Crystal structure of Middle East respiratory syndrome coronavirus helicase

Hao, Wei; Wojdyla, Justyna Aleksandra; Zhao, Rong; Han, Ruiyun; Das, Rajat; Zlatev, Ivan; Manoharan, Muthiah; Wang, Meitian; Cui, Sheng

The CH domain of MERS-CoV nsp13 (residues 1-112) is a compact domain with three zincbinding motifs stabilizing the fold ( Fig 3A) . The CH domain contains an N-terminal RINGlike module (1-46aa, β1-5 and α1) with two zinc fingers and C-terminal RING-like module with single treble-clef zinc finger (47-87aa, β6-8). A long loop (88-100aa, between β8-α2) spanning across the entire height of CH domain connects zinc coordination modules. The first zinc (Zn1) is coordinated within a CCCC type treble-clef zinc finger with four cysteine ligands (Cys5, Cys8 Cys26 and Cys29) ( Fig 3B) . Cys5 and Cys8 are located on a zinc knuckle between β1-β2 strands, whereas Cys26 and Cys29 are placed in the N-terminal region of α1 helix. The second zinc (Zn2) is coordinated by a two-Cys, two-His (C2H2) zinc finger motif. The cysteine ligands (Cys16 and Cys19) are forming part of a zinc knuckle between β3-β4 strands, while histidine ligands His33 and His39 are located on α1 helix and the loop between α1 and β5, respectively. The third zinc (Zn3) is coordinated by the C-terminal treble-clef zinc finger motif (Fig 3C) . A zinc-knuckle on a hairpin loop between β5 and β6 strands provides Cys50 and Cys55 for Zn3 coordination, whereas Cys72 from the C-terminal region of β7 strand and His75 from the loop between β7 and β8 strands provide two more ligands for the zinc. All Cys/His involved in zinc coordination are invariant in CoVs (Fig 4C) , indicating the zinc binding is essential for the structure and function of nsp13.