DNA replication licensing factor MCM4 is a component of the MCM2-MCM7 complex which consists of six sequence-related AAA+ type ATPases/helicases that form a heterohexameric ring essential for 'once per cell cycle' DNA replication initiation and elongation in eukaryotic cells [1-3]. This is the winged helix domain (WHD) found at the C-terminal of MCM4 [1-3]. Upon ssDNA binding the MCM5-WHD moves out of the central channel and becomes disordered, the MCM6-WHD moves backward and away from the channel, and the MCM4-WHD now partially occludes the channel opening. These changes enable CMG (Cdc45, Mcm2-7, GINS, the eukaryotic replicative helicase) to encircle dsDNA, and is consistent with reports that showed the MCM2-7 matures into two fully assembled CMGs on dsDNA before origin DNA unwinding occurs [3].

1: Structure of the eukaryotic MCM complex at 3.8 A. Li N, Zhai Y, Zhang Y, Li W, Yang M, Lei J, Tye BK, Gao N; Nature. 2015;524:186-191. PMID:26222030

2: Structure of the eukaryotic replicative CMG helicase suggests a pumpjack motion for translocation. Yuan Z, Bai L, Sun J, Georgescu R, Liu J, O'Donnell ME, Li H; Nat Struct Mol Biol. 2016;23:217-224. PMID:26854665

3: Structure of eukaryotic CMG helicase at a replication fork and implications to replisome architecture and origin initiation. Georgescu R, Yuan Z, Bai L, de Luna Almeida Santos R, Sun J, Zhang D, Yurieva O, Li H, O'Donnell ME; Proc Natl Acad Sci U S A. 2017;114:E697. PMID:28096349