Mechanism of DNA unwinding by MCM8-9 in complex with HROB

In a new study published in "Nature Communications" the Cejka laboratory and collaborators identified the molecular mechanisms by which the HROB protein enhances the DNA unwinding function of the MCM8-9 helicase, crucial for DNA repair and replication processes.

The MCM8-9 helicase and the HROB proteins function together in DNA repair. Acharya and colleagues show how HROB interacts with MCM8-9. They show that HROB makes important yet transient contacts with both MCM8 and MCM9 proteins individually, but binds the MCM8-9 complex with the highest affinity. MCM8-9 and HROB proteins preferentially unwind branched DNA structures. The researchers demonstrate that the MCM8-9 helicase functions as a hexamer, which assembles from dimers on single-stranded DNA in the presence of ATP. The hexamer involves two repeating protein-protein interfaces between the alternating MCM8 and MCM9 subunits, which forms a protein ring around ssDNA. One of these interfaces is quite stable and forms an obligate heterodimer. The other interface is labile and mediates hexamer assembly. The authors show that HROB promotes DNA unwinding downstream of MCM8-9 loading and ring formation on ssDNA. The study was primarily carried out by Dr. Ananya Acharya in the Cejka laboratory at the Institute for Research in Biomedicine in Bellinzona, affiliated with the Faculty of Biochemical Sciences of Università della Svizzera italiana. Additional important collaborators include Raphael Guerois (Institute for Intergrative Biology of the Cell, France), together with additional researchers from the ETH Zurich, Columbia University (USA) and the University of Leipzig (Germany).