Structures of the Cmr-β Complex Reveal the Regulation of the Immunity Mechanism of Type III-B CRISPR-Cas

Abstract

AbstractCmr-β is a Type III-B CRISPR-Cas complex that upon target RNA recognition unleashes a multifaceted immune response against invading genetic elements, including ssDNA cleavage, cyclic oligoadenylate synthesis, and also a unique UA-specific ssRNA hydrolysis by the Cmr2 subunit. Here, we present the structure-function relationship of Cmr-β unveiling how binding of the target RNA regulates the Cmr2 activities. CryoEM analysis revealed the unique subunit architecture of Cmr-β and captured the complex in different conformational stages of the immune response, including the non-cognate and cognate target-RNA bound complexes. The binding of the target RNA induces a conformational change of Cmr2, which together with the complementation between the 5’-handle in the crRNA and the 3’-antitag of the target RNA, activate different configurations in a unique loop of the Cmr3 subunit, which acts as an allosteric sensor signaling the self vs. non-self recognition. These findings highlight the diverse defense strategies of Type III complexes.