Motifs of the C-terminal domain of MCM9 direct localization to sites of mitomycin-C damage for RAD51 recruitment

The MCM8/9 complex is implicated in aiding fork progression and facilitating homologous recombination (HR) in response to several DNA damage agents. MCM9 itself is an outlier within the MCM family containing a long C-terminal extension (CTE) comprising 42% of the total length, but with no known functional components and high predicted disorder. In this report, we identify and characterize two unique motifs within the primarily unstructured CTE that are required for localization of MCM8/9 to sites of mitomycin C (MMC)-induced DNA damage. First, an unconventional "bipartite-like" nuclear localization (NLS) motif consisting of two positively charged amino acid stretches separated by a long intervening sequence is required for the nuclear import of both MCM8 and MCM9. Second, a variant of the BRC motif (BRCv) similar to that found in other HR helicases is necessary for localization to sites of MMC damage. The MCM9-BRCv directly interacts with and recruits RAD51 downstream to MMC-induced damage to aid in DNA repair. Patient lymphocytes devoid of functional MCM9 and discrete MCM9 knockout cells have a significantly impaired ability to form RAD51 foci after MMC treatment. Therefore, the disordered CTE in MCM9 is functionally important in promoting MCM8/9 activity and in recruiting downstream interactors; thus, requiring full-length MCM9 for proper DNA repair.