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J. Biol. Chem., Vol. 281, Issue 50, 38358-38364, December 15, 2006

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Ethanolamine Phosphate Linked to the First Mannose Residue of Glycosylphosphatidylinositol (GPI) Lipids Is a Major Feature of the GPI Structure That Is Recognized by Human GPI Transamidase^*

From the Department of Biochemistry, Weill Medical College of Cornell University, New York, New York 10021

Glycosylphosphatidylinositol (GPI) anchoring of proteins is catalyzed by GPI transamidase (GPIT), a multisubunit, endoplasmic reticulum (ER)-localized enzyme. GPIT recognizes ER-translocated proteins that have a GPI-directing C-terminal signal sequence and replaces this sequence with a preassembled GPI anchor. Although the GPI signal sequence has been extensively characterized, little is known about the structural features of the GPI lipid substrate that enable its recognition by GPIT. In a previous study we showed that mature GPIs could be co-immunoprecipitated with GPIT complexes containing functional subunits (Vainauskas, S., and Menon, A. K. (2004) J. Biol. Chem. 279, 6540–6545). We now use this approach, as well as a method that reconstitutes the interaction between GPIs and GPIT, to define the basis of the interaction between GPI and human GPIT. We report that (i) human GPIT can interact with GPI biosynthetic intermediates, not just mature GPIs competent for transfer to protein, (ii) the ethanolamine phosphate group on the third mannose residue of the GPI glycan is not critical for GPI recognition by GPIT, (iii) the ethanolamine phosphate residue linked to the first mannose of the GPI structure is a major feature of GPIs that is recognized by human GPIT, and (iv) the simplest GPI recognized by human GPIT is EtN-P-2Man1–4GlcN-(acyl)-phosphatidyl-inositol. These studies define the molecular characteristics of GPI that are recognized by GPIT and open the way to identifying GPIT subunits that are involved in this process.

Received for publication, September 18, 2006 , and in revised form, October 18, 2006.

^* This work was supported by National Institutes of Health Grant GM55427. 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: Dept. of Biochemistry, Weill Medical College of Cornell University, 1300 York Ave., New York, NY 10021. Tel.: 212-746-6432; Fax: 212-746-8875; E-mail: akm2003{at}med.cornell.edu.

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