of CRISPR-Cas genes in Thermococcus onnurineus reveals the association of the Cas2 gene with the putative type IV system that lacks Cas1 or its homologous genes. Here, we present a crystal structure of T. onnurineus Cas2 (Ton_Cas2) that exhibits a deep and wide cleft at an interface lined with positive residues (Arg16, Lys18, Lys19, Arg22, and Arg23). The obvious DNA recognizing loops in Cas2 from E. coli (Eco_Cas2) are absent in Ton_Cas2 and have significantly different shapes and electrostatic potential distributions around the putative nucleotide binding region. Furthermore, Ton_Cas2 lacks the hairpin motif at the C-terminus that is responsible for Cas1 binding in Eco_Cas2. These structural features could be a unique signature and indicate an altered functional mechanism in the adaptation stage of Cas2 in type IV CRISPR-Cas systems.