Maintenance-type DNA methyltransferase (Dnmt1) is usually down-regulated in nonproliferating cells, In the present study, we detected significant expression of Dnmt1 protein in adult mouse brain where the majority of the cells are in a post-mitotic state. A significant amount of Dnmt1 protein was fractionated into the post-nuclear fraction for both cerebrum and cerebellum. The Dnmt1 in this fraction was enzymatically active. An immunofluorescence study revealed that Dnmt1 protein was mainly expressed in neurons and seemed to be localized in the cytoplasmic compartment, Primary culturing of neurons confirmed the expression and localization of Dnmt1 in the cytoplasmic compartment, The findings that the Dnmt1 transcript in the brain utilized the somatic-type exon and that the apparent size of the Dnmt1 protein in the cytoplasm was identical to that in proliferating culture cells indicate that the cytoplasmic Dnmt1 in neurons was of the somatic-type.
