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J. Biol. Chem., Vol. 282, Issue 12, 8613-8621, March 23, 2007

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Regulation of the Transport and Protein Levels of the Inositol Phosphorylceramide Mannosyltransferases Csg1 and Csh1 by the Ca²⁺-binding Protein Csg2^*

Satoshi Uemura^¶, Akio Kihara^¶1, Soichiro Iwaki^¶, Jin-ichi Inokuchi^¶, and Yasuyuki Igarashi^¶||

From the Division of Glycopathology, Institute of Molecular Biomembrane and Glycobiology, Tohoku Pharmaceutical University, 4-4-1, Komatsushima, Aoba-ku, Sendai, Miyagi 981-8558, Japan, the Core Research for Evolutional Science and Technology Program (CREST), Japan Science and Technology Agency (JST), 4-1-8, Honcho Kawaguchi, Saitama 332-0012, Japan, the ^¶Laboratory of Biomembrane and Biofunctional Chemistry, Faculty of Pharmaceutical Sciences, Hokkaido University, Kita 12-jo, Nishi 6-choume, Kita-ku, Sapporo 060-0812, Japan, and the ^||Laboratory of Biomembrane and Biofunctional Chemistry, Faculty of Advanced Life Science, Hokkaido University, Kita 21-jo, Nishi 11-choume, Kita-ku, Sapporo 001-0021, Japan

Complex sphingolipids in yeast are known to function in cellular adaptation to environmental changes. One of the yeast complex sphingolipids, mannosylinositol phosphorylceramide (MIPC), is produced by the redundant inositol phosphorylceramide (IPC) mannosyltransferases Csg1 and Csh1. The Ca²⁺-binding protein Csg2 can form a complex with either Csg1 or Csh1 and is considered to act as a regulatory subunit. However, the role of Csg2 in MIPC synthesis has remained unclear. In this study, we found that Csg1 and Csh1 are N-glycosylated with core-type and mannan-type structures, respectively. Further identification of the glycosylated residues suggests that both Csg1 and Csh1 exhibit membrane topology with their C termini in the cytosol and their mannosyltransferase domains in the lumen. After complexing with Csg2, both Csg1 and Csh1 function in the Golgi, and then are delivered to the vacuole for degradation. However, uncomplexed Csh1 cannot exit from the endoplasmic reticulum. We also demonstrated that Ca²⁺ stimulates IPC-to-MIPC conversion, because of a Csg2-dependent increase in Csg1 levels. Thus, Csg2 has several regulatory functions for Csg1 and Csh1, including stability, transport, and gene expression.

Received for publication, July 13, 2006 , and in revised form, December 8, 2006.

^* This work was supported by a Grant-in-Aid for Young Scientists (A) (17687011) from the Ministry of Education, Culture, Sports, Sciences and Technology of Japan. 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 To whom correspondence should be addressed: Laboratory of Biomembrane and Biofunctional Chemistry, Faculty of Pharmaceutical Sciences, Hokkaido University, Kita 12-jo, Nishi 6-choume, Kita-ku, Sapporo 060-0812, Japan. Tel.: 81-11-706-3971; Fax: 81-11-706-4986; E-mail: kihara{at}pharm.hokudai.ac.jp.

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