Correct option is C
Explanation:
The pathway of presenting extracellular pathogens to cytotoxic CD8+ T cells via MHC class I involves several key steps. Let's analyze the correct sequence:
Step 1: Viral entry and protein release
A. Fusion of endosome membrane with the virus and escape of RNA and protein in the cytosol.
The virus is engulfed into an endosome, and after fusion, its RNA and proteins are released into the cytoplasm.
Step 2: Proteolysis of viral proteins
C. Proteolysis of viral proteins by the proteasome.
The proteasome breaks down the viral proteins into small peptide fragments, which will later be loaded onto MHC class I molecules.
Step 3: Peptide loading onto MHC class I
D. Binding of the peptide to the α chain and stabilization of the assembly of α chain and β₂-microglobulin.
The viral peptide is transported to the endoplasmic reticulum (ER), where it binds to the MHC class I α chain, and the complex is stabilized by β₂-microglobulin.
Step 4: MHC class I complex assembly in the Golgi
B. Assembly of class I MHC protein with bound viral peptide in the Golgi apparatus.
The MHC class I-peptide complex is further processed and packaged in the Golgi before being transported to the cell surface.
Step 5: Recognition by cytotoxic T cells
E. Recognition of viral peptide by cytotoxic T cell.
The processed MHC class I-peptide complex is transported to the cell surface, where it is recognized by cytotoxic T lymphocytes (CTLs, CD8+ T cells), leading to an immune response.
Correct Sequence:
A → C → D → B → E
