Molecule of the Month: Major Histocompatibility Complex
MHC displays peptides on the surfaces of cells, allowing the immune system to sense the infection inside
Each cell has a second line of defense that it uses to signal to the immune system when something goes wrong inside. Cells continually break apart a few of their old, obsolete proteins and display the pieces on their surfaces. The small peptides are held in MHC, the major histocompatibility complex, which grips the peptides and allow the immune system to examine them. In this way, the immune system can monitor what is going on inside the cell. If all the peptides displayed on the cell surface are normal, the immune system leaves the cell alone. But if there is a virus multiplying inside the cell, many of the MHC molecules carry unusual peptides from viral proteins, and the immune system kills the cell.
MHC in Action
The Cancer Connection
Types and Terminology
A Family of Folds
Exploring the Structure
MHC I with Viral Peptides
The entire MHC system poses a problem: each cell has thousands of different peptides to display, but each cell only builds a few types of MHC. The solution to this dilemma was revealed in the early structures of MHC with different peptides. The two structures shown here, PDB ID 2vaa and 2vab, have peptides from two different viruses bound to the same MHC. Another similar series can be found in PDB entries 1hhg, 1hhh, 1hhi, 1hhj and 1hhk. Looking at these structures, you can see that the peptides (red), which are nine amino acids long, are held in a similar extended conformation in a groove between two long alpha helices. The structures revealed that the protein primarily grips the ends of the peptide, leaving most of the variable portions of the peptide exposed on the surface. Click on the image to view an interactive JSmol. In addition, you can explore how the immune system recognizes these exposed portions of the peptides in the Molecule of the Month on T-cell receptors.
- S. Sell (2001) Immunology, Immunopathology and Immunity. ASM Press, Washington, D. C.
- K. Natarajan, H. Li, R. A. Mariuzza and D. H. Margulies (1999) MHC Class I Molecules, Structure and Function. Reviews in Immunogenetics 1, 32-46.
- I. A. York and K. L. Rock (1996) Antigen Processing and Presentation by the Class I Major Histocompatibility Complex. Annual Review of Immunology 14, 369-396.
- M. Matsumura, D. H. Fremont, P. A. Peterson and I. A. Wilson (1992) Emerging Principles for the Recognition of Peptide Antigens by MHC Class I Molecules. Science 257, 927-934.
February 2005, David Goodsellhttp://doi.org/10.2210/rcsb_pdb/mom_2005_2