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Vol. 16, Issue 3, 1439-1448, March 2005

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Mammalian PIG-X and Yeast Pbn1p Are the Essential Components of Glycosylphosphatidylinositol-Mannosyltransferase I

Hisashi Ashida ^*, Yeongjin Hong , Yoshiko Murakami ^*, Nobue Shishioh ^*, Nakaba Sugimoto , Youn Uck Kim , Yusuke Maeda ^*, and Taroh Kinoshita ^*

^* Department of Immunoregulation, Research Institute for Microbial Diseases, Osaka University, Osaka 565-0871, Japan; Genomic Research Center for Enteropathogenic Bacteria and Department of Microbiology, Chonnam National University Medical School, Gwangju, Korea; Division of Advanced Medical Bacteriology, Department of Molecular and Applied Medicine, Graduate School of Medicine, Osaka University, Osaka 565-0871, Japan; and Division of Applied Biological Sciences, Sunmoon University, Asan, Korea

Submitted September 14, 2004; Revised December 10, 2004; Accepted December 16, 2004
Monitoring Editor: Reid Gilmore

Within the endoplasmic reticulum (ER), mannoses and glucoses, donated from dolichol-phosphate-mannose and -glucose, are transferred to N-glycan and GPI-anchor precursors, and serine/threonine residues in many proteins. Glycosyltransferases that mediate these reactions are ER-resident multitransmembrane proteins with common characteristics, forming a superfamily of >10 enzymes. Here, we report an essential component of glycosylphosphatidylinositol-mannosyltransferase I (GPI-MT-I), which transfers the first of the four mannoses in the GPI-anchor precursors. We isolated a Chinese hamster ovary (CHO) cell mutant defective in GPI-MT-I but not its catalytic component PIG-M. The mutant gene, termed phosphatidylinositolglycan-class X (PIG-X), encoded a 252-amino acid ER-resident type I transmembrane protein with a large lumenal domain. PIG-X and PIG-M formed a complex, and PIG-M expression was <10% in the absence of PIG-X, indicating that PIG-X stabilizes PIG-M. We found that Saccharomyces cerevisiae Pbn1p/YCL052Cp, which was previously reported to be involved in autoprocessing of proproteinase B, is the functional homologue of PIG-X; Pbn1p is critical for Gpi14p/YJR013Wp function, the yeast homologue of PIG-M. This is the first report of an essential subcomponent of glycosyltransferases using dolichol-phosphate-monosaccharide.

Abbreviations used: CHO, Chinese hamster ovary; DAF, decay accelerating factor; Dol-P, dolichol-phosphate; ER, endoplasmic reticulum; EtNP, phosphoethanolamine; FACS, fluorescence-activated cell sorter; GlcN-PI(C8), glucosaminyl-PI with di-octanoyl groups; GPI, glycosylphosphatidylinositol; MT, mannosyltransferase; PI, phosphatidylinositol.

Address correspondence to: Taroh Kinoshita (tkinoshi{at}biken.osaka-u.ac.jp).

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