Analyses of the interactions of tapasin and ERp57-tapasin proteins with PaSTa 1 and PaSTa 2 antibodies and MHC-I molecules

Abstract The intracellular loading of major histocompatibility complex class I (MHC-I) molecules with high affinity peptides is a pre-requisite for their cell surface display as ligands for T and NK cells. Peptide acquisition occurs in the ER within the peptide loading complex (PLC) and is accomplished through the concerted action of several proteins, most notably tapasin. In the absence of tapasin, peptide loading is compromised, resulting in decreased cell surface expression of some, though not all, MHC-I allelomorphs. In order to gain a mechanistic understanding of tapasin function as well as its apparent allelomorph specificity we have produced recombinant Fab fragments of two anti-tapasin antibodies, PaSTa 1 and PaSTa 2, and investigated their binding to human recombinant soluble tapasin and ERp57/tapasin by SPR methods. Both antibody Fabs bind with nanomolar affinities characterized by slow off rates. Based on these results we developed an MHC-I binding assay employing PaSTa 1-captured tapasin or ERp57/tapasin which reveals affinities in the micromolar range for selected MHC-I alleles. These affinities are notably weaker than those obtained for the structurally related but PLC-independent chaperone, TAPBPR (TAP-binding protein, related), and may reflect distinct evolutionarily derived functions for the two related molecules.