Abstract
More than 200 diverse secretory proteins from Arabidopsis thaliana carry a glycosylphosphatidylinositol (GPI) lipid anchor covalently attached to their carboxyl-terminus. The GPI-anchor contains a lipid-linked glycan backbone that is preassembled in the endoplasmic reticulum (ER) of plants and subsequently transferred to distinct proteins, which provides them with specific features. The GPI-anchored proteins exit the ER and are transported through the Golgi apparatus to the plasma membrane. In the Golgi, the glycan moiety can be further modified by the specific attachment of sugar residues. While these biosynthetic steps are already quite well understood in mammals and yeast, comparatively little is known in plants. In this perspective, we discuss the current knowledge about the biosynthesis of the GPI-anchor glycan moiety in the light of recent findings for mammalian GPI-anchor glycan modifications.
Highlights
The attachment of glycosylphosphatidylinositol (GPI) is a common posttranslational modification for anchoring of proteins to the outer surface of the plasma membrane in eukaryotes
The detection of a single hexose as GPI side chain modification raises several questions: which glycosyltransferase catalyzes this step, which donor substrate is used in the reaction and in which subcellular compartment is the side chain modified
The lack of a PIG-Z homolog in plants (Ellis et al, 2010; Luschnig and Seifert, 2011) is consistent with the absence of a fourth mannose residue attached to the GPI glycan core in the endoplasmic reticulum (ER) unless ABNORMAL POLLEN TUBE GUIDANCE1 (APTG1) transfers a second mannose residue as it has been suggested for PIG-B (Wang et al, 2020)
Summary
The attachment of glycosylphosphatidylinositol (GPI) is a common posttranslational modification for anchoring of proteins to the outer surface of the plasma membrane in eukaryotes. Glycosylphosphatidylinositol biosynthesis is initiated at the cytosolic side of the ER by the transfer of GlcNAc from the nucleotide sugar UDP-GlcNAc to inositol to generate GlcNAc-PI (Figure 1) This step is catalyzed by the GPI-GlcNAc-transferase (GPI-GnT), a complex consisting of seven protein subunits in mammalian cells. The resulting structure is competent for transfer to proteins, but can be further modified by the attachment of a fourth mannose residue catalyzed by the GPI α1,2-mannosyltransferase PIG-Z. These findings are consistent with the structure from AGP isolated from pear cells (Oxley and Bacic, 1999) indicating that attachment of a single galactose in β-linkage is a common side chain formation of the GPI core glycan in plants
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