Where does loading of antigens onto MHC class I molecules occur?

Loading of antigens onto MHC class I molecules occurs in the endoplasmic reticulum (ER). Within the ER, newly synthesized MHC class I molecules bind to peptide fragments, which are typically derived from proteins that have been degraded by the proteasome. These peptides are then transported into the ER through a transporter associated with antigen processing (TAP).

Once in the ER, the peptides are loaded onto the MHC class I molecules, which are subsequently transported to the cell surface. This process is critical for presenting endogenous antigens, such as viral proteins or altered self-proteins, to CD8+ T cells, enabling the immune system to detect and respond to infected or abnormal cells.

The other locations mentioned do not play a role in the primary loading of MHC class I molecules. The Golgi apparatus is involved in the further processing and trafficking of MHC molecules after they have been loaded with peptide. Phagolysosomes are primarily associated with MHC class II molecules for processing exogenous antigens, while the cell membrane is where MHC class I molecules present the loaded peptides to T cells, but it is not the site of loading.