DNA methylation is one of the most important epigenetic marks and plays a central role in gene silencing in high eukaryotes. It is established by DNA methyltransferases (DNMTs), which are classified into three subfamilies. In contrast to DNMT1 and DNMT3, the biological function of DNMT2 remains elusive to date. DNMT2 was first identified in mouse and human and appears to be well conserved among eukaryotes (Okano et al., 1998; Yoder and Bestor, 1998). DNMT2 contains all 10 characteristic motifs of DNA cytosine MTases and a distinctive target recognition domain (TRD) between motifs VIII and IX, but lacks the N-terminal regulatory domains present in DNMT1 and DNMT3 (Dong et al., 2001; Goll and Bestor, 2005). Nevertheless, only weak residual DNA MTase activity could be detected in human DNMT2, and no DNA methylation changes could be observed in DNMT2 knockout mouse ES cells (Okano et al., 1998; Hermann et al., 2003). Intriguingly, human DNMT2 exhibits a notable MTase activity towards C38 of tRNAAsp (Goll et al., 2006).
To explore the biological function(s) of DNMT2 and the underlying molecular mechanism, we identified and cloned a DNMT2 homolog from the insect Spodoptera frugiperda (SfDNMT2). SfDNMT2 shares moderate to high sequence identities with other known DNMT2s and possesses all 10 characteristic motifs of DNA cytosine MTases (Supplementary Figure S1). We generated the SfDNMT2 specific antiserum in mouse for the immunostaining experiments. SfDNMT2 is found to exist in both the nucleus and the cytoplasm of the Sf9 cells (Figure 1A).