The secret of a fast base editor
CRISPR-Cas9 base editors contain Cas proteins bound to RNA fused to an enzyme that can deaminate the nucleoside DNA. No natural enzyme deaminates adenine in DNA, and so a breakthrough occurred when natural transfer RNA deaminase was fused to Cas9 and developed to form an adenine base editor (ABE) that works on DNA. Further development has provided the enzyme ABE8e, which catalyzes deamination more than 1000 times faster than early ABE. Lapinaite et al. now represent the 3,2-angstromic DNA-bound distinguishing structure of ABE8e, in which the target adenine is replaced by an analog designed to capture the catalytic conformation. The structure, together with the kinetic data comparing ABE8e with previous ABEs, explains how ABE8e edits DNA bases and could inform about the future design of the base editor.
science, this edition with. 566
CRISPR-Cas-led editors convert A • T to G • C or C • G to T • A in cellular DNA for accurate genome editing. To understand the molecular basis for DNA adenosine deamination by adenine base (ABE) editors, we determined a cryo-electron microscopic structure with a resolution of 3.2 angstroms of ABE8e in a substrate-bound state in which the deaminase domain intervenes DNA exposed in CRISPR-Cas9. R-loop complex. Kinetic and structural data suggest that ABE8e catalyzes DNA deamination up to 1100100-fold faster than previous ABEs due to mutations that stabilize DNA substrates in a restricted RNA-like conformation. The accelerated DNA deamination of ABE8e further suggests a previously unrecognized transient melting of DNA that may occur during double-stranded DNA monitoring with CRISPR-Cas9. These results explain the results of ABE8e-mediated database editing and inform about the future design of database editors.