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Eur J Immunol. 2010 Jan;40(1):214-24. doi: 10.1002/eji.200939342.

Tapasin edits peptides on MHC class I molecules by accelerating peptide exchange.

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Biochemistry and Cell Biology, Jacobs University Bremen, Bremen, Germany.


The endoplasmic reticulum (ER) protein tapasin is essential for the loading of high-affinity peptides onto MHC class I molecules. It mediates peptide editing, i.e. the binding of peptides of successively higher affinity until class I molecules pass ER quality control and exit to the cell surface. The molecular mechanism of action of tapasin is unknown. We describe here the reconstitution of tapasin-mediated peptide editing on class I molecules in the lumen of microsomal membranes. We find that in a competitive situation between high- and low-affinity peptides, tapasin mediates the binding of the high-affinity peptide to class I by accelerating the dissociation of the peptide from an unstable intermediate of the binding reaction.

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