Mass spectrometric profiling of HLA-B44 peptidomes provides evidence for Tapasin-mediated Tryptophan editing

Abstract

Abstract Activation of CD8 +T cells against pathogens and cancers involves the recognition of antigenic peptides presented in complex with human leukocyte antigen (HLA) class-I proteins. Peptide binding to HLA-I proteins is coordinated by a multi-protein complex called the peptide loading complex (PLC). Tapasin, a key PLC component, facilitates the binding and optimization of HLA-I peptides. However, different HLA-I allotypes have variable requirements for tapasin for their assembly and surface expression. HLA-B*44:02 and HLA-B*44:05, which differ only at position 116 of their heavy chain sequences, fall at the opposite ends of the tapasin-dependency spectrum. HLA-B*44:02 (D116) is highly tapasin-dependent, whereas HLA-B*44:05 (Y116) is highly tapasin-independent. To better understand the influences of tapasin upon HLA-B44 peptidome compositions, we undertook mass spectrometric comparisons of HLA-B*44:02/05 peptidomes under tapasin-sufficient and/or tapasin-deficient conditions. Analyses of the HLA-B*44:05 peptidomes in the presence vs. absence of tapasin reveal that tapasin increases the frequency of C-terminal tryptophan residues. In the presence of tapasin, C-terminal tryptophans are also more highly represented among peptides unique to B*44:02 and those shared between B*44:02 and B*44:05, compared with peptides unique to B*44:05. Overall, our findings demonstrate that tapasin influences the C-terminal composition of HLA-I-bound peptides. For the HLA-B44 family, the presence of tapasin or high tapasin-dependence of an allotype results in better binding of peptides with C-terminal tryptophans, consistent with the role for tapasin in stabilizing an open conformation to accommodate bulky C-terminal residues. Supported by grants from NIH to Malini Raghavan (R21 AI164025, R21 AI126054 and R01AI044115)