Csm2

Csm2 is a protein component of the type III CRISPR-Cas interference system found in some bacteria and archaea. In these organisms, type III CRISPR-Cas systems form multi-subunit effector complexes, often referred to as Csm (Cas10–Csm) complexes, that use a CRISPR RNA (crRNA) to recognize complementary transcripts from invading genetic elements. Csm2 is one of several conserved subunits that assemble with Cas10 (Csm1), Csm3, Csm4, and Csm5 to form the functional complex.

The precise function of Csm2 is not fully resolved, but it is generally considered to contribute to the proper assembly and stability of the effector complex and to influence the positioning of crRNA within the complex. Biochemical and structural studies indicate that Csm2 interacts with neighboring subunits, such as Csm3, helping to organize the RNA-binding surface that enables target recognition. While Csm2 itself does not carry the catalytic activity associated with interference, its presence supports the overall architecture required for target RNA sensing by the complex.

Mechanistically, type III Csm complexes recognize transcripts that are complementary to the crRNA, triggering downstream activities primarily driven by Cas10. Activation can lead to transcription-dependent interference and, in many systems, production of signaling molecules that recruit auxiliary RNases, contributing to the degradation of invading RNA and, in some contexts, DNA interference. Csm2’s contribution is therefore linked to proper complex assembly and functional coordination rather than direct nuclease activity.

Csm2 is encoded within csm gene clusters and is present in a subset of bacteria and archaea that harbor type III CRISPR-Cas systems. Its exact conservation and roles may vary among species.