Proteasomes are multi-subunit proteins found in eukaryotes and some archaebacteria. The primary role of proteasomes is to destroy other proteins. Proteasomes degrade proteins that are either old or improperly made, into short polymers about 8-9 amino acids in length. Their ability to do this means they serve several useful purposes within the cell.
1. Cleaning up the "Junk"
Sometimes proteins are improperly made and other proteins are just meant to be short-lived and must be degraded when their time is up. Old, misfolded, truncated and otherwise abnormal proteins are conjugated (bound) to ubiquitin (Ub; a short, highly conserved protein) by a type of enzyme called Ub ligase. Different ligases recognize various "signals" indicating a protein is not functional. Ubiquitin-protein conjugates are then recognized by proteasomes which destroy them.
Among the proteins destroyed by proteasomes are those involved in transcription and translation. By destroying certain regulatory proteins, for example, transcription factors, proteasomes play a role in controlling gene expression.
Since proteasomes degrade proteins that have been marked for destruction by ubiquitin, and help control gene expression, they also wind up playing an important role in epigenetics. This is because the proteins they destroy include those that turn on, or off, specific genes that encode proteins involved in cellular operations like histone modifications or DNA methylation.
4. Immune System Response
Proteosomes cleave antigens within cells and release peptides to the cytoplasm. The peptides bind to MHC class I proteins in the endoplasmic recticulum (ER) of the cell, and that entire complex is sent to the cell surface to trigger T-cell responses. This is called antigen processing and is necessary for T-cells of the immune system to recognize antigens.