MCM8 Is an MCM2-7-Related Protein that Functions as a DNA Helicase during Replication Elongation and Not Initiation

MCM2-7 proteins are replication factors required to initiate DNA synthesis and are currently the best candidates for replicative helicases. We show that the MCM2-7-related protein MCM8 is required to efficiently replicate chromosomal DNA in Xenopus egg extracts. MCM8 does not associate with the soluble MCM2-7 complex and binds chromatin upon initiation of DNA synthesis. MCM8 depletion does not affect replication licensing or MCM3 loading but slows down DNA synthesis and reduces chromatin recruitment of RPA34 and DNA polymerase-(alpha). Recombinant MCM8 displays both DNA helicase and ATPase activities in vitro. Reconstitution experiments show that ATP binding in MCM8 is required to rescue DNA synthesis in MCM8-depleted extracts. MCM8 colocalizes with replication foci and RPA34 on chromatin. We suggest that MCM8 functions in the elongation step of DNA replication as a helicase that facilitates the recruitment of RPA34 and stimulates the processivity of DNA polymerases at replication foci.