Structure of mouse DNA (cytosine-5-)-methyltransferase

DNA (cytosine-5-)-methyltransferase was purified as a single polypeptide (190 kDa by SDS-PAGE) from mouse P815 mastocytoma cells. This enzyme transfers methyl groups to unmethylated as well as to hemimethylated DNA sites with a strong preference for the hemimethylated substrate. A structural analysis of the isolated enzyme by electron microscopical techniques was undertaken. On the basis of the results obtained, we propose a model for the enzyme structure. This model describes the enzyme as a hemi-elliptical globular structure with dimensions of 5.4-6.7 nm for the height h and 10.3-10.8 nm for the diameter d, respectively; this globular structure bears a small appendix at the flat side. A molecular mass of 235-250 kDa is calculated from the measured dimensions. Limited trypsin digestion of the enzyme led to a 160-kDa fragment which preserved the gross morphology of the original material. The possible structure function relationships are discussed.