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Tapasin Increases Efficiency of MHC I Assembly in the Endoplasmic Reticulum but Does Not Affect MHC I Stability at the Cell Surface¹

Department of Biology, Johns Hopkins University, Baltimore, MD 21218

Cell surface expression of MHC I molecules depends on the chaperone tapasin; how tapasin functions is not fully understood. We created three fluorescent tapasin constructs: wild-type tapasin, soluble tapasin, which does not interact with TAP, and N300 tapasin, which does not interact with MHC I. In contrast to earlier reports, all three constructs localize to the endoplasmic reticulum (ER), though soluble tapasin is more mobile than wild type and N300. Soluble tapasin does not increase MHC I surface levels to the same extent as wild type, which suggests that proximity to TAP is necessary for full tapasin function. N300 acts as a dominant-negative perhaps by blocking wild-type tapasin access to TAP. None of the constructs affects MHC I stability at the cell surface, although stability of ER resident MHC I is decreased in tapasin-negative cells. We propose that tapasin acts primarily to increase efficiency of assembly of MHC I within the ER.

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¹ This work was supported by National Institutes of Health Grant AI14584.

² Address correspondence and reprint requests to Dr. Michael Edidin, Johns Hopkins University, 144 Mudd Hall, 3400 North Charles Street, Baltimore, MD 21218. E-mail address: edidin{at}jhu.edu

³ Abbreviations used in this paper: ER, endoplasmic reticulum; CFP, cyan fluorescent protein; YFP, yellow fluorescent protein; BFA, brefeldin A; FRAP, fluorescence recovery after photobleaching.