Final answer:
While ubiquitination typically signals a protein for degradation by proteasomes, there are exceptions where ubiquitin tagging alters protein function or location instead of leading to its destruction.
Step-by-step explanation:
The exception to the general rule that a protein tagged with ubiquitin is marked for degradation can be found in some signaling pathways where ubiquitination does not lead to protein decay. Protein degradation is a well-regulated cellular process, involving the tagging of a protein with ubiquitin to signal its lifespan is complete and its removal by proteasomes. However, ubiquitin can also play a role in altering a protein's function or location in a cell without causing its degradation, which is an exception to the ubiquitin-proteasome pathway's usual role in protein removal.