Molecular basis for the inhibition of de novo DNA methylation by TCL1A

DNA methyltransferases DNMT3A/B mediate de novo DNA methylation, essential for embryonic development and cell fate determination. Dysregulation of DNMT3A/B causes developmental defects and tumorigenesis. TCL1A is critical for embryogenesis but promotes lymphomagenesis when deregulated. Previous studies suggested TCL1A binds DNMT3A/B and inhibits their activity, but the mechanism remained unclear. Here, we report the cryo-EM structure of the DNMT3A-TCL1A complex, which comprises a DNMT3A dimer bound by two TCL1A dimers. TCL1A interacts with the catalytic domain of DNMT3A, overlapping with the DNMT3L-binding site, and induces extended conformational rearrangements. The target recognition domain and catalytic loop shift markedly, reducing DNA accessibility, while the catalytic loop occupies the SAM-binding pocket, thereby blocking methyltransferase activity. Supported by biochemical assays and molecular dynamics simulations, we propose a dynamic inhibition mechanism in which TCL1A exploits DNMT3A conformational plasticity to suppress de novo DNA methylation.