Abstract
The replication fork helicase in eukaryotes is a large complex that is composed of Mcm2-7, Cdc45, and GINS. The Mcm2-7 proteins form a heterohexameric ring that hydrolyzes ATP and provide the motor function for this unwinding complex. A comprehensive study of how individual Mcm subunit biochemical activities relate to unwinding function has not been accomplished. We studied the mechanism of the Mcm4-Mcm6-Mcm7 complex, a useful model system because this complex has helicase activity in vitro. We separately purified each of three Mcm subunits until they were each nuclease-free, and we then examined the biochemical properties of different combinations of Mcm subunits. We found that Mcm4 and Mcm7 form an active unwinding assembly. The addition of Mcm6 to Mcm4/Mcm7 results in the formation of an active Mcm4/Mcm6/Mcm7 helicase assembly. The Mcm4-Mcm7 complex forms a ringed-shaped hexamer that unwinds DNA with 3' to 5' polarity by a steric exclusion mechanism, similar to Mcm4/Mcm6/Mcm7. The Mcm4-Mcm7 complex has a high level of ATPase activity that is further stimulated by DNA. The ability of different Mcm mixtures to form rings or exhibit DNA stimulation of ATPase activity correlates with the ability of these complexes to unwind DNA. The Mcm4/Mcm7 and Mcm4/Mcm6/Mcm7 assemblies can open to load onto circular DNA to initiate unwinding. We conclude that the Mcm subunits are surprisingly flexible and dynamic in their ability to interact with one another to form active unwinding complexes.
Highlights
Ecules in Saccharomyces cerevisiae, in vast excess of what is required to license origins of DNA replication (9 –13)
The eukaryotic Mcms are somewhat unusual in this regard, because they form heterohexamers. It is currently not known how the different eukaryotic Mcm subunits function in coordination to unwind DNA, and a comprehensive study of how individual Mcm subunit biochemical activities relate to unwinding activity has not been accomplished
The Mcm4-Mcm6-Mcm7 complex is a useful model system in this regard, because the complex purified from recombinant proteins is active as a helicase in vitro
Summary
Mcm4/Mcm6/Mcm rings spontaneously open to load onto circular ssDNA and unwind an annealed oligonucleotide. The ability of different Mcm mixtures to form ring shapes and exhibit DNA stimulation of ATPase activity correlates well with the ability of these Mcm mixtures to unwind DNA. We conclude that Mcm subunits are surprisingly flexible and dynamic in their ability to form active unwinding complexes
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