Peptide-dependent trafficking of MHC class I molecules

I have investigated the trafficking of peptide-receptive MHC class I molecules using confocal fluorescence microscopy. I have found that both peptide-free and -bound forms can leave the endoplasmic reticulum and become transported to the cis-Golgi, from where only those forms that are bound to high-affinity peptides can proceed to the cell surface, while peptide-receptive forms are retrieved to the ER. I have then checked different peptides in their ability to move class I to the cell surface, and I have found that the carboxy terminus of the peptide is important for the export of the class I to the cell surface. I have also investigated the effect of tapasin and calreticulin, components of the peptide loading complex, on the trafficking of class I. In both tapasin-negative and calreticulin-negative cell lines, class I remains intracellular. Interestingly, introduction of high-affinity peptides can partially overcome this defect in both cell lines. That confirms the role of tapasin and calreticulin in the exchange of the peptides bound to class I from low-affinity to higher-affinity peptides.