Deletion of GPI7, a yeast gene required for addition of a side chain to the glycosylphosphatidylinositol (GPI) core structure, affects GPI protein transport, remodeling, and cell wall integrity

Gpi7 was isolated by screening for mutants defective in the surface express ion of glycosylphosphatidylinositol (GPI) proteins. Gpi7 mutants are defici ent in YJL062w, herein named GP17. GPI7 is not essential, but its deletion renders cells hypersensitive to Calcofluor White, indicating cell wall frag ility. Several aspects of GPI biosynthesis are disturbed in Delta gpi7. The extent of anchor remodeling, i.e. replacement of the primary lipid moiety of GPI anchors by ceramide, is significantly reduced, and the transport of GPI proteins to the Golgi is delayed, Gpi7p is a highly glycosylated integr al membrane protein with 9-11 predicted transmembrane domains in the C-term inal part and a large, hydrophilic N-terminal ectodomain. The bulk of Gpi7p is located at the plasma membrane, but a small amount is found in the endo plasmic reticulum. GP17 has homologues in Saccharomyces cerevisiae, Caenorh abditis elegans, and man, but the precise biochemical function of this prot ein family is unknown. Based on the analysis of M4, an abnormal GPI lipid a ccumulating in gpi7, we propose that Gpi7p adds a side chain onto the GPI c ore structure. Indeed, when compared with complete GPI lipids, M4 lacks a p reviously unrecognized phosphodiester-linked side chain, possibly an ethano lamine phosphate. Gpi7p contains significant homology with phosphodiesteras es suggesting that Gpi7p itself is the transferase adding a side chain to t he alpha 1,6-linked mannose of the GPI core structure.