#GPI-anchored proteins

Lipid Link Protein

Which technique is used to determine whether a protein is GPI-anchored?

Distinguishing GPI anchoring from transmembrane helices or other lipid modifications requires an enzymatic strategy that specifically targets the glycolipid linkage chemistry. A mature GPI anchor contains phosphoethanolamine in amide bond to the protein C-terminus, a conserved tetrasaccharide core of three mannose residues and glucosamine, and phosphatidylinositol whose diacylglycerol or alkylacyl side chains embed in the exoplasmic leaflet. Bacterial phosphatidylinositol-specific phospholipase C from Bacillus thuringiensis cleaves the phosphodiester between glycerol and phosphate, releasing p

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 10, GPI-anchored proteins and PI-PLC assay.

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Lipid Link Protein

Which of the following statements is true regarding GPI-anchored proteins?

Glycosylphosphatidylinositol anchored proteins are synthesized in endoplasmic reticulum via en bloc transfer of preassembled GPI moiety to C-terminus after cleavage of hydrophobic signal, trafficked through Golgi where lipid remodeling occurs, and delivered to extracellular leaflet of plasma membrane where they reside in ordered lipid rafts enriched in cholesterol and sphingolipid. Attachment chemistry involves phosphoethanolamine bridging peptide carboxyl to glycan tetramannosyl glucosamine core linked via phosphodiester to phosphatidylinositol lipid with two fatty acyl chains inserted into o

Ref: Kinoshita & Fujita, Biochim Biophys Acta 2016, GPI-anchored proteins localization and bonds.

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Lipid Link Protein

The lipid anchor in GPI-anchored proteins is attached to which part of the protein?

Glycosylphosphatidylinositol anchoring is complex post-translational modification processed in endoplasmic reticulum for about 150 human proteins destined for extracellular leaflet. Nascent proteins contain C-terminal hydrophobic signal peptide that is recognized by transamidase complex, cleaved after a specific omega residue, and replaced en bloc with preformed GPI moiety consisting of phosphatidylinositol lipid, glucosamine, three mannoses, phosphoethanolamine and galactose modifications. Attachment occurs to new C-terminus via amide linkage between protein carboxyl group and ethanolamine ph

Ref: Fujita & Kinoshita, J Lipid Res 2012, GPI anchor biosynthesis and attachment.

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Protein Sorting and Trafficking

GPI-anchored proteins are synthesized in:

Glycosylphosphatidylinositol anchoring provides alternative membrane attachment without transmembrane span, enabling rapid lateral diffusion within lipid rafts, apical sorting and regulated release by phospholipases. Precursors contain N-terminal ER targeting signal and C-terminal GPI attachment signal comprising small residues at omega cleavage site followed by moderately polar spacer and hydrophobic tail of fifteen to twenty residues. Co-translationally inserted into ER lumen via Sec61, nascent chain may receive N-glycans, then GPI transamidase complex, pentamer of PIG-K catalytic cysteine p

Ref: Kinoshita T, Annu Rev Biochem 89: 2020, GPI-Anchored Protein Biosynthesis in ER.

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