Previously, the purification of
DNA methyltransferase from murine P815
mastocytoma cells by immunoaffinity chromatography was described (Pfeifer, G.P., Grünwald, S., Palitti, F., Kaul, S., Boehm, T.L.J., Hirth, H.P. and Drahovsky, D. (1985) J. Biol. Chem. 260, 13787-13793).
Proteins that stimulate the enzymatic activity of
DNA methyltransferase have been purified from the same cells. These
proteins, which partially coelute with
DNA methyltransferase from
DEAE-cellulose and
heparin-agarose, are separated from the
enzyme during the immunoaffinity purification step. A further purification of the stimulating
proteins was achieved by
butanol extraction,
DEAE-cellulose chromatography and gel filtration on
Superose 12. Two
DNA methyltransferase-stimulating
protein fractions were obtained. SDS-
polyacrylamide gel electrophoresis of one fraction showed a single
polypeptide with a molecular mass of 29 kDa. The second fraction consisted of 5 or 6
polypeptides with molecular masses 78-82 and 51-54 kDa. The
proteins stimulate both de novo and maintenance activity of
DNA methyltransferase about 3-fold. They enhance the methylation of any natural
DNA and of
poly[(dI-dC).(dI-dC)] but inhibit the methylation of
poly[(dG-dC).(dG-dC)]. The purified
proteins do not form a tight complex with
DNA methyltransferase; however, they bind both to double-stranded and
single-stranded DNA. The sequence specificity of
DNA methyltransferase is obviously altered in presence of these
proteins.