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

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 MHCclass 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.