Which molecule generates peptide fragments for MHC class I presentation in the cytosol?

Which molecule generates peptide fragments for MHC class I presentation in the cytosol?

• A. Proteasome
• B. Lysosome
• C. Ribosome
• D. Endosome

Answer: (A)

The proteasome is responsible for generating peptide fragments for MHC class I presentation in the cytosol. It degrades ubiquitin-tagged endogenous proteins into short peptides (about 8–10 amino acids) that can be transported into the endoplasmic reticulum by TAP. Inside the ER, these peptides are loaded onto MHC class I molecules with the help of chaperones, forming peptide-MHC complexes that travel to the cell surface for recognition by CD8+ T cells. The other options don’t fit this cytosolic peptide-generation step: lysosomes are part of the endosomal-lysosomal pathway feeding MHC class II; ribosomes synthesize proteins rather than generate degradation peptides; endosomes are involved in trafficking and processing of internalized material rather than producing cytosolic peptide fragments for MHC I.

The proteasome is responsible for generating peptide fragments for MHC class I presentation in the cytosol. It degrades ubiquitin-tagged endogenous proteins into short peptides (about 8–10 amino acids) that can be transported into the endoplasmic reticulum by TAP. Inside the ER, these peptides are loaded onto MHC class I molecules with the help of chaperones, forming peptide-MHC complexes that travel to the cell surface for recognition by CD8+ T cells. The other options don’t fit this cytosolic peptide-generation step: lysosomes are part of the endosomal-lysosomal pathway feeding MHC class II; ribosomes synthesize proteins rather than generate degradation peptides; endosomes are involved in trafficking and processing of internalized material rather than producing cytosolic peptide fragments for MHC I.