This is the C-temrinal domain of Cas3 proteins. The C-terminal domain (CTD) is shown to completely wrap ssDNA inside the helicase. Deletion of the CTD (aa 819-924) reduced CRISPR interference. It is suggested that the CTD regulates the N-terminal HD nuclease activity by functioning as a substrate filter [1].

1: Structures of CRISPR Cas3 offer mechanistic insights into Cascade-activated DNA unwinding and degradation. Huo Y, Nam KH, Ding F, Lee H, Wu L, Xiao Y, Farchione MD Jr, Zhou S, Rajashankar K, Kurinov I, Zhang R, Ke A; Nat Struct Mol Biol. 2014;21:771-777. PMID:25132177

2: Molecular insights into DNA interference by CRISPR-associated nuclease-helicase Cas3. Gong B, Shin M, Sun J, Jung CH, Bolt EL, van der Oost J, Kim JS; Proc Natl Acad Sci U S A. 2014;111:16359-16364. PMID:25368186