Tapasin-related protein TAPBPR is an additional component of the MHC class I presentation pathway

Abstract

Tapasin is an integral component of the peptide-loading complex (PLC) important for efficient peptide loading onto MHC class I molecules. We investigated the function of the tapasin-related protein, TAPBPR. Like tapasin, TAPBPR is widely expressed, IFN-γ-inducible, and binds to MHC class I coupled with β2-microglobulin in the endoplasmic reticulum. In contrast to tapasin, TAPBPR does not bind ERp57 or calreticulin and is not an integral component of the PLC. β2-microglobulin is essential for the association between TAPBPR and MHC class I. However, the association between TAPBPR and MHC class I occurs in the absence of a functional PLC, suggesting peptide is not required. Expression of TAPBPR decreases the rate of MHC class I maturation through the secretory pathway and prolongs the association of MHC class I on the PLC. The TAPBPR: MHC class I complex trafficks through the Golgi apparatus, demonstrating a function of TAPBPR beyond the endoplasmic reticulum/ cis-Golgi. The identification of TAPBPR as an additional component of the MHC class I antigen-presentation pathway demonstrates that mechanisms controlling MHC class I expression remain incompletely understood.

The Binding of TAPBPR and Tapasin to MHC Class I Is Mutually Exclusive

“…The following Abs were used: rabbit anti-TAPBPR R014 raised against aa 22-406 of human TAPBPR (27); rabbit anti-TAPBPR R021 raised against the cytoplasmic tail of human TAPBPR; a conformational specific mAb raised against aa 22-406 of human TAPBPR (PeTe4) (27); a mouse anti-TAPBPR raised against aa 23-122 of full-length human TAPBPR (ab57411; Abcam); the tapasin-specific mAbs Pasta1 and Rgp48N (both kind gifts from Peter Cresswell, Yale University School of Medicine, New Haven, CT); rabbit anti-GFP (Ab290; Abcam); mouse anti-GFP (Roche); rabbit anti-calnexin (Enzo Life Sciences); mAb HC10, which recognizes HLA-A, HLA-B, and HLA-C containing a PxxWDR motif at aa 57-62 in the a1 domain (33,34); mAb specific for conformational HLA-A2 and HLA-A68 (One Lambda); and HLA-A2-specific mAb BB7.2 (35). IgG1 and IgG2a isotype-control Abs were also used (Dako).…”

“…This panel of TAPBPR molecules was cloned into a lentiviral expression vector with a bicistronic GFP reporter and transduced into HeLa cells. Because endogenous TAPBPR expression is undetectable in HeLa cells (27), this cell line provides an ideal system for testing the effect of alterations to TAPBPR on its ability to bind to MHC class I. To produce stable HeLa cells expressing comparable TAPBPR levels, cell lines were sorted based on emerald expression encoded bistronically from the TAPBPR protein.…”

“…Overexpression of TAPBPR in HeLa cells results in downregulation of MHC class I expression on the cell surface (27). This phenotypic effect was used as an assay to determine the effect of the TAPBPR mutants on MHC class I surface expression.…”

“…Although the amino acid sequence of TAPBPR is only 22% identical to tapasin, TAPBPR also binds to MHC class I H chain/b2m heterodimers in the ER (27). However, in contrast to tapasin, human TAPBPR does not associate with TAP, ERp57, or calreticulin and is not essential for peptide loading onto MHC class I molecules.…”

“…However, in contrast to tapasin, human TAPBPR does not associate with TAP, ERp57, or calreticulin and is not essential for peptide loading onto MHC class I molecules. TAPBPR decreases the rate at which MHC class I molecules mature through the secretory pathway (27). Although it is not a component of the peptide loading complex, TAPBPR is necessary to maintain prolonged contact of MHC class I with the peptide loading complex, a role that might be important for peptide selection by MHC class I molecules.…”