Ethanolamine Phosphate Linked to the First Mannose Residue of Glycosylphosphatidylinositol (GPI) Lipids Is a Major Feature of the GPI Structure That Is

Glycosylphosphatidylinositol (GPI) anchoring of proteins is catalyzed by GPI transamidase (GPIT), a multisubunit, endo-plasmic reticulum (ER)-localized enzyme. GPIT recognizes ER-translocated proteins that have a GPI-directing C-terminal sig-nal sequence and replaces this sequence with a preassembled GPI anchor. Although the GPI signal sequence has been exten-sively characterized, little is known about the structural features of theGPI lipid substrate that enable its recognition byGPIT. In a previous study we showed that mature GPIs could be co-im-munoprecipitated with GPIT complexes containing functional subunits (Vainauskas, S., andMenon, 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 wit