Mechanistic insights into how CMG helicase facilitates replication past DNA roadblocks

Abstract

Before leaving the house, it is a good idea to check for road closures that may affect the morning commute. Otherwise, one may encounter significant delays arriving at the destination. While this is commonly true, motorists may be able to consult a live interactive traffic map and pick an alternate route or detour to avoid being late. However, this is not the case if one needs to catch the train which follows a single track to the terminus; if something blocks the track, there is a delay. Such is the case for the DNA replisome responsible for copying the genetic information that provides the recipe of life. When the replication machinery encounters a DNA roadblock, the outcome can be devastating if the obstacle is not overcome in an efficient manner. Fortunately, the cell's DNA synthesis apparatus can bypass certain DNA obstructions, but the mechanism(s) are still poorly understood. Very recently, two papers from the O'Donnell lab, one structural (Georgescu et al., 2017 [1]) and the other biochemical (Langston and O'Donnell, 2017 [2]), have challenged the conventional thinking of how the replicative CMG helicase is arranged on DNA, unwinds double-stranded DNA, and handles barricades in its path. These new findings raise important questions in the search for mechanistic insights into how DNA is copied, particularly when the replication machinery encounters a roadblock.

Keywords: CMG; DNA damage; DNA replication; Helicase; Interstrand cross-link; MCM.

Figure 1.. Models of hexameric helicase activation and unwinding.

A double hexamer MCM complex forms with the N-terminal domain (NTD) head to head at the origin of replication where single-stranded DNA selection and unwinding can proceed either with the A) NTD or B) C-terminal domain (CTD) leading the way. During active unwinding, the MCM helicase proceeds in the 3′-5′ direction on the translocating strand C) excluding single-stranded DNA from the central channel that can D) interact on the exterior surface to control the unwinding rate and efficiency. E) A modification of these models includes initial double-stranded DNA separation occurring within the central channel.

Figure 2.. ICL lesion bypass by a CMG helicase in a unidirectional manner.

A) CMG converts from a single-stranded DNA translocase to a double-stranded DNA translocase as it passes the ICL. B) CMG helicase opens its ring upon encounter with the ICL, and then closes again once the ICL is traversed. C) An accessory DNA helicase (triangle) may promote CMG bypass of the ICL.

Figure 3.. ICL lesion bypass by CMG helicases at converging forks.

p97 and BRCA1, together with leading strand DNA synthesis, promote CMG helicases to unload at the converging fork. Reloading occurs on the distal side of the ICL.