Acts as component of the mcm2-7 complex (mcm complex) which is the putative replicative helicase essential for 'once per cell cycle' DNA replication initiation and elongation in eukaryotic cells. The active ATPase sites in the mcm2-7 ring are formed through the interaction surfaces of two neighboring subunits such that a critical structure of a conserved arginine finger motif is provided in trans relative to the ATP-binding site of the Walker A box of the adjacent subunit. The six ATPase active sites, however, are likely to contribute differentially to the complex helicase activity. The existence of maternal and zygotic forms of mcm3 and mcm6 suggests that specific forms of mcm2-7 complexes may be used during different stages of development. May replace mmcm6 in the mcm2-7 complex. ATP + H2O = ADP + H(+) + phosphate Component of the mcm2-7 complex (RLF-M). The complex forms a toroidal hexameric ring with the proposed subunit order mcm2-mcm6-mcm4-mcm7-mcm3-mcm5 (By simililarity). Begins to associate with zmcm3, mcm4 and mcm7 into mcm complexes at the neurula stage. May replace mmcm6 in the complex that functions during licensing of DNA replication. Associated with chromatin before the formation of nuclei and detaches from it as DNA replication progresses. Expressed zygotically. Expression begins after the midblastula transition (MBT) at the neurula stage. Belongs to the MCM family.