Structure and Methyl-lysine Binding Selectivity of the HUSH Complex Subunit MPP8.

Area of Science:

• Molecular Biology
• Epigenetics
• Genomics

Background:

• The Human Silencing Hub (HUSH) complex protects the genome from retroelement expression.
• HUSH functions by recognizing specific nascent transcripts and recruiting epigenetic modifiers.

Purpose of the Study:

• To determine the structural basis of MPP8's role in HUSH complex assembly and function.
• To elucidate the molecular mechanisms underlying HUSH-mediated transcriptional repression.

Main Methods:

• Crystal structure determination of the MPP8 C-terminal domain (CTD).
• AlphaFold3 modeling of the MPP8-TASOR complex.
• Cell-based reporter assays to assess HUSH activity after mutagenesis.

Main Results:

• The MPP8 CTD structure reveals ankyrin repeats and a PINIT-like domain.
• AlphaFold3 accurately predicted interaction interfaces between MPP8 and TASOR, validated by mutation studies.
• MPP8's chromodomain binds H3K9 methyltransferases, facilitating heterochromatin formation.

Conclusions:

• Novel structural elements in MPP8 are essential for HUSH complex assembly and silencing.
• MPP8 recruits H3K9 methyltransferases to promote heterochromatinization and control retrotransposons.
• This work provides a structural framework for understanding HUSH-mediated genome defense.