Abstract
The adaptive immune function of CRISPR-CRISPR-associated protein (Cas) systems in bacteria and archaea is mediated through Cas. The adaptation module, typically involving Cas1, Cas2, and Cas4, helps integrate viral "spacer" sequences into the host genome. Cas4 proteins are classified into two types based on neighboring genes: CAS-Cas4, flanked by other cas genes, and Solo-Cas4, which exists independently. While CAS-Cas4 proteins are implicated in adaptation, they remain biochemically uncharacterized in archaea, unlike archaeal Solo-Cas4 proteins. This study biochemically characterizes TON_0321, a CAS-Cas4 protein from the type IV-C CRISPR cassette of Thermococcus onnurineus. TON_0321 exhibits 5' to 3' exonuclease activity and unique structure-dependent endonuclease activity, shedding light on CAS-Cas4 functional diversity. A distinct spatial organization of the catalytic site, angled with the positively charged patch on the protein surface, enables TON_0321 to recognize branching points in DNA substrates. Furthermore, this spatial arrangement facilitates cleavage 2 to 3 nucleotides away from the branch point in the 5' direction, demonstrating structure-specific endonuclease activity.