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Different MHC Class I Heavy Chains Compete with Each Other for Folding Independently of ₂-Microglobulin and Peptide¹

Sophie Tourdot^2,*, Mohamed Nejmeddine^*, Simon J. Powis and Keith G. Gould^3,*

^* Department of Immunology, Wright-Fleming Institute, Imperial College London, London, United Kingdom; and Division of Cell Biology and Immunology, University of Dundee, Dundee, United Kingdom

We reported previously that different MHC class I molecules can compete with each other for cell surface expression in F₁ hybrid and MHC class I transgenic mice. In this study, we show that the competition also occurs in transfected cell lines, and investigate the mechanism. Cell surface expression of an endogenous class I molecule in Chinese hamster ovary (CHO) cells was strongly down-regulated when the mouse K^d class I H chain was introduced by transfection. The competition occurred only after K^d protein translation, not at the level of RNA, and localization studies of a CHO class I-GFP fusion showed that the presence of K^d caused retention of the hamster class I molecule in the endoplasmic reticulum. The competition was not for ₂-microglobulin, because a single chain version of K^d that included mouse ₂-microglobulin also had a similar effect. The competition was not for association with TAP and loading with peptide, because a mutant form of the K^d class I H chain, not able to associate with TAP, caused the same down-regulation of hamster class I expression. Moreover, K^d expression led to a similar level of competition in TAP2-negative CHO cells. Competition for cell surface expression was also found between different mouse class I H chains in transfected mouse cells, and this competition prevented association of the H chain with ₂-microglobulin. These unexpected new findings show that different class I H chains compete with each other at an early stage of the intracellular assembly pathway, independently of ₂-microglobulin and peptide.

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