The immune system continuously monitors the body’s cells for internal threats like viral infections or cancerous transformations. A specialized mechanism samples proteins produced inside the cell and displays fragments, known as antigens, on the surface. These antigens can be pieces of cellular debris, viral components, or abnormal tumor proteins. Major Histocompatibility Complex Class I (MHC I) molecules serve as the display platform, presenting these internal contents to cytotoxic T lymphocytes (CTLs). This display is accomplished through the endogenous pathway of antigen processing, which begins in the cell’s cytoplasm.
Antigen Creation through Proteasomal Activity
The initial step in preparing antigens occurs when proteins in the cytosol are selected for degradation. This process frequently targets damaged, misfolded, or foreign proteins. Selection is initiated by tagging the protein with a small regulatory protein called ubiquitin, a process known as ubiquitination. The addition of multiple ubiquitin molecules, or polyubiquitination, acts as a molecular flag, signaling the protein for destruction.
The flagged protein is directed to the 26S proteasome, a large, barrel-shaped complex that functions as the cell’s primary degradation unit. The 19S regulatory cap recognizes the ubiquitin tag, unfolds the target protein, and feeds it into the 20S core for cleavage. In immune-activated cells, specialized immunosubunits form the immunoproteasome. This variant alters cleavage specificity, preferentially generating peptides with hydrophobic or basic residues at the C-terminus, which are better suited for binding to MHC I molecules.
The proteasome yields short peptide fragments, typically ranging from 8 to 10 amino acids in length. This size is specifically suited to fit the binding groove of the MHC I molecule. While the 26S proteasome is the dominant source, some longer fragments are also produced, which can be further trimmed by cytosolic aminopeptidases.
Peptide Movement into the Endoplasmic Reticulum
Antigenic peptides are created in the cytoplasm, but MHC I molecules are synthesized and assembled in the Endoplasmic Reticulum (ER). This spatial separation requires active transport, performed by the Transporter associated with Antigen Processing (TAP) complex. TAP is a heterodimer embedded in the ER membrane, belonging to the ATP-binding cassette (ABC) transporter family. It uses energy derived from ATP hydrolysis to pump peptides across the membrane.
Peptide binding to the TAP transporter occurs on the cytosolic side and does not require ATP. However, the subsequent translocation of the peptide into the ER lumen is strictly dependent on ATP hydrolysis. The TAP complex exhibits selectivity for the peptides it transports, favoring those with lengths between 8 and 16 amino acids. Transport efficiency is highest for peptides in the 8 to 12 amino acid range.
The TAP transporter also displays a preference for the amino acid composition, particularly at the peptide’s C-terminus, favoring hydrophobic or basic residues. This selectivity ensures that the peptides delivered to the ER are likely to bind MHC I molecules. Upon successful transport, the peptides enter the ER lumen, where they encounter the waiting MHC I molecules and the specialized loading machinery.
Assembly and Loading of the MHC Class I Molecule
Within the ER, the MHC I heavy chain is newly synthesized and begins folding, initially associating with the chaperone protein calnexin. Calnexin stabilizes the heavy chain until the light chain, \(\beta_2\)-microglobulin (\(\beta_2\)m), binds to form a partially folded heterodimer. Following \(\beta_2\)m association, the MHC I molecule is released from calnexin and rapidly integrates into a larger, multi-component structure known as the Peptide Loading Complex (PLC).
The PLC is an intricate machinery designed to capture, check, and load the optimal peptide onto the MHC I molecule. Key components of this complex include the chaperone calreticulin, the thiol oxidoreductase ERp57, and the specialized bridging protein called tapasin. Tapasin is a non-covalent linker that physically connects the MHC I heterodimer to the TAP transporter, effectively positioning the MHC I binding groove directly in the path of incoming peptides.
Tapasin actively modulates the conformation of the MHC I binding groove, holding it in an open or peptide-receptive state. This sustained open conformation significantly increases the MHC I molecule’s affinity for the peptides being translocated by TAP. As peptides arrive, they attempt to bind to the MHC I molecule in a process termed “peptide editing.” Peptides that bind with low affinity are quickly released and may be further trimmed by the ER-resident aminopeptidase ERAP1 to potentially improve their fit.
The loading process is completed when a peptide binds with high affinity to the MHC I molecule, causing a stable conformational change that closes the binding groove. This stabilization is the signal for the dissociation of the entire PLC. The now fully assembled and peptide-loaded MHC I complex is structurally rigid and ready to exit the ER. This quality control mechanism ensures that only stable, high-affinity peptide-MHC I complexes proceed to the cell surface.
Surface Expression and Immune Surveillance
After the MHC I molecule binds a high-affinity antigen peptide and dissociates from the PLC, the stabilized complex exits the ER. It moves through the secretory pathway, first traversing the Golgi apparatus. This trafficking ensures proper post-translational modifications and packaging before the molecule reaches its final destination.
The peptide-loaded MHC I molecule is then displayed prominently on the plasma membrane of nearly all nucleated cells. This continuous surface display allows for constant immune surveillance by circulating CD8+ cytotoxic T lymphocytes (CTLs).
CTLs patrol the body, using their T cell receptors to inspect the peptides presented by MHC I molecules. If the displayed peptide is a fragment of a normal, self-protein, the CTL recognizes the cell as healthy. If the MHC I molecule is presenting a non-self-peptide, such as a viral fragment or a mutation-derived tumor protein, the CTL recognizes the abnormality and is activated to destroy the infected or cancerous cell.