Introduction to Polycomb and Trithorax group proteins

The chromatin environment is crucial for maintaining cell-type-specific gene expression patterns and thereby cell identity. Different chromatin-modifying protein complexes can influence the accessibility of chromatin and local patterns of chromatin modifications. Two such protein families, the Polycomb group (PcG) and Trithorax group (TrxG), have been widely studied for their roles in modulating transcription and influencing cell fate.

PcG and TrxG proteins were originally classified based on their distinct developmental phenotypes in genetic studies in Drosophila. Molecular mechanisms of the encoded proteins have since been studied, and homologs have been identified in many metazoan species. Both classes of proteins associate with several large, multimeric protein complexes, encompassing diverse molecular mechanisms and catalytic activities. PcG proteins are involved in maintaining transcriptional repression, while TrxG proteins modulate chromatin contributing to active transcription¹.

Polycomb and Trithorax group proteins modulate gene expression through the establishment of distinct chromatin states. Transcription initiation leads to the recruitment of TrxG proteins, facilitating an open chromatin environment permissive for active transcription. Termination of transcription leads to PcG recruitment to facilitate a more compact, repressive chromatin environment.

References

1. Schuettengruber, B., Bourbon, H. M., Di Croce, L., et al. Genome regulation by polycomb and trithorax: 70 years and counting Cell 171 ,34-57 (2017)