During MHC Class I Synthesis and Folding in the Endoplasmic

question 3

Multiple Choice

During MHC class I synthesis and folding in the endoplasmic reticulum (ER) , a process of peptide editing takes place as the newly synthesized MHC class I protein is held in a 'peptide receptive' state by binding to the calreticulin:ERp57:tapasin complex. Peptide editing ensures that the MHC class I molecules that reach the cell surface have stable, high affinity binding for their peptide cargo. Peptide editing is important to the immune response because it:

• A) Maintains high levels of surface MHC class I expression
• B) Ensures that MHC class I molecules are not degraded in the ER
• C) Retains the nascent MHC class I molecule in a peptide receptive state
• D) Allows surface MHC class I molecules to bind new peptides from the extracellular milieu
• E) Prevents surface MHC class I molecules from undergoing peptide exchange at the cell surface

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