Nearest neighbour analysis of MCM protein complexes in Drosophila melanogaster

The MCM proteins are a group of six proteins whose action is vital for DNA replication in eukaryotes. It has been suggested that they constitute the r eplicative helicase, with a subset of the proteins forming the catalytic he licase (MCM4,6,7) while the others have a loading or control function. In t his paper we show that all six MCM proteins are present in equivalent amoun ts in soluble extracts and on chromatin. We have also analysed soluble and chromatin-associated MCM protein complexes under different conditions. This suggests that all six MCM proteins are always found in a complex with each other, although the interaction between the individual MCM proteins is not equivalent as stringent salt conditions are able to break the intact compl ex into a number of stable subcomplexes. These data contribute to the ongoi ng debate about the nature of MCM complexes, supporting the hypothesis that they act as a heterohexamer rather than as a number of different subcomple xes. Finally, using protein-protein cross-linking we have shown that MCM2 i nteracts directly with MCM5 and MCM6; MCM5 with MCM3 and MCM2; and MCM6 wit h MCM2 and MCM4. This provides the first direct information about specific subunit contacts in the MCM complex.