FIGURE 2.

Digestion rate of synthetic peptide precursors of HLA-B27 ligands by ERAP1 variants. A, combined digestion (%) of peptide species longer than the natural ligand, excluding the original substrate. It was calculated as: 100% − the combined yield of the corresponding species; B, yield of the natural ligand; C, yield of peptides shorter than the natural ligand, resulting from destructive cleavages. ♦, Arg-528/Asp-575; ■, Lys-528/Asp-575; ▴, Arg-528/Asn-575; ●, Lys-528/Asn-575. Yields are relative to the total amount of peptide, which was estimated as the added intensity of the ion peaks corresponding to each peptide species in the MALDI-TOF MS spectrum of the digestion mixture. The data are mean ± S.D. of 3–5 experiments. Note that for any given substrate and time point, the value in A is essentially accounted for by the added yield of the corresponding ligand and its digestion product(s) in B and C, respectively.