The Class II molecule's path toward the cell surface is somewhat different. In the rough endoplasmic reticulum (RER)the alpha and beta chains associate with each other and a third protein, called the "invariant chain" [embl: X00497] which stabilizes the complex. Without the invariant chain, the alpha and beta proteins will not associate. Invariant chain (Ii) which trimerizes in the ER, associates with MHC class II molecules and is released from the ER as a nine subunit complex. This MHC-invariant complex passes from the RER to, and out of, the Golgi body. Before moving to the cell surface, the vesicle containing this complex fuses with an endocytic compartment where an external protein has been broken into fragments. Here the invariant chain is proteolytically degraded and a peptide from the external protein associates with the MHC II molecule in the channel between the alpha-1 and beta-1 domains. The resulting MHC II-peptide complex proceeds to the surface where it is expressed. MHC II molecules appear to be expressed on the surface of cells in pairs.
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