Csm1Cas10

Csm1, also known as Cas10, is the large subunit of the Type III CRISPR-Cas effector complexes, including the Csm complex of Type III-A systems and the Cmr complex of Type III-B systems. These effector assemblies use a CRISPR-derived RNA guide to recognize complementary target RNA. Csm1 is the central scaffolding protein that interacts with other subunits such as Csm2, Csm3, Csm4, and Csm5 to form a ribonucleoprotein complex bound to a CRISPR RNA (crRNA).

Structurally, Cas10/Csm1 is a multi-domain protein, typically containing an N-terminal HD nuclease domain and a palm-domain region capable of synthesizing cyclic oligoadenylates (cOA) when the complex binds a target RNA. The cOA signal acts as a second messenger to activate auxiliary ribonucleases such as Csm6, enabling a robust antiviral state.

Functionally, upon recognition of an RNA target complementary to the crRNA, the Cas10-containing complex cleaves the target RNA and stimulates cOA production. The resulting cOA molecules recruit and activate non-Cas nucleases, broadening the immune response. In some organisms, Cas10 may contribute to DNA degradation via its HD domain, but in Type III systems DNA cleavage is not required for interference and this activity is not universally observed.

Distribution and significance: Csm1/Cas10 is found in bacteria and archaea that harbor Type III CRISPR-Ca systems. It has been studied to understand how Type III effector complexes coordinate RNA targeting with cOA signaling, and to explore potential biotechnological applications involving programmable RNA responses or diagnostic tools.