Goethe University: X-ray structure analysis shows how MHC I molecules are loaded with peptides
For an appropriate immune response, it is essential that infected or degenerated cells are recognized by T lymphocytes. T lymphocytes recognize such cells using antigenic peptides that these cells present using specialized surface molecules (MHC I molecules). A Frankfurt research team has now been able to use X-ray structural analysis to show how the MHC I molecules are loaded with peptides and how suitable peptides are selected for this.
As the task force of the adaptive immune system, T lymphocytes are responsible for attacking and killing virus-infected or cancerous cells. Like all body cells, such cells present on their surface fragments of all the proteins that they produce inside. If these include peptides that a T-lymphocyte recognizes as foreign, it is activated and kills the cell in question. It is therefore important for a robust T cell response that the T lymphocyte is presented with suitable protein fragments. The research team led by Simon Trowitzsch and Robert Tampé from the Institute of Biochemistry at Goethe University Frankfurt has now elucidated how the cell selects these protein fragments or peptides.
The peptides are presented on so-called major histocompatibility molecules of class I (MHC I). MHC I molecules are a group of very diverse surface proteins that can bind a large number of different peptides accordingly. MHC I molecules are anchored in the cell membrane and form a peptide binding pocket with their portion facing outwards. Like all surface proteins, the MHC I molecules go through the so-called secretory pathway: They are synthesized in the cavity system (end plasmatic reticulum (ER) and Golgi apparatus) of the cell and folded there. Small bubbles (vesicles) then pinch off from the cavity system, migrate to the cell membrane and fuse with it.
In order for a T lymphocyte (orange) to be able to destroy a cancer cell (green), the cancer cell must present suitable protein fragments on its surface.
The maturation process of the MHC I molecules is very strictly controlled: In the ER, chaperones – proteins known as “chaperones” – support their folding. The chaperone tapasin is crucial for loading with peptides. “When an MHC I molecule has bound a peptide, Tapasin checks how tight the bond is,” Trowitzsch explains the task of the chaperone. “If the binding is unstable, the peptide is removed and replaced by a tightly binding one.” How exactly tapasin performs this task has not yet been clarified – mainly because the loading process is extremely fast.
The team led by Trowitzsch and Tampé has now succeeded for the first time in making the short-lived interaction between the chaperone and the MHC I molecule visible using an X-ray structure analysis. To do this, they produced variants of the two interaction partners that were no longer in the membrane, cleaned them and brought them together. A trick helped to capture the loading complex in action for crystallization: First, the research team loaded the MHC I molecule with a high-affinity peptide so that a stable bond was formed. A light signal was able to trigger cleavage of the peptide, which greatly reduced the ability to bind the MHC I molecule. Tapasin immediately came on the scene and joined forces with the peptide-free MHC I molecule. “The light-induced cleavage of the peptide was crucial for the success of our experiment,” says Tampé. “With the help of this new type of optochemical biology, we can now specifically simulate complex cell biological processes individually.”
X-ray structure analysis of the crystals revealed how tapasin widens the peptide binding pocket of the MHC I molecule and thereby tests the strength of the peptide bond. For this purpose, the interaction partners form a large contact area; a loop of the tapasin protrudes into the widened binding pocket for stabilization. “We are thus showing an important process within the antigen loading in high resolution for the first time,” says Tampé happily. According to the biochemist, the recordings also reveal how a single chaperone can interact with the enormous variety of MHC I molecules: “Tapasin binds precisely to the non-variable areas of the MHC I molecules.” However, the new structure does not improve anything only the understanding of the complex processes involved in the loading of the MHC I molecules. It should also help