What is recognized by the proteasome during degradation?

The proteasome is a large protein complex responsible for degrading unneeded, damaged, or misfolded proteins within the cell. The recognition and targeting of substrates for degradation by the proteasome is primarily mediated by ubiquitin, a small regulatory protein.

In particular, the proteasome recognizes and binds to proteins that have been tagged with chains of ubiquitin molecules, a process known as ubiquitination. These polyubiquitin chains signal to the proteasome that the tagged protein should be processed for destruction. The presence of multiple ubiquitin molecules linked together (as chains) is crucial because it forms a strong signal for recognition by the proteasome. When the protein is polyubiquitinated, it undergoes unfolding and then translocates into the proteasome’s catalytic core to be degraded into smaller peptides.

Other options, such as monoubiquitin or phosphorylated proteins, do not serve as effective signals for proteasomal degradation. Free amino acids are the products of protein degradation and are not involved in the recognition process by the proteasome. Therefore, the correct identification of ubiquitin chains as the signal for proteasome recognition is essential for understanding how proteins are regulated and removed in the cell.