A human TAPBP (TAPASIN)-related gene, TAPBP-R

TAPASIN, a V-C1 (variable-constant) immunoglobulin superfamily (IgSF) molecule that links MHC class I molecules to the transporter associated with antigen processing (TAP) in the endoplasmic reticulum (ER) is encoded by the TAPBP gene, located near to the MHC at 6p21.3. A related gene was identified at chromosome position 12p13.3 between the CD27 and VAMP1 genes near a group of MHC-paralogous loci. The gene, which we have called TAPBP-R (R for related), also encodes a member of the IgSF, TAPASIN-R. Its putative product contains similar structural motifs to TAPASIN, with some marked differences, especially in the V domain, transmembrane and cytoplasmic regions. By using the mouse ortholog to screen tissue, we revealed that the TAPBP-R gene was broadly expressed. Sub-cellular localization showed that the bulk of TAPASIN-R is located within the ER but biotinylation experiments were consistent with some expression at thecell surface. TAPASIN-R lacks an obvious ER retention signal. The function of TAPASIN-R will be of interest in regards to the evolution of the immune system as well as antigen processing.