Clustered, regularly interspaced, short palindromic repeat (CRISPR) loci play a pivotal role in the prokaryotic host defense system against invading genetic materials. The CRISPR loci are transcribed to produce CRISPR RNAs (crRNAs), which form interference complexes with CRISPR-associated (Cas) proteins to target the invading nucleic acid for degradation. The interference complex of the type III-A CRISPR-Cas system is composed of five Cas proteins (Csm1-Csm5) and a crRNA, and targets invading DNA. This entry represents the C-terminal domain found in Csm4. Csm4 structurally resembles Cmr3, a component of the type III-B CRISPR-Cas interference complex. Studies indicate that Csm3-Csm4 complex binds single-stranded RNA in a non-sequence-specific manner. Structural analysis show , Csm3 and Csm4 have one and two ferredoxin-like folds (also known as an RRM-like fold), respectively. The long beta-hairpin inserted into the C-terminal ferredoxin-like fold of Csm4, is well-conserved in the Cmr3 structure. The corresponding beta-hairpin of Cmr3 binds the D1 domain of Cmr2, as observed in the Cmr2-Cmr3 complex structure. Furthermore, it is suggested that the hairpin of Csm4 is responsible for the interaction with Csm1 (ortholog of Cmr2) [1].

1: Crystal structure of the Csm3-Csm4 subcomplex in the type III-A CRISPR-Cas interference complex. Numata T, Inanaga H, Sato C, Osawa T; J Mol Biol. 2015;427:259-273. PMID:25451598