Ethanolamine phosphate on the second mannose as bridge in GPI anchored proteins: Towards understanding inherited PIGG deficiency
AbstractGlycosylphosphatidylinositols (GPIs) are glycolipids that anchor many proteins (GPI-APs) on the cell surface. GPI precursor has three mannoses, which in mammalian cells, are all modified by the addition of ethanolamine-phosphate (EthN-P). It is postulated that EthN-P on the third mannose (EthN-P-Man3) is the bridge between GPI to the protein and the second (EthN-P-Man2) is removed after GPI-protein attachment. However, EthN-P-Man2 may not be always transient as postulated, as mutations of PIGG, the enzyme that transfers EthN-P to Man2, result in inherited GPI deficiencies (IGDs), characterized by neuronal dysfunctions. We hypothesized that EthN-P-Man2 plays a role beyond what is now postulated. Indeed, EthN-P on Man2 is not only the alternate bridge in some GPI-APs but the actual bridge in others, among them, the ect-5’-nucleotidase and netrin G2. We found that CD59, a GPI-AP, is attached via EthN-P-Man2 in both PIGB-knockout cells, in which GPI lacks Man3 and with a small fraction, in wild type cells. Our findings modify the current view of GPI anchoring and provide mechanistic bases of IGDs caused by PIGG mutations.