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 nucleotide-binding pocket of MERS-CoV nsp13 is located between RecA1 and RecA2 domains. The RecA1 (241-443aa) contains a seven-stranded parallel β-sheet sandwiched by two α-helices located near the Stalk domain on one side and three α-helices on the opposite side. RecA2 (444-596aa) has a five-stranded parallel β-sheet surrounded by four helices on one side and three helices on the other side. Seven helicase motifs conserved in SF1/SF2 families are located in the cleft between RecA1-RecA2. RecA1 contains motifs I, Ia, II and III, whereas RecA2 includes motifs IV, V and VI (Fig 3E) . Sulfate, crystallization condition precipitant, was found bound to the P-loop mimicking binding of the NTP's phosphate moiety. Residues Gln404, Arg443 and Arg567 from helicase motifs III, IV and VI form hydrogen bonds with the sulfate suggesting their involvement in NTP hydrolysis. The corresponding residues in human Upf1 helicase are Gln665, Arg703 and Arg865 [24] , while in EAV nsp10 Gln267, Arg296 and Arg381 [25] . Residues Arg865 and Gln665 of Upf1 helicase act as the "arginine finger" and "γ-phosphate sensor" during ATP hydrolysis [24] , suggesting that MERS-CoV nsp13 Gln404 and Arg567 have the same function. Moreover, Tyr442 of MERS-CoV nsp13 is structurally equivalent to Tyr702 of Upf1, which stabilizes adenosine base (Fig 3E) .