Working in Xenopus egg extracts, the team found that, although the prevention of rereplication is dependent on high concentrations of Ran-GTP and Cdk2 kinase activity, it does not require nuclear export of MCM. That means that Crm1 may have a new mechanism of action, since all of its previously known functions involved nuclear transport.
“We think this makes sense,” says Newport, “because it is an extremely rapid mechanism for inactivating MCM.” If the repression required transporting MCM out of the nucleus, the process would be considerably slower and more mistakes would occur in a process that can't tolerate errors.
The target of Cdk2 in the system is not yet clear, but the group thinks it may be MCM itself. Given the observation that the inhibition requires Ran-GTP, a protein known to stabilize interactions between Crm1 and its target proteins, the authors hypothesize that Crm1, the phosphorylated MCM protein, and Ran-GTP form a heterotrimeric complex. But, says Newport, this is still a work in progress and it will take more experiments to unveil just how Crm1 blocks MCM activity. ▪