Fig. 6. The proposed mechanism of tapasin-mediated peptide exchange.

Step 1: pMHC-I molecules sample conformations, which determine peptide binding and tapapsin binding. Tapasin recognizes a certain state of pMHC-I by approaching pMHC-I from the α₂-helix side of the pMHC-I. Interaction between its loop_187-196 with the loop_126-133 of heavy chain provides initial binding which alters the dynamics of the peptide binding groove and subsequently together with the interaction between its E72 in strands β3 and Y84 of the heavy chain promote the C-terminus of peptide to adopt loose binding in the F-pocket accompanied. Now pMHC-I switches to the unlocked state. MHC-I charged with very low affinity peptide can switch to peptide unlocked state automatically by the motion of the bound peptide adopting loose binding state. Step 2: The scoop loop_11-20 interacts with the F-pocket region and stabilize the F-pocket with side chain of L18. Now pMHC-I switches to the peptide receptive state. For allotypes bearing a negatively charged F-pocket, tapasin can also use the residue K16 in the scoop loop_11-20 to stabilize the charged F-pocket, which increases the efficiency of tapasin-mediated peptide exchange. Step 3: The upcoming peptide competes for the F-pocket until the affinity is high enough to replace the side chain. Step 4: Once the high-affinity peptide binds, tapasin is released from the pMHC-I.