Secrets of a Quick Base Editor
The CRISPR-Cas9 base editors contain Cas RNA-directed proteins fused to an enzyme that can deamine a DNA nucleoside. No natural enzyme deaminates adenine in DNA, and so a breakthrough was made when a transfer RNA dezinase was joined to Cas9 and evolved to provide an adenine base (ABE) editor that works on DNA. Further evolution provided the enzyme ABE8e, which catalyzes deamination more than 1000 times faster than early ABEs. Lapinaite et al. now introduce a DNA-bound ABE8e 3.2-angstrom structure in which the target adenine is replaced with an analogue created to block catalytic conformance. The structure, along with the kinetic data comparing ABE8e with previous ABEs, explain how ABE8e edits the bases of DNA and can inform the future model of the basic editor.
science, this issue p. 566
CRISPR-led database editors convert A • T to G • C, or C • G to T • A, to cellular DNA for accurate genome editing. To understand the molecular basis for DNA adenosine deamination by adenine-based (ABE) editors, we defined a 3.2 angstrom microscopic structure of ABE8e in a substrate-bound state in which the field of deaminase engages DNA exposed within CRISPR-Cas9 R-loop linkage complex. Kinetic and structural data suggest that ABE8e catalyzes DNA deamination up to 1100-fold faster than previous ABEs due to mutations that stabilize DNA substrates in a constraint, transferring RNA-like conformity. Furthermore, the accelerated deamination of ABE8e DNA suggests a previous previously uncontrolled transient DNA fusion that may occur during double-level DNA surveillance by CRISPR-Cas9. These results explain the results of ABE8e-mediated database editing and inform the future model of basic editors.