Once expressed on the surface of host cells, an MHC protein remains stably associated with its bound peptide for several days. This highly stable peptide binding behavior is important because, it prevents peptide exchanges on the cell surface, ensuring that peptide :MHC major histocompatibility complexes are reliable indicators of the proteins present inside that host cell.
Pathogen-derived peptides are loaded onto MHC class I and MHC class II molecules at various locations inside the cell, depending on whether the pathogen is located in the cytosol or the vesicular compartment of the cell. Before they may be expressed at the cell surface in a stable manner, both classes of MHC molecules need to join forces with a peptide.
In contrast, MHC class II molecules are prevented from binding peptides in the endoplasmic reticulum and are instead transported to an endosomal compartment where vesicular peptide loading takes place. Peptide-binding completes the folding and assembly of freshly synthesized MHC class I molecules in the endoplasmic reticulum.
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