peptide-loading complex §

Components:

• calreticulin
• tapasin
• ERp57
• TAP

Assortment of chaperone protein used in MHC class I production. After β₂-microglobulin association and calnexin disassociation, the unloaded MHC class I will bind to the chaperones (calreticulin and ERp57) and binds to TAP embedded in the membrane via tapasin.

tapasin is specific to antigen processing while calnexin, ERp57 and calreticulin bind various other glycoproteins assembling in the ER and seem to be part of the cell’s QC machinery.

Note

The PLC facilitates the exchange of already bound low-affinity peptides, for those of higher affinity, a process called peptide editing. This is supported by the ERp57:tapasin heterodimer.

MHC class I molecules that are miss-folded or unable to bind a peptide are transported bay into the cytosol via the ERAD system.