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. 2015 Apr 30;39(3):442–463. doi: 10.1093/femsre/fuv019

Figure 2.

Figure 2.

Assembly of the type I-E Cascade structure. The I-E Cascade complex has a seahorse-shaped structure and consists of 11 protein subunits ((Cse1)1-(Cse2)2-(Cas5)1-(Cas7)6-(Cas6e)1) and a single 61-nt crRNA (pdb: 4TVX). Cas6e is tightly bound to the 3′ stem-loop structure of the mature crRNA and positioned at the head of the complex. Cas5e directly caps the 5′ handle of the crRNA, which leads to the hook-like structure of the crRNA. The structure of Cas5 and Cas7 displays a conserved palm-thumb domain arrangement, highlighting the intertwined assembly of the Cascade backbone. The thumb of either Cas5 or each of the six Cas7 subunits (Cas7.1-Cas7.6) kinks the crRNA at position −1 in the 5′ handle and every sixth position in the spacer sequence, and buries the base between the thumb and the palm of the adjacent Cas7 subunit. The two small Cse2 subunits (Cse2.1–Cse2.2) are connected to the crRNA backbone via protein:protein interactions to the Cas7 subunits. The large subunit Cse1 is positioned at the Cascade tail and interacts with Cas5, Cas7 and Cse2.