Xenopus eggs express an identical DNA methyltransferase, Dnmt1, to somatic cells

In mouse, an oocyte-specific short isoform of DNA methyltransferase-1 (Dnmt1) lacking amino terminal 118 amino acid residues exists and plays a crucial role in maintaining the methylation state of imprinted genes during early embryogenesis [Howell et al. (2001) Cell 104, 829-838]. To address the question of whether or not Xenopus oocyte expresses such a short isoform, we raised monoclonal antibodies against the amino-terminal portion of Xenopus Dnmt1. Two of the isolated monoclonal antibodies, 3C6 and 4A8, were determined to recognize (1-32) and (115-126) of Xenopus Dnmt1, respectively. The amounts of Dnmt1 in Xenopus eggs were determined to be similar, 10.0 2.5, 8.0 0.8, and 8.2 0.2 ng per egg with monoclonal antibodies 3C6 and 4A8, and polyclonal antibodies, respectively. This indicated that Dnmt1 in Xenopus mature eggs had an identical amino-terminal sequence to the amino acid sequence deduced from the cDNA. Together with the fact that Dnmt1 in A6 cells immuno-reacted with all the monoclonal antibodies isolated and with the polyclonal antibodies, we concluded that Dnmt1 expressed in Xenopus mature eggs possesses an identical amino-terminal sequence to that in somatic cells. Immuno-purified Xenopus Dnmt1 in mature eggs showed similar specific activity to that in proliferating A6 cells and that of mouse recombinant Dnmt1.