Steric exclusion and wrapping of the excluded DNA strand occurs along discrete external binding paths during MCM helicase unwinding

The minichromosome maintenance (MCM) helicase complex is essential for the initiation and elongation of DNA replication in both the eukaryotic and archaeal domains. The archaeal homohexameric MCM helicase from Sulfolobus solfataricus serves as a model for understanding mechanisms of DNA unwinding. In this report, the displaced 5'-tail is shown to provide stability to the MCM complex on DNA and contribute to unwinding. Mutations in a positively charged patch on the exterior surface of the MCM hexamer destabilize this interaction, alter the path of the displaced 5'-tail DNA and reduce unwinding. DNA footprinting and single-molecule fluorescence experiments support a previously unrecognized wrapping of the 5'-tail. This mode of hexameric helicase DNA unwinding is termed the steric exclusion and wrapping (SEW) model, where the 3'-tail is encircled by the helicase while the displaced 5'-tail wraps around defined paths on the exterior of the helicase. The novel wrapping mechanism stabilizes the MCM complex in a positive unwinding mode, protects the displaced single-stranded DNA tail and prevents reannealing.