Structural basis for targeting the chromatin repressor Sfmbt to Polycomb response elements

Area of Science:

• Chromatin biology
• Molecular genetics
• Structural biology

Background:

• Polycomb group (PcG) proteins are essential for gene silencing during development.
• The assembly and recruitment mechanisms of PcG protein complexes remain incompletely understood.
• Pleiohomeotic (Pho)-repressive complex (PhoRC) is a core component involved in PcG-mediated repression.

Purpose of the Study:

• To elucidate the structural basis of PhoRC complex formation.
• To understand the interaction between Pho and Sfmbt in Drosophila and their human orthologs.
• To investigate the role of the Pho-Sfmbt interaction in Polycomb repression.

Main Methods:

• X-ray crystallography to determine the structure of the Drosophila Pho:Sfmbt complex.
• Structural comparison with human YY1:MBTD1 complexes.
• Site-directed mutagenesis to assess the functional importance of the Pho-Sfmbt interaction.

Main Results:

• The crystal structure reveals a tight complex between the Pho spacer region and the 4MBT domains of Sfmbt.
• Human YY1 binds human 4MBT proteins with lower affinity, explained by structural differences.
• Disrupting the Pho-Sfmbt interaction abolishes ternary complex formation and gene repression in vivo.

Conclusions:

• Pho directly tethers Sfmbt to Polycomb Response Elements (PREs).
• This tethering is essential for the recruitment of other PcG proteins and gene silencing.
• The Pho-Sfmbt interaction serves as a critical hub for PcG complex assembly at target genes.