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MHC Class I-Like MILL Molecules Are ₂-Microglobulin-Associated, GPI-Anchored Glycoproteins That Do Not Require TAP for Cell Surface Expression¹

Mizuho Kajikawa^*,, Tomohisa Baba, Utano Tomaru, Yutaka Watanabe^*, Satoru Koganei, Sachiyo Tsuji-Kawahara^¶, Naoki Matsumoto, Kazuo Yamamoto, Masaaki Miyazawa^¶, Katsumi Maenaka, Akihiro Ishizu^,|| and Masanori Kasahara^2,*,

^* Department of Biosystems Science, School of Advanced Sciences, Graduate University for Advanced Studies (Sokendai), Hayama, Japan; Division of Structural Biology, Medical Institute of Bioregulation, Kyushu University, Fukuoka, Japan; Department of Pathology, Hokkaido University Graduate School of Medicine, Sapporo, Japan; Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Chiba, Japan; ^¶ Department of Immunology, Kinki University School of Medicine, Osaka, Japan; and ^|| Department of Medical Technology, Hokkaido University School of Medicine, Sapporo, Japan

MILL (MHC class I-like located near the leukocyte receptor complex) is a family of MHC class I-like molecules encoded outside the MHC, which displays the highest sequence similarity to human MICA/B molecules among known class I molecules. In the present study, we show that the two members of the mouse MILL family, MILL1 and MILL2, are GPI-anchored glycoproteins associated with ₂-microglobulin (₂m) and that cell surface expression of MILL1 or MILL2 does not require functional TAP molecules. MILL1 and MILL2 molecules expressed in bacteria could be refolded in the presence of ₂m, without adding any peptides. Hence, neither MILL1 nor MILL2 is likely to be involved in the presentation of peptides. Immunohistochemical analysis revealed that MILL1 is expressed in a subpopulation of thymic medullary epithelial cells and a restricted region of inner root sheaths in hair follicles. The present study provides additional evidence that MILL is a class I family distinct from MICA/B.

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 grants-in-aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan, Uehara Memorial Foundation, the Naito Foundation, and the Takeda Science Foundation.

² Address correspondence and reprint requests to Dr. Masanori Kasahara, Department of Pathology, Hokkaido University Graduate School of Medicine, North-15, West-7, Sapporo 060-8638, Japan. E-mail address: mkasaha{at}med.hokudai.ac.jp

³ Abbreviations used in this paper: ₂m, ₂-microglobulin; MILL, MHC class I-like located near the leukocyte receptor complex; PI-PLC, phosphatidylinositol-specific phospholipase C; PNGase F, peptide:N-glycosidase F.

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