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Interaction of Bap31 and MHC Class I Molecules and Their Traffic Out of the Endoplasmic Reticulum ¹

Fumiyoshi Abe^2,*,, Nancy Van Prooyen^2,3,*, John J. Ladasky^* and Michael Edidin^4,*

^* Department of Biology, Johns Hopkins University, Baltimore, MD 21218; and Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan

The endoplasmic reticulum (ER) protein Bap31 associates with nascent class I MHC molecules. It appears to mediate the export of class I MHC molecules from the ER and may also be involved in their quality control. In this study, we use Förster resonance energy transfer and quantitative fluorescence imaging to show that in human, HeLa cells, Bap31 clusters with MHC class I (HLA-A2) molecules in the ER, and traffics via export vesicles to the ER/Golgi intermediate compartment. Förster resonance energy transfer between Bap31 and HLA-A2 and forward traffic increases when MHC class I molecules are loaded with a pulse of peptide. The increased forward traffic is blocked by overexpression of Bap29, a partner protein for Bap31, which localizes to the ER. Thus, in HeLa cells, Bap31 is involved in the exit of peptide-loaded MHC class I from the ER, and its function is regulated by its interaction with its homologue, Bap29.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ This work was supported by a grant from the National Institutes of Health, AI-14584 (to M.E.).

² F.A. and N.V.P. contributed equally to this work.

³ N.V.P. is a Gilliam Graduate Fellow of the Howard Hughes Medical Institute.

⁴ Address correspondence and reprint requests to Dr. Michael Edidin, Biology Department, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218. E-mail address: Edidin{at}jhu.edu

⁵ Abbreviations used in this paper: ER, endoplasmic reticulum; A:D, acceptor-to-donor ratio; CFP, cyan fluorescent protein; ERGIC, ER/Golgi intermediate compartment; FRET, Förster resonance energy transfer; PCCF, pair cross-correlation function; ROI, region of interest; YFP, yellow fluorescent protein.

⁶ The online version of this article contains supplemental material.