What kind of proteins are associated with the proteasome's active site?

The association of ubiquitinated proteins with the proteasome's active site is a fundamental aspect of the protein degradation process within cells. Proteins that are marked for degradation are typically tagged with a small protein called ubiquitin. This ubiquitination signals that the protein is destined for destruction, and the proteasome plays a crucial role in this process by recognizing and binding these ubiquitinated proteins.

Once a ubiquitinated protein is recognized by the proteasome, it is unfolded and translocated into the proteolytic core of the proteasome, where it is broken down into smaller fragments, usually peptides. This mechanism is essential for regulating protein levels within the cell, removing damaged or misfolded proteins, and maintaining cellular homeostasis.

Other types of proteins, such as chaperon-bound proteins, degraded proteins, or non-functional proteins, may interact with various cellular systems, but they are not specifically associated with the active site of the proteasome in the same manner that ubiquitinated proteins are. The ubiquitin-proteasome pathway is a targeted process, and only proteasomal substrates that have been tagged with ubiquitin are directed to this complex for degradation.