Bridge helix of Cas12a is an allosteric regulator of R-loop formation and RuvC activation

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• CRISPR-Cas12a is a versatile RNA-guided DNA endonuclease used in genome editing and diagnostics.
• Off-target DNA cleavage by Cas12a necessitates strategies for enhanced specificity.
• A previously developed Francisella novicida Cas12a variant (FnCas12a^KD2P) with bridge helix (BH) mutations showed reduced off-target activity.

Purpose of the Study:

• To elucidate the molecular mechanisms underlying bridge helix (BH)-mediated activation of Cas12a for DNA cleavage.
• To understand how specific mutations in the BH influence Cas12a conformational dynamics and DNA binding.
• To compare the structural diversity of the BH in Cas12 and Cas9 enzyme families.

Main Methods:

• Combinatorial approach and structural biology (crystallography) to capture different activation states of FnCas12a^KD2P.
• Biochemical activity assays to quantify DNA cleavage efficiency and specificity.
• Computational simulations to analyze conformational changes and allosteric signaling pathways.

Main Results:

• Five distinct structural states of FnCas12a^KD2P revealed the loop-to-helical transition and bending of the BH during activation.
• This BH conformational change acts as an allosteric trigger for RNA-DNA hybrid propagation and subsequent DNA cleavage.
• The BH transition is coupled to RuvC motif-II remodeling and REC lobe movements, facilitating DNA access to the active site.

Conclusions:

• The bridge helix (BH) plays a critical allosteric role in Cas12a DNA cleavage activation through conformational transitions.
• Understanding BH mechanics provides insights into Cas12a specificity and potential for engineering improved genome editing tools.
• Structural comparisons highlight conserved and divergent mechanisms across Cas12 and Cas9 nucleases.