PMID: 29880725

Authors:

Xiao Y, Luo M, Dolan AE, Liao M, Ke A

Title:

Structure basis for RNA-guided DNA degradation by Cascade and Cas3.

Journal:

Science. 2018 Jun 7. pii: science.aat0839. doi: 10.1126/science.aat0839.

Abstract:

Type I CRISPR-Cas system features a sequential target-searching and degradation process on double-stranded DNA, by the RNA-guided Cascade complex and the nuclease-helicase fusion enzyme Cas3, respectively. Here we present a 3.7 A resolution cryo-EM structure of the Type I-E Cascade/R-loop/Cas3 complex, poised to initiate DNA degradation. Cas3 distinguishes Cascade conformations and only captures the R-loop forming Cascade, to avoid cleaving partially complementary targets. Its nuclease domain recruits the non-target strand (NTS) DNA at a bulged region for the nicking of single-stranded DNA. An additional 4.7 A resolution cryo-EM structure captures the post-nicking state, in which the severed NTS retracts to the helicase entrance, to be threaded for ATP-dependent processive degradation. These snapshots form the basis for understanding RNA-guided DNA degradation in Type I-E CRISPR-Cas systems.