Dimerization of DNA methyltransferase 1 is mediated by its regulatory domain.

DNA methylation is a major epigenetic modification and plays a crucial role in the regulation of gene expression. Within the family of DNA methyltransferases (Dnmts), Dnmt3a and 3b establish methylation marks during early development, while Dnmt1 maintains methylation patterns after DNA replication. The maintenance function of Dnmt1 is regulated by its large regulatory N-terminal domain that interacts with other chromatin factors and is essential for the recognition of hemi-methylated DNA. Gelfiltration analysis showed that purified Dnmt1 elutes at an apparent molecular weight corresponding to the size of a dimer. With protein interaction assays we could show that Dnmt1 interacts with itself through its N-terminal regulatory domain. By deletion analysis and co-immunoprecipitations we mapped the dimerization domain to the targeting sequence TS that is located in the center of the N-terminal domain (amino acids 310-629) and was previously shown to mediate replication independent association with heterochromatin at chromocenters. Further mutational analyses suggested that the dimeric complex has a bipartite interaction interface and is formed in a head-to-head orientation. Dnmt1 dimer formation could facilitate the discrimination of hemi-methylated target sites as has been found for other palindromic DNA sequence recognizing enzymes. These results assign an additional function to the TS domain and raise the interesting question how these functions are spatially and temporarily co-ordinated.