What is the primary function of TAP in antigen processing?

What is the primary function of TAP in antigen processing?

• A. They bind to class II molecules to help block the antigen-binding site
• B. They bind to class I proteins in proteasomes
• C. They transport peptides into the lumen of the endoplasmic reticulum
• D. They help cleave peptides for transport to endosomes

Answer: (C)

Antigen processing for MHC class I relies on delivering cytosolic peptide fragments into the endoplasmic reticulum so they can be loaded onto MHC class I molecules. TAP is the transporter that does this: a TAP1/TAP2 complex in the ER membrane uses ATP to move peptide fragments generated by the proteasome from the cytosol into the ER lumen. Once inside the ER, these peptides (often trimmed to the right length by ER aminopeptidases) bind to MHC class I molecules, which then travel to the cell surface to present the peptides to CD8+ T cells. Without TAP, peptides can’t reach the ER and MHC class I molecules can’t be loaded, impairing endogenous antigen presentation. The other described roles pertain to different parts of antigen presentation (such as class II pathways or protease activity in other compartments) and are not TAP’s function.

Antigen processing for MHC class I relies on delivering cytosolic peptide fragments into the endoplasmic reticulum so they can be loaded onto MHC class I molecules. TAP is the transporter that does this: a TAP1/TAP2 complex in the ER membrane uses ATP to move peptide fragments generated by the proteasome from the cytosol into the ER lumen. Once inside the ER, these peptides (often trimmed to the right length by ER aminopeptidases) bind to MHC class I molecules, which then travel to the cell surface to present the peptides to CD8+ T cells. Without TAP, peptides can’t reach the ER and MHC class I molecules can’t be loaded, impairing endogenous antigen presentation. The other described roles pertain to different parts of antigen presentation (such as class II pathways or protease activity in other compartments) and are not TAP’s function.