1.3 Antigen processing and presentation

As previously noted, a cell samples its internal proteins and presents them on MHC class I molecules. The way it does this is called ‘antigen processing’ and is illustrated in Figure 3. Cells have an organelle called a proteasome, which breaks down cytosolic proteins into peptides that are transported into the endoplasmic reticulum by a transporter (TAP1/2), where they can associate with MHC class I molecules.

Figure _unit3.2.3 Figure 3 Antigen processing by the proteasome

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This diagram shows antigens in the cytosol entering the proteasome to be degraded. The resultant peptides are transported across the endoplasmic reticulum membrane into the ER lumen. Here each antigenic peptide molecule binds to an MHC class I molecule attached to the inner surface of the endoplasmic reticulum. Part of the ER membrane is then pinched off to form a vesicle containing the peptide-MHC class I complex. The vesicle moves through the cytosol to fuse with the plasma membrane, allowing it to present the antigen on the cell surface.

Figure 3 Antigen processing by the proteasome

The antigenic peptides are trimmed to size by enzymes in the endoplasmic reticulum before the MHC molecule is transported to the plasma membrane in order to present the antigen.