Structural insights into CpG-specific DNA methylation by human DNA methyltransferase 3B

Area of Science:

• Epigenetics
• Molecular Biology
• Structural Biology

Background:

• DNA methyltransferases (DNMTs) are crucial for epigenetic gene regulation by methylating CpG sites.
• DNMT3A and DNMT3B are de novo methyltransferases essential for establishing DNA methylation patterns during development.
• The differential mechanisms by which DNMT3A and DNMT3B establish genomic methylation patterns remain incompletely understood.

Purpose of the Study:

• To elucidate the structural basis for the differential substrate recognition and catalytic mechanism of human DNMT3B.
• To understand how DNMT3B specifically recognizes and methylates DNA sequences, particularly CpGpG sites.
• To provide structural insights into DNMT3B mutations linked to immunodeficiency, centromere instability, and facial anomalies syndrome.

Main Methods:

• X-ray crystallography was employed to determine the structures of the catalytic domain of the human DNMT3B-3L complex.
• Structures were obtained for the complex bound noncovalently with DNA of varying sequences.
• Comparative analysis with DNMT3A was performed to highlight differences in substrate recognition.

Main Results:

• Crystal structures reveal DNMT3B utilizes flexible loops to enclose DNA and a catalytic loop to flip out the cytosine base for methylation.
• DNMT3B specifically recognizes CpGpG DNA sites through interactions involving Asn779 and Lys777 residues in its target recognition domain loop.
• A proton wire water channel involved in the final deprotonation step of cytosine methylation by DNMT3B was identified.

Conclusions:

• The study reveals the complete working mechanism of cytosine methylation by DNMT3B at a structural level.
• DNMT3B's specific recognition of CpGpG sites facilitates processive methylation of tandemly repeated CpG sites.
• The findings provide a structural foundation for understanding DNMT3B-associated hypomethylation disorders.