Cas_St_Csn2 is a family of Csn2 CRISPR-associated (Cas) proteins found in Firmicutes, largely Streptococcus and Enterococcus. CRISPR-associated (Cas) proteins are the main executioners of the process whereby prokaryotes acquire immunity against foreign genetic material. Cas allow short segments of this DNA, called spacer, to become incorporated into chromosomal loci as clustered regularly interspaced short palindromic repeats or CRISPRs; the resulting encoded RNAs are then processed into small fragments that guide the silencing of the invading genetic elements. Thus Cas are involved in the acquisition of new spacers. This family of St_Csn2 is longer than the canonical Csn2, Pfam:PF09711 through the addition of a large C-terminal domain. The central domain present in both families appears to be a channel that selectively interacts with dsDNA [1].

1: Identification, structural, and biochemical characterization of a group of large Csn2 proteins involved in CRISPR-mediated bacterial immunity. Lee KH, Lee SG, Eun Lee K, Jeon H, Robinson H, Oh BH; Proteins. 2012;80:2573-2582. PMID:22753072