What marks proteins for degradation?

Proteins are marked for degradation primarily through a process involving ubiquitin. Ubiquitin is a small regulatory protein that is ubiquitously expressed in nearly all tissues of eukaryotic organisms. The process of tagging a protein with ubiquitin is known as ubiquitination. This modification typically occurs on lysine residues of the target protein and serves as a signal that the protein is to be directed towards the proteasome, a multi-subunit protease complex responsible for degrading and recycling damaged or unneeded proteins.

The ubiquitination process is crucial for maintaining cellular health and homeostasis, as it allows for the removal of misfolded, damaged, or regulatory proteins at the right time, thus preventing potential toxicity and regulating various cellular processes. When proteins are marked by ubiquitin, they are recognized and processed by the proteasome, where they are unfolded and subsequently degraded into smaller peptides.

The other options pertain to protein interactions and modifications but do not serve the specific function of marking proteins for degradation. Folding agents and chaperones assist in proper protein folding and prevent aggregation, while phosphorylation serves as a regulatory mechanism influencing protein activity and function rather than signaling for degradation directly.