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Alessandro Costa : Structure and dynamics of the MCM motor of the replicative helicase

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The MCM is a ring-shaped complex formed of six homologous polypeptides sharing the same domain organization. They contain an N-terminal domain forming a collar that encircles DNA, and an ATPase motor domain belonging to the superfamily of AAA+ ATPases.

The MCM is functionally asymmetric with only certain ATPase sites being required for DNA translocation. MCM is loaded onto origin DNA in a process that requires ATP hydrolysis and depends on a set of loading factors named ORC, Cdc6 and Cdt1. Using yeast proteins, we have established that Cdt1 stabilises an open configuration of MCM allowing helicase loading onto DNA.

We determined the structure of the inactive MCM double hexamer loaded onto origin DNA, showing that helicase loading alone does not result in melting of the DNA double helix. We also determined the structure of the activated Cdc45-MCM-GINS (CMG) complex, revealing the conformational changes in MCM that lead to activation of the helicase motor. In the structure of the CMG bound to a DNA fork, we found that the ATPase domain of MCM interacts with single-stranded DNA, not duplex DNA.

We now want to establish how the CMG employs the energy derived from ATP hydrolysis to promote DNA translocation.

 

Structure of the inactive MCM double hexamer loaded onto duplex DNA

Figure 1: Structure of the inactive MCM double hexamer loaded onto duplex DNA

Structure of the active CMG helicase bound to DNA

Figure 2: Structure of the active CMG helicase bound to DNA.