Figure 1. Schematic of the core GPI anchor glycan and its side chain modifications.

GPI anchors consist of a highly conserved carbohydrate sequence that is linked to the inositol head group (hexagon) of phosphatidylinositol (hexagon + LIPID). The core GPI glycan contains glucosamine (blue/white square) and three mannoses (Man I-III, green circles). A protein is linked to the third mannose via an ethanolamine phosphate. R indicates positions where modification of the core GPI structure can occur, and possible R modifications in various species are described in Table 1. The red R₁ indicates the position of fourth mannose addition.