Structure of Csx1-cOA₄ complex reveals the basis of RNA decay in Type III-B CRISPR-Cas

Area of Science:

• Molecular Biology
• Biochemistry
• Structural Biology

Background:

• Type III CRISPR-Cas systems provide immunity against foreign genetic elements.
• These systems generate cyclic oligoadenylate (cOA) second messengers.
• cOA activates CRISPR-associated RNases, such as those in the Csm/Csx families, leading to RNA decay.

Purpose of the Study:

• To elucidate the structural basis of RNase activation by cOA in Type III CRISPR-Cas systems.
• To present the structure of the Sulfolobus islandicus (Sis) Csx1-cOA4 complex.
• To understand the mechanism of allosteric activation of RNase activity.

Main Methods:

• X-ray crystallography
• Biochemical assays
• Structural analysis of protein-nucleic acid interactions

Main Results:

• The SisCsx1 complex is a hexamer composed of a trimer of dimers.
• Each dimer contains a binding site for cOA4 and a catalytic pocket for ssRNA.
• cOA4 binding induces a conformational change, allosterically activating ssRNA degradation via an HTH domain.
• The RNase exhibits sequential, cooperative hydrolysis of phosphodiester bonds at 5'-C-C-3' motifs.
• Ring nucleases degrade cOA4, leading to deactivation of SisCsx1.

Conclusions:

• The structure reveals the allosteric mechanism of RNase activation by cOA4 in SisCsx1.
• This activation involves conformational changes transmitted through an HTH domain.
• The enzyme's activity is regulated by cOA4 degradation, suggesting potential biotechnological applications.