The Invariant chain

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.

Suggested further reading

Castellino F, 1997
Antigen presentation by MHC class II molecules: invariant chain function, protein trafficking, and the molecular basis of diverse determinant capture.
Hum Immunol 54(2), 159-169 (1997)

Krensky AM, 1997
The HLA system, antigen processing and presentation.
Kidney Int Suppl 58, S2-S7 (1997)

Stumptner P, 1997
Interaction of MHC class II molecules with the invariant chain: role of the invariant chain (81-90) region.
EMBO J 16(19), 5807-5818 (1997)