Fig. 3. The role of tapasin scoop loop_11–20 in mediating peptide exchange of MHC-I.

a The loop_11–20 of tapasin in the modeled MHCI-ERp57-Tsn complex (ERp57 is not shown here). Residue 16 and 18 were colored red. The sequence of tapasin scoop loop_11–20 mutants are shown besides. b Representative peptide exchange profiles of 500 nM A*03:01/photoKK9, B*27:05/photoRL9, B*27:09/photoRL9 in the absence (black) and presence of equimolar amounts of ERp57-Tsn^WT (light green), ERp57- Tsn^L18G (sky blue), ERp57-Tsn^K16G (purple), and ERp57-Tsn^GGGGG (golden). FITC-KK9 for A*03:01 and FITC-RL9 for B*27:05. FTIC-IF9 (IRAAK^FITCPPLF) was used for better resolution of the kinetics for B*27:09. dFP means the baseline was subtracted. c Relative catalytic activity of ERp57-Tsn loop_11–20 mutants compared to ERp57-Tsn^WT towards three allotypes. The observed association rates (K_{on_Tsn_mutant}) were determined for each mutant on the corresponding MHC-I allotype and relative activity was calculated by the formula of (K_{on_Tsn_mutant} – K_{on_Tsn}^-)/(K_{on_Tsn_WT} – K_{on_ Tsn}^-). Error bar (SD) was calculated from five independent experiments. d Normalized free energy values of the modeled tapasin mutants calculated in Rosetta Energy Unit (REU). Error bar (SD) was calculated from 100 data points. In c comparison of loop mutants to WT, the two-sample unequal variance Student’s t test was performed, in d mean comparison method is Tukey, *p < 0.05, **p < 0.01, ***p < 0.001.