Fig. 5. The TAPBPR G24-R36 loop promotes peptide binding on empty MHC-I.

a Schematic of the TAPBPR-mediated MHC-I peptide exchange cycle. The dissociation constant (K_D) of each step is noted. b ITC performed at different stages of the peptide exchange cycle (K_D2, K_D3, K_D4) for TAX9/HLA-A*02:01/hβ₂m with TAPBPR^WT or TAPBPR^∆G24-R36 (see “Methods”). K_D1 was not measured directly, but inferred from thermodynamic balance along the cycle shown in (a). Due to experimental limitations, the measured K_D1 and K_D2 are “apparent” and thus denoted with K_D1,app and K_D2,app. K_D and n values determined by ITC are noted in Table 1. c Apparent K_D ratios determined by ITC for TAPBPR^∆G24-R36/TAPBPR^WT for TAX8, TAX9, and KLL15 peptides. The dotted line represents no effect. d FP performed under substoichiometric (left) and stoichiometric (right) conditions. Millipolarization (mP) values are plotted as a function of the log₁₀ peptide concentration for TAX8, TAX9, and KLL15 competitor peptides. The insets show a comparison of the ratio of FP-determined IC₅₀ values for TAPBPR^∆G24-R36 or TAPBPR^∆ALAS versus TAPBPR^WT. The dotted line represents no effect. e X-ray structures (TAX8, TAX9) and RosettaCM models (TAX10, TAX11, TAX12) show peptide bulging within the HLA-A*02:01 groove. The open and closed conformation of the TAPBPR G24-R36 loop from MD simulations are shown in gray and black, respectively. f Comparison of FP-determined IC₅₀ values under stoichiometric conditions for TAX length variants versus pMHC-I thermal stability. The R² value of the linear regression fit (black line) is shown. g Comparison of FP-determined IC₅₀ values under stoichiometric conditions for TAX length variants for TAPBPR^WT versus TAPBPR^∆ALAS or TAPBPR^∆G24-R36. The dotted red line represents a conceptual 1:1 correlation (no effect). Data presented in panels (d), (f), and (g) are mean ± SD for n = 3 independent experimental replicates.