Ethanolamine‐phosphate on the second mannose is a preferential bridge for some GPI‐anchored proteins

Abstract

Glycosylphosphatidylinositols (GPIs) are glycolipids that anchor many proteins (GPI-APs) on the cell surface. The core glycan of GPI precursor has three mannoses, which in mammals, are all modified by ethanolamine-phosphate (EthN-P). It is postulated that EthN-P on the third mannose (EthN-P-Man3) is the bridge between GPI and the protein and the second (EthN-P-Man2) is removed after GPI-protein attachment. However, EthN-P-Man2 may not be always transient, as mutations of PIGG, the enzyme that transfers EthN-P to Man2, result in inherited GPI deficiencies (IGDs), characterized by neuronal dysfunctions. Here, we show that EthN-P on Man2 is the preferential bridge in some GPI-APs, among them, the Ect-5'-nucleotidase and Netrin G2. We find that CD59, a GPI-AP, is attached via EthN-P-Man2 both in 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 a mechanistic basis for IGDs caused by PIGG mutations.

Structures of liganded glycosylphosphatidylinositol transamidase illuminate GPI-AP biogenesis

“…Second, GPI-T exhibits promiscuity for the GPI substrate. For example, EtNP3 on GPI is long thought to be the sole physiological bridge for GPI attachment, but a recent study 14 shows EtNP2 is the preferred choice for some GPI-APs. What is more, GPI-T exhibits activities with non-GPI amines including hydroxylamine 15 .…”

Sequential hydrolysis of FAD by ecto-5′ nucleotidase CD73 and alkaline phosphatase is required for uptake of vitamin B2 into cells

“…In future, we are planning to analyze the metabolic conditions in vivo using IGD model mice ( 29 ). CD73 expression is decreased in phosphatidylinositol glycan anchor biosynthesis class G (PIGG) KO cells ( 30 ) and some cases with null mutation of PIGG also showed decreased expression of CD73 and mitochondrial dysfunction ( 31 ), suggesting that CD73 expression is important for uptake of vitamin B 2 in PIGG deficiency. Similar to HPP, some patients with IGD show decreased vitamin B 6 uptake and suffer from PN-dependent seizures ( 32 ).…”

Requirement of sequential hydrolysis by CD73 and ALP for uptake of vitamin B2 into cells

“…This is consistent with reports that some severe IGD cases show mitochondrial dysfunction (Tarailo-Graovac et al , 2015). CD73 expression is decreased in phosphatidylinositol glycan anchor biosynthesis class G (PIGG) knockout cells (Ishida et al , 2022) and some cases with null mutation of PIGG also showed decreased expression of CD73 and mitochondrial dysfunction (Tremblay-Laganière et al , 2021), suggesting that CD73 expression is important for uptake of vitamin B 2 in PIGG deficiency. Similar to HPP, some patients with IGD show decreased vitamin B 6 uptake and suffer from pyridoxine-dependent seizures (Kuki et al , 2013).…”