A novel multi-functional role for the MCM8/9 helicase complex in maintaining fork integrity during replication stress

The minichromosome maintenance (MCM) 8/9 helicase is a AAA+ complex involved in DNA replication-associated repair. Despite high sequence homology to the MCM2-7 helicase, an active role for MCM8/9 has remained elusive. We interrogated fork progression in cells lacking MCM8 or 9 and find there is a functional partitioning. Loss of MCM8 or 9 slows overall replication speed and increases markers of genomic damage and fork instability, further compounded upon treatment with hydroxyurea. MCM8/9 acts upstream and antagonizes the recruitment of BRCA1 in nontreated conditions. However, upon treatment with fork stalling agents, MCM9 recruits Rad51 to protect and remodel persistently stalled forks. The helicase function of MCM8/9 aids in normal replication fork progression, but upon excessive stalling, MCM8/9 directs additional stabilizers to protect forks from degradation. This evidence defines novel multifunctional roles for MCM8/9 in promoting normal replication fork progression and promoting genome integrity following stress.