Mechanistic and evolutionary insights into a type V-M CRISPR-Cas effector enzyme

Area of Science:

• Molecular Biology
• Microbiology
• Genetics

Background:

• RNA-guided CRISPR-Cas12 effectors confer adaptive immunity in prokaryotes.
• Cas12m2, a compact type V effector, possesses a unique RuvC active site but lacks dsDNA cleavage ability.
• Its mechanism for RNA-guided genome inactivation and protection against mobile genetic elements (MGEs) was previously unknown.

Purpose of the Study:

• To elucidate the molecular mechanism of RNA-guided genome inactivation by the Cas12m2 effector.
• To determine the structural basis for Cas12m2's strong DNA-binding affinity and its role in adaptive immunity.
• To understand the evolutionary relationship between Cas12m2 and its putative ancestor, TnpB.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine the structures of Cas12m2 complexes.
• Structures of the Cas12m2-crRNA binary complex and two states of Cas12m2-crRNA-target DNA ternary complexes were resolved.
• Comparative structural analysis was performed between Cas12m2 and TnpB.

Main Results:

• Cryo-EM structures revealed the dynamic process of crRNA-target DNA heteroduplex formation.
• Cas12m2 exhibits strong DNA binding due to interactions with an arginine-rich cluster and its non-canonical RuvC active site.
• The REC2 insertion in Cas12m2 plays a critical role in binding the protospacer-adjacent motif-distal region, essential for adaptive immunity.

Conclusions:

• Cas12m2 employs a unique mechanism of transcriptional silencing via strong DNA binding for adaptive immunity.
• Structural insights clarify the function of Cas12m2's non-canonical active site and its interaction with target DNA.
• Findings contribute to understanding type V CRISPR-Cas systems and the evolution from TnpB to Cas12 enzymes.