Component of the antigen processing and presentation pathway, which binds to MHC class I coupled with beta2-microglobulin/B2M. Association between TAPBPR and MHC class I occurs in the absence of a functional peptide-loading complex (PLC). Interacts with peptide-free HLA-A*02-B2M complexes or those loaded with low affinity peptides, likely facilitating peptide exchange onto higher affinity peptides. Mainly found in endoplasmic reticulum but a minority is found on the cell surface (PubMed:11920573). By interferon gamma. Has reduced cell surface expression, and does not down-regulate MHC class I surface expression as efficiently as isoform alpha. Does not interact with MHC class I. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay.