Crystal structure of a CRISPR RNA-guided surveillance complex, Cascade, bound to a ssDNA target[1]
The . The body is formed by a helical filament of six Cas7 subunits (Cas7.1 to 7.6) wrapped around the crRNA guide, with a head-to-tail dimer of Cse2 (Cse2.1 and Cse2.2) at the belly. Cas6e and the 3′ handle of crRNA cap the Cas7 filament at the head while Cas5 and the 5′ handle cap the tail. The N-terminal base of Cse1 is positioned at the tail of the filament; the C-terminal four-helix bundle contacts Cse2.2. The ssDNA target is juxtaposed to the guide region of the crRNA in a groove formed by the Cas7 filament, the four-helix bundle of Cse1, and the Cse2 dimer.
The do not twist around one another in a helix, but instead adopt an underwound ribbon-like structure reminiscent of a ladder. The 5′ and 3′ ends of the curved target strand are ~102Å apart, roughly the length of straight B-form dsDNA with an identical sequence (~107 Å). The crRNA (green) and ssDNA target (orange) are displayed in a spheres representation. Underwinding is facilitated by (ribbon representation of the crRNA and ssDNA). . Disrupted RNA and DNA nucleotides are colored red and blue, respectively.
Structure of the Cas7 subunit.
Within Cascade, the , with a pitch of ~135Å, around the guide target hybrid. The filament is arranged such that the . .
Stabilization of the guide-target hybrid by Cas7, Cse1, and Cse2.
. 5-bp segment is colored red. Extensive contacts between the guide region of the crRNA and the Cas7 filament bury a large portion of the crRNA backbone, leaving the bases solvent-exposed. The absence of direct contacts between protein side chains and bases of the crRNA explains the lack of sequence specificity by Cascade for the guide sequence. . The DNA target has been removed for clarity. Intercalation by Met166 from Cas7 is highlighted.
