The CRISPR-Cas systems can be divided into six types (Types I-VI) according to different interference mechanisms. Type III CRISPR-Cas system can be classified into two main subtypes: Type III-A and Type III-B.  For Type III-A system, Csm (1-5) subunits assemble with crRNA to form an active effector complex, which can both degrade RNA and ssDNA.

Type III effector complexes were recently demonstrated to synthesize a novel second messenger, cyclic oligonucleotide, for activating the nonspecific RNA degradation activity of Csm6/Csx1. To date, only low resolution electron microscopy (EM) structures of Csm effector complexes from different species have been reported because of the lack of the atomic resolution structure.

In a recent study published in Cell Research, Prof. JIANG Tao's group at the Institute of Biophysics of Chinese Academy of Sciences determined the atomic resolution Cryo-EM structure of Type III-A CRISPR-Cas effector complex.

Rsearchers found that the ToCsm effector complex is arranged in a ‘boot’ shape with the stoichiometry of Csm1₁2₁3₂4₁5₁:crRNA, significantly smaller than previously reported Type III-A effectors.

Csm1 is located at the body of the boot and interacts with Csm4 and Csm2. The C-terminus of Csm1 forms a six-helix bundle, interacting with Csm2. Above Csm4, two Csm3 subunits and Csm5 are arranged along the tube of the boot, forming nearly a double-helical backbone.

A 24-nt crRNA single strand was constructed running throughout the whole complex. The thumb-like β-hairpin of Csm4, Csm3.1 and Csm3.2 induce the protrusion of nucleotides from the backbone at the 8^th nt, the 14^th nt and the 20^th nt, resulting in 6-nt intervals in the crRNA, indicating the degraded site, which was confirmed by in vitro assay.

Besides, they solved the crystal structure of ToCsm1 binding two ATP molecules at 1.69 angstrom resolution.

Taken together, the study showed the first atomic resolution structure of Type III-A CRISPR-Cas effector complex, which provides detailed structural basis for functional understanding.

On the other hand, previous studies have shown that Type III-A CRISPR effector complex can accommodate different crRNA in length by altering the number of Csm3 and Csm2. In this structure, there are only two Csm3 and one Csm2, thus this is the smallest Csm effector complex reported to date. This may be beneficial for modifying ToCsm effector complex into an effective gene-editing tool.

In bacteria and archaea, CRISPR-Cas proteins constitute a crRNA-guided surveillance complex that interferes with invading nucleic elements. Up to now, several CRISPR-Cas systems have been developed and applied for gene editing.