Molecule of the Month: MHC I Peptide Loading Complex

Our immune system is constantly making decisions. It is tasked with a difficult job: it must seek out infections and cancers and destroy them, but at the same time, ignore the many normal cellular processes that are needed for healthy life. So, when the immune system finds an infected cell or tumor, it needs to make absolutely sure that there is a problem. If it makes a wrong decision and attacks a healthy cell, it can lead to life-threatening inflammation or autoimmune disease. MHC I (“major histocompatibility complex”) helps the immune system make these decisions. Most of our cells chop up a few copies of their internal proteins and use MHC I to display the small peptide pieces on the cell surface. That way the immune system can monitor what’s happening inside the cell. The peptide loading complex (PLC) shown here helps our cells load only the most interesting peptides onto MHC I.

The peptide loading complex has many functional parts that find an empty MHC I, load it with a peptide, check the stability of the peptide-MHC I complex, and then release it for delivery to the cell surface. Most of the action happens in the endoplasmic reticulum. The protein chaperone calreticulin starts building the complex by recognizing a distinctive glycan on the surface MHC I. It then recruits the chaperone tapasin, the disulfide isomerase ERp57, and the transporter TAP (“transporter associated with antigen processing”). When the whole thing is assembled, a peptide is loaded. Finally, the complex disassembles and a glucose is clipped off the end of the MHC I glycan, giving the signal that peptide-MHC I complex is ready for transport.

The illustration shown here includes two experimentally-determined atomic structures of different parts of the peptide loading complex. PDB ID 7qpd shows what’s happening inside the endoplasmic reticulum. Calreticulin, tapasin and ERp57 surround MHC I. Tapasin changes the shape of the peptide-binding groove, making it a bit wider at one end, and also forms a small lid over part of the groove, making it more difficult for weakly-binding peptides to find their way in. As a result, the complex accelerates binding of peptides and favors strong-binding, optimal peptides. PDB ID 5u1d includes TAP, which has the task of transporting peptides into the endoplasmic reticulum. It is an ATP-driven transporter similar to P-glycoprotein and other multidrug transporters, consisting of two similar protein subunits that form a passageway through the endoplasmic reticulum membrane. In this structure, the transporter is frozen by a small protein from herpes simplex virus, ICP47, which blocks transport of viral proteins and allows the virus to hide from the immune system.

Amazingly, a second level of quality control is employed to ensure that only the most relevant peptides are displayed on the cell surface. As the complex of peptide and MHC I moves through the transport process, it is examined by another set of proteins. TAPBPR (“TAP-binding protein-related,” shown here from PDB entry 5opi) binds to MHC I in a similar way as tapasin and checks if the peptide is binding tightly. If the peptide doesn’t pass the test, the protein UGGT (“UDP-glucose:glycoprotein glucosyltransferase,” shown here from PDB ID 5mzo) adds glucose back to the MHC I glycan, giving the cell the signal to recycle the emptied MHC I back to the endoplasmic reticulum for another try.

The TAP transporter delivers a wide range of peptides to the endoplasmic reticulum, from about 8 to 40 amino acids in length. However, MHC I prefers smaller peptides, with about 8 to 10 amino acids. So, two similar enzymes, ERAP1 and ERAP2 (“endoplasmic reticulum aminopeptidase,” with ERAP2 shown here from PDB ID 5ab0) trim the peptides to the proper size. A recent study of historical human DNA reveals how important this process is. The Black Death, which struck in the Middle Ages, is the single greatest mortality event in recorded history, killing somewhere between a third and half of the human population. A recent study of several hundred DNA extracts from people of the time revealed that individuals with enhanced ERAP2 activity were 40% more likely to survive. This may have been an example of human evolutionary natural selection that occurred in the time of recorded history, leaving a population that for centuries often showed reduced mortality rates in later bubonic plague pandemics.