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¹ Laboratory of Alzheimer's Disease, RIKEN Brain Science Institute, Wako, Saitama, Japan
² Molecular Membrane Biology Laboratory, RIKEN Advanced Science Institute, Wako, Saitama 351-0198, Japan
³ Department of Biological Sciences, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

In an attempt to express human β-amyloid precursor protein (APP) in yeast, we fortuitously found that this protein is only O-glycosylated in yeast. APP was effectively expressed in yeast, processed by yeast -secretases, members of the Yapsin family, to produce N-terminal (sAPP) and C-terminal (CTF) domains, when its signal sequence was replaced by that of the yeast -mating factor. APP is known to acquire N- and O-glycosylation through the endoplasmic reticulum (ER) and the Golgi apparatus and is transported to the plasma membrane in mammalian cells. In spite of the presence of canonical N-glycosylation consensus sequences, APP was not N-glycosylated in the yeast system. Pulse-chase experiments demonstrated that APP received only O-mannosylation in yeast. Examination of yeast pmt mutants, which are defective in the initiation of O-mannosylation in the ER, revealed that Pmt4p is most responsible for the oligosaccharide modification of APP. Maturation of APP was slowed down and aggregated forms of APP were observed by sucrose density gradient fractionation of the pmt4 mutant lysate. This caused decreased production of CTF. We conclude that O-mannosylation is required for the solubilization of exogenously expressed human APP.

^aPresent address: Department of Health Sciences, Graduate School of Medicine, Tohoku University, 2-1 Seiryo, Aoba-ku, Sendai 980-8575, Japan

^bPresent address: Laboratory of Molecular Traffic, Department of Molecular Cellular Biology, Institute of Molecular and Cellular Regulation, Gunma University, Maebashi, Gunma 371-8512, Japan

^cPresent address: Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan

^* Correspondence: nakano{at}riken.jp

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