The double lysine motif of tapasin is a retrieval signal for retention of unstable MHC class I molecules in the endoplasmic reticulum.

Tapasin (tpn), an essential component of the MHC class I (MHC I) loading complex, has a canonical double lysine motif acting as a retrieval signal, which mediates retrograde transport of escaped endoplasmic reticulum (ER) proteins from the Golgi back to the ER. In this study, we mutated tpn with a substitution of the double lysine motif to double alanine (GFP-tpn-aa). This mutation abolished interaction with the coatomer protein complex I coatomer and resulted in accumulation of GFP-tpn-aa in the Golgi compartment, suggesting that the double lysine is important for the retrograde transport of tpn from late secretory compartments to the ER. In association with the increased Golgi distribution, the amount of MHC I exported from the ER to the surface was increased in 721.220 cells transfected with GFP-tpn-aa. However, the expressed MHC I were less stable and had increased turnover rate. Our results suggest that tpn with intact double lysine retrieval signal regulates retrograde transport of unstable MHC I molecules from the Golgi back to the ER to control the quality of MHC I Ag presentation.