Mohammed Al-Balushi

• MHC class I molecules are proteins are central to the defence of the body against viruses. They pick up fragments of proteins (peptides) from the inside of a cell and present them at the cell surface so that the immune system (cytotoxic T lymphocytes) can detect and kill them. Most class I molecules that appear at the cell surface are loaded with specific peptides, however, some empty (peptide-receptive) class I molecules are able to escape the ER QC. This raises the question whether these peptide-receptive class molecules cycle between the ER and cis-Golgi or whether they are retained in the ER. To address this question, I used radiolabeled mammalian ER membranes to generate COPII vesicles in vitro. From these vesicles, I investigated the packaging of different forms of class I molecules using specific antibodies. I found that peptide-receptive class I molecules are packaged into COPII vesicles that are generated from wild type lymphocytes as well as from TAP deficient lymphocytes. This is interesting, since for the first time we are able to look at the empty class I molecules in a situation similar to that of the living cell. Using my established in vitro vesicle generation system (budding assay) is advantageous for looking at the packaging of proteins that escape the ER QC, and their characteristics. I used the same system to study the trafficking of the peptide loading complex (PLC) in the COPII vesicles to find out whether the PLC proteins (which are chaperones that help in class I peptide binding) are packaged into COPII vesicles. My results indicate that the TAP peptide transporter and calreticulin are present in the COPII vesicles, and that these molecules may be retrieved from the ERGIC in COPI transport vesicles. This finding indicates that the PLC cycles between the ER and post ER compartments and the cycling may play a role in the retrieval of empty class I molecules.