Final answer:
Ubiquitin is the signal for proteasomal degradation. It marks proteins for destruction, leading to their degradation by the proteasome into peptides and free amino acids.
Step-by-step explanation:
The signal for proteasomal degradation of a protein is the addition of ubiquitin. Ubiquitin acts as a flag, marking the protein as ready for destruction. The proteasome, a complex organelle responsible for protein degradation, recognizes these ubiquitinated proteins and proceeds to degrade them.
The process begins with the activation of ubiquitin by an ubiquitin-activating enzyme (E1), which, through ATP hydrolysis, attaches ubiquitin to a substrate protein. Following this, an ubiquitin-conjugating enzyme (E2) takes over from E1, and the protein destined for degradation takes the place of E2. Several more ubiquitins bind to this complex, creating a poly-ubiquitinated protein that is delivered to the proteasome for degradation into short peptides and eventually, free amino acids.
In summary, ubiquitination is the modification that targets proteins for degradation by the proteasome. This process is critical for regulating various cellular processes, including gene expression and responses to stress.