Final answer:
Proteins in the exogenous pathway are endocytosed and degraded by digestive enzymes within endosomes, which later form lysosomes containing proteases such as cathepsins. The ubiquitin-proteasomal system is another pathway for protein degradation in the cytosol.
Step-by-step explanation:
In the exogenous pathway of antigen processing, proteins are endocytosed and later degraded within endosomes. Once internalized, the vesicles shed their coats and fuse with early endosomes, eventually becoming late endosomes and proceeding to form lysosomes. Within these lysosomes, digestive enzymes known as proteases, specifically cathepsins, catalyze the hydrolysis of proteins into peptides suitable for presentation to T cells via MHC II molecules. Importantly, these lysosomal proteases are essential for breaking down both extracellular and intracellular proteins acquired through various pathways, including phagocytosis.
Proteins can also be degraded via the ubiquitin-proteasomal system in the cytosol. This complex pathway employs ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2), and ubiquitin ligases (E3), which tag proteins with ubiquitin, marking them for degradation. The regulated function of E3 is crucial in determining the specificity of the protein for degradation.