Vps13-like proteins provide phosphatidylethanolamine for GPI anchor synthesis in the ER

Abstract

Glycosylphosphatidylinositol (GPI) is a glycolipid membrane anchor found on surface proteins in all eukaryotes. It is synthesized in the ER membrane. Each GPI anchor requires three molecules of ethanolamine phosphate (P-Etn), which are derived from phosphatidylethanolamine (PE). We found that efficient GPI anchor synthesis in Saccharomyces cerevisiae requires Csf1; cells lacking Csf1 accumulate GPI precursors lacking P-Etn. Structure predictions suggest Csf1 is a tube-forming lipid transport protein like Vps13. Csf1 is found at contact sites between the ER and other organelles. It interacts with the ER protein Mcd4, an enzyme that adds P-Etn to nascent GPI anchors, suggesting Csf1 channels PE to Mcd4 in the ER at contact sites to support GPI anchor biosynthesis. CSF1 has orthologues in Caenorhabditis elegans (lpd-3) and humans (KIAA1109/TWEEK); mutations in KIAA1109 cause the autosomal recessive neurodevelopmental disorder Alkuraya-Kučinskas syndrome. Knockout of lpd-3 and knockdown of KIAA1109 reduced GPI-anchored proteins on the surface of cells, suggesting Csf1 orthologues in human cells support GPI anchor biosynthesis.

Structural and biochemical insights into lipid transport by VPS13 proteins

“…For example, these could be part of the lipid biosynthetic machinery, allowing lipids to be removed from the membrane leaflet in which they are made to another compartment as they are made. Such a scenario was recently proposed for a newly identified member of the VPS13 family, Csf1p ( Toulmay et al, 2022 ). We predict that this model of a partnership between scramblases, or other classes of multipass integral membrane proteins, and lipid transporters will also be applicable to more recently identified members of the VPS13 protein family, including SHIP164, KIAA1109/Csf1p, and Hobbit/Hob1p/Hob2p ( Gomes Castro et al, 2021 ; Hanna et al, 2021 ; John Peter et al, 2021 ; Neuman et al, 2022 ; Toulmay et al, 2022 ).…”

A possible role for VPS13-family proteins in bulk lipid transfer, membrane expansion and organelle biogenesis

“…Thus far, in vitro biochemistry approaches indicate that VPS13 and ATG2 proteins can extract and transfer a wide range of glycerophospholipids, suggesting that these proteins are not inherently selective, and the structure of a long hydrophobic bridge is most consistent with a non-selective greasy slide. However, recently published work on the bridge-like protein Csf1 in yeast implicates loss of this protein in a failure to move sufficient amounts of phosphatidylethanolamine (PE) from the mitochondria to the ER to support glycosylphosphatidylinositol (GPI)-anchor formation ( Toulmay et al, 2022 ). In the same work, the authors establish that Csf1 directly interacts with the GPI synthesis protein Mcd4 and suggest that Csf1 therefore may be used to funnel mitochondrially derived PE directly into the synthesis enzyme, perhaps implying that PE is the main or only substrate during this event.…”

“…Although the early work connecting bridge proteins to lipid transport has focused on VPS13 proteins and ATG2, the family of chorein-domain-containing proteins is growing rapidly to include many other transporters. These include yeast Csf1 and its mammalian ortholog KIAA1109, as well as Fmp27 and Ypr117w (also known as Hob1 and Hob2, respectively; yeast homologs of Drosophila Hobbit) ( Gomes Castro et al, 2021 preprint ; John Peter et al, 2021 preprint ; Neuman et al, 2022 ; Toulmay et al, 2022 ), each of which associate with ER–mitochondria or ER–plasma membrane contact sites in the cytoplasm, as well as Mdm31 and Mdm32, which function within the mitochondria intramembrane space ( Levine, 2019 ; Miyata et al, 2017 ). To elucidate whether these proteins are functionally redundant, how they are targeted to specific contact sites and how differences in their structures are used in the cell will require additional work.…”

Structural and biochemical insights into lipid transport by VPS13 proteins

“…For example, these could be part of the lipid biosynthetic machinery, allowing lipids to be removed from the membrane leaflet in which they are made to another compartment as they are made. Such a scenario was recently proposed for a newly identified member of the VPS13 family, Csf1p (Toulmay et al, 2022). We predict that this model of a partnership between scramblases, or other classes of multi-pass integral membrane proteins, and lipid transporters will also be applicable to more recently identified members of the VPS13 protein family, including SHIP164, KIAA1109/Csf1p, and Hobbit/Hob1p/Hob2p (Gomes Castro et al, 2021; Hanna et al, 2021; John Peter et al, 2021; Neuman et al, 2022; Toulmay et al, 2022).…”