This entry represents the Csn2 superfamily of Cas proteins, which are found only in CRISPR-containing species, near other CRISPR-associated proteins (cas), as part of the NMENI subtype of CRISPR/Cas loci. The species range so far for this subtype is animal pathogens and commensals only. This protein is present in some but not all NMENI CRISPR/Cas loci [].The structure of Csn2 contains two domains: an alpha/beta domain and an α-helical domain with significant hinge motion between these two domains []. Sn2 is a ds-DNA-binding protein functioning at the quaternary structure level and regulated by Ca(2+) ions [].
This 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.Proteins in this family are usually longer than the canonical Csn2 () 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 [].
The CRISPR-Cas system is a prokaryotic defense mechanism against foreign genetic elements. The key elements of this defense system are the Cas proteins and the CRISPR RNA. Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) are a family of DNA direct repeats separated by regularly sized non-repetitive spacer sequences that are found in most bacterial and archaeal genomes []. CRISPRs appear to provide acquired resistance against mobile genetic elements (viruses, transposable elements and conjugative plasmids). CRISPR clusters contain sequences complementary to antecedent mobile elements and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA).The defense reaction is divided into three stages. In the adaptation stage, the invader DNA is cleaved, and a piece of it is selected to be integrated as a new spacer into the CRISPR locus, where it is stored as an identity tag for future attacks by this invader. During the second stage (the expression stage), the CRISPR RNA (pre-crRNA) is transcribed and subsequently processed into the mature crRNAs. In the third stage (the interference stage), Cas proteins, together with crRNAs, identify and degrade the invader [, , ].The CRISPR-Cas systems have been sorted into three major classes. In CRISPR-Cas types I and III, the mature crRNA is generally generated by a member of the Cas6 protein family. Whereas in system III the Cas6 protein acts alone, in some class I systems it is part of a complex of Cas proteins known as Cascade (CRISPR-associated complex for antiviral defense). The Cas6 protein is an endoribonuclease necessary for crRNA production whereas the additional Cas proteins that form the Cascade complex are needed for crRNA stability []. This entry represents the Csn2 family of Cas proteins, which are found only in CRISPR-containing species, near other CRISPR-associated proteins (cas), as part of the NMENI subtype of CRISPR/Cas loci. The species range so far for this subtype is animal pathogens and commensals only. This protein is present in some but not all NMENI CRISPR/Cas loci.
