Structure of Csm2 elucidates the relationship between small subunits of CRISPR-Cas effector complexes

Abstract

Type I and type III CRISPR-Cas effector complexes share similar architecture and have homologous key subunits. However, the relationship between the so-called small subunits of these complexes remains a contentious issue. Here, it is shown that the recently solved structure of Thermotoga maritima Csm2 represents a dimer with the extensive structure swapping between monomers. Unswapping the structure generates a compact globular monomer which shares similar structure and surface properties with Cmr5, the small subunit of a related Cmr complex. Detailed analysis of available structures of small subunits reveals that they all have a common fold suggesting their common origin.