MCM2-7 hexamer functions as a DNA helicase that unwinds DNA duplex at the DNA replication fork. Here we examined the effects of forcedly expressed MCM (minichromosome maintenance) 2-7 proteins on nuclear structure in HeLa cells. Expressed wild-type MCM3 and MCM5 proteins, which were mainly localized in nuclei, induced the formation of micronuclei-like structure and multi-nuclei, respectively. Mutagenesis at ATP-binding motifs in MCM3 cancelled the abnormal nuclear structure formation but mutagenesis in MCM5 did not. Expression of wild-type MCM2 and MCM7 resulted in the formation of the micronuclei-like structure and/or the multi-nuclei at lesser extents in a ATP-binding site-dependent manner. The results suggest that the presence of extra-amounts of MCM3 and MCM5 exerts strong negative effects on MCM2-7 function. Expressed wild-type MCM2-7 proteins except for MCM6 were largely localized in nuclei but MCM4 and MCM7 mutated at ATP-binding sites were mainly localized in cytoplasm. These results suggest that the ATP-binding activities of MCM4 and MCM7 are required for nuclear localization and/or chromatin binding of MCM2-7 complex.
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