Mechanism of stimulation of catalytic activity of Dnmt3A and Dnmt3B DNA-(cytosine-C5)-methyltransferases by Dnmt3L.

Dnmt3L has been identified as a stimulator of the catalytic activity of de novo DNA methyltransferases. It is essential in the development of germ cells in mammals. We show here that Dnmt3L stimulates the catalytic activity of the Dnmt3A and Dnmt3B enzymes by directly binding to their respective catalytic domains via its own C-terminal domain. The catalytic activity of Dnmt3A and -3B was stimulated approximately 15-fold, and Dnmt3L directly binds to DNA but not to S-adenosyl-L-methionine (AdoMet). Complex formation between Dnmt3A and Dnmt3L accelerates DNA binding by Dnmt3A 20-fold and lowers its K(m) for DNA. Interaction of Dnmt3L with Dnmt3A increases the binding of the coenzyme AdoMet to Dnmt3A, and it lowers the K(m) of Dnmt3A for AdoMet. On the basis of our data we propose a model in which the interaction of Dnmt3A with Dnmt3L induces a conformational change of Dnmt3A that opens the active site of the enzyme and promotes binding of DNA and the AdoMet. We demonstrate that the interaction of Dnmt3A and Dnmt3L is transient, and after DNA binding to Dnmt3A, Dnmt3L dissociates from the complex. Following dissociation of Dnmt3L, Dnmt3A adopts a closed conformation leading to slow rates of DNA release. Therefore, Dnmt3L acts as a substrate exchange factor that accelerates DNA and AdoMet binding to de novo DNA methyltransferases.