Presumptive replicative helicase. Has ATPase and DNA helicase activities. The latter preferentially melts 5'-tailed oligonucleotides and is stimulated by the SSB protein (single-stranded DNA binding protein). The active ATPase sites in the MCM 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 helicase function is proposed to use a partially sequential mode of ATP hydrolysis; the complex appears to tolerate multiple catalytically inactive subunits. ATP + H2O = ADP + H(+) + phosphate Inhibited by the cdc6-2 protein. The ATPase activity is not stimulated in the presence of DNA molecules. Homohexamers. Forms a complex with SSB. Belongs to the MCM family.