Structural insights into the CRISPR-Cas-associated ribonuclease activity of Staphylococcus epidermidis Csm3 and Csm6

The Clustered Regularly Interspaced Short Palindromic Repeats(CRISPR)-CRISPR-associated(Cas) system is an adaptive immune system in bacteria and archaea that resists exogenous invasion through nucleic acid-mediated cleavage. In the type III-A system, the Csm complex contains five effectors and a CRISPR RNA, which edits both single stranded RNA and double stranded DNA. It has recently been demonstrated that cyclic oligoadenylates(cOAs), which are synthesized by the Csm complex, act as second messengers that bind and activate Csm6. Here, we report the crystal structures of Staphylococcus epidermidis Csm3(SeCsm3) and an N-terminally truncated Csm6(SeCsm6 AN) at 2.26 and 2.0 , respectively. The structure of SeCsm3 highly resembled previously reported Csm3 structures from other species; however, it provided novel observations allowing further enzyme characterization. The homodimeric SeCsm6 AN folds into a compact structure. The dimerization of the HEPN domain leads to the formation of the ribonuclease active site, which is consistent with the reported Csm6 structures. Altogether, our studies provide a structural view of the ribonuclease activity mediated by Csm3 and Csm6 of the type Ⅲ-A CRISPR-Cas system.