Class II histocompatibility molecules consist of
- two transmembrane polypeptides and
- a third molecule nestled in the groove they form.
All three components of this complex must be present in the endoplasmic reticulum for proper assembly.
But antigenic peptides are not transported to the endoplasmic reticulum, so a protein called the invariant chain ("Ii") temporarily occupies the groove.
The steps:- The two chains of the class II molecule are inserted into the membrane of the endoplasmic reticulum.
- They bind (in their groove) one molecule of invariant chain.
- This trimolecular complex is transported through the Golgi apparatus and into vesicles called lysosomes.
- Foreign antigenic material is engulfed by endocytosis forming endosomes.
- These also fuse with lysosomes.
- The antigen is digested into fragments.
- The invariant (Ii) chain is digested.
- This frees the groove for occupancy by the antigenic fragment.
- The vesicles move to the plasma membrane and the complex is displayed at the cell surface.
- a receptor (TCR) able to bind the peptide and flanking portions of the histocompatibility molecule (the hot dog in the bun) and
- CD4 molecules that bind the CD4 receptor (shown above as a yellow triangle) found on all class II histocompatibility molecules.
