IDENTIFICATION OF A MISSING LINK IN GLYCOSYLPHOSPHATIDYLINOSITOL ANCHOR BIOSYNTHESIS IN MAMMALIAN-CELLS

A large number of mammalian proteins are anchored to the cell membrane by a glycosylphosphatidylinositol (GPI) anchor. Biosynthetic intermed iates of the GPI anchor have been identified in mammalian cells. The e arly GPI precursors are sensitive to phosphatidylinositol (PI)-specifi c phospholipase C (PLC). However, all of the later GPI precursors, whi ch contain 1 or more mannose residues, are PI-PLC-resistant, suggestin g that there is another unidentified precursor. Here, we report the id entification of this missing link. This GPI precursor can only be labe led with glucosamine and inositol, and is resistant to PI-PLC but sens itive to GPI-phospholipase D. It accumulates in large quantity only in mutants which are defective in the addition of the first mannose resi due to the elongating GPI core. Thus, fatty acylation of glucosaminylp hosphatidylinositol, to render it PI-PLC-resistant, is an obligatory s tep in the biosynthesis of mammalian GPI anchor precursors.