Mcm10 and RecQL4 Synergize to Activate the Eukaryotic Replicative DNA Helicase

Chromosomes are copied from thousands of origins. At each origin, two replicative DNA helicases are first assembled, then activated to begin unwinding DNA. Several replication proteins are subsequently recruited to the active helicase, forming a replisome. The helicase must undergo dramatic conformational changes during its activation and this process is poorly understood, especially in metazoa. How the metazoan replicative helicase is activated, and which proteins promote this essential process are long-standing questions. Using a combination of single-molecule imaging and ensemble biochemistry, we show that Mcm10 and RecQL4 act in a concerted manner to activate replicative helicases. Mcm10 first binds to inactive helicases, then recruits RecQL4, which synergizes with Mcm10 to promote helicase activation. Mcm10 is not incorporated into replisomes and dissociates from origins during replication initiation. In the absence of Mcm10, RecQL4 is recruited to origins via an interaction with the Mcm7 subunit of the helicase. Our data reveal that Mcm10 and RecQL4 play partially redundant roles during helicase activation, help resolve long-standing controversies about the roles of Mcm10 and RecQL4 in DNA replication, and reveal replication initiation defects caused by pathologic RecQL4 mutations.