Final answer:
Proteins destined for lysosomal delivery are typically tagged with mannose-6-phosphate (M6P), although ubiquitin tags proteins for degradation by the proteasome rather than the lysosome.
Step-by-step explanation:
The signal that tags proteins for lysosomal delivery is ubiquitin. Ubiquitin serves as a marker that signals a protein's life cycle has completed. When a protein is tagged with ubiquitin, it is recognized and transported to the proteasome, a structure that is responsible for degrading proteins. It's important to distinguish these processes observed in the cytosol, where proteins are tagged for degradation via ubiquitination, from those targeted to lysosomes. However, lysosomal targeting typically involves the mannose-6-phosphate (M6P) tag on glycoproteins that are directed to the lysosomes. In the cell, the synthesis of these proteins occurs in the endoplasmic reticulum (ER). After synthesis, they are packaged into vesicles and transported via the Golgi apparatus, where the M6P tag is added to ensure their correct delivery to the lysosomes.