TAPBPR isoforms exhibit altered association with MHC class I

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The tapasin-related protein TAPBPR is a novel component of the antigen processing and presentation pathway, which binds to MHC class I coupled withβ2-microglobulin. We describe six alternatively spliced TAPBPR transcripts from theTAPBPLgene and investigate three of these at a protein level. TAPBPR transcripts lacking exon 5 result in loss of the membrane proximal IgC domain and loss of ability to bind to MHC class I. Alternative acceptor and donor splice sites in exon 4 of TAPBPR altered the reading frame in the IgV domain and produced a truncated TAPBPR product. An additional exon in theTAPBPLgene was identified that encodes extra residues in the cytoplasmic tail of TAPBPR. This longer TAPBPR protein interacted with MHC class I but was attenuated in its ability to down-regulate surface expression of MHC class I. The abundance of these alternative transcripts in peripheral blood mononuclear cells and dendritic cells suggests an important role of TAPBPR isoformsin vivo.

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