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MBC in Press, published online ahead of print January 5, 2005
Mol. Biol. Cell 10.1091/mbc.E04-09-0802

A more recent version of this article appeared on March 1, 2005
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Submitted on September 14, 2004
Revised on December 10, 2004
Accepted on December 16, 2004

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

Hisashi Ashida,* Yeongjin Hong,{dagger} Yoshiko Murakami,* Nobue Shishioh,* Nakaba Sugimoto,{ddagger} Youn Uck Kim,{sect} Yusuke Maeda,* and Taroh Kinoshita*||

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

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 multi-transmembrane proteins with common characteristics, forming a superfamily of more than ten 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 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 S. 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 utilizing dolichol-phosphate-monosaccharide.

||Corresponding author. E-mail: tkinoshi{at}biken.osaka-u.ac.jp






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