Practice question

Classification of lipid-anchored proteins includes GPI anchor addition in endoplasmic reticulum lumen via multi-step pathway conserved across eukaryotes. GPI precursor synthesis starts on cytoplasmic face adding N-acetylglucosamine to phosphatidylinositol by PIG-A, deacetylation by PIG-L, flipping to luminal side by flippase, addition of three mannose residues from dolichol-phosphate-mannose by PIG-M PIG-V PIG-B, addition of phosphoethanolamine by PIG-O PIG-F, final transfer en bloc to C-terminal GPI signal peptide cleavage site containing omega site small residues by GPI transamidase comprising PIG-K catalytic cysteine protease forming amide bond between protein carboxyl and phosphoethanolamine linked to terminal mannose glycan mannose core attached via glucosamine to inositol diacylglycerol embedding in outer leaflet. Proteins remain tethered high lateral mobility localized lipid rafts cleavable by phospholipase C or D releasing soluble forms. Examples hydrolytic enzymes alkaline phosphatase ecto-5 nucleotidase, adhesion molecules Thy-1 CD90, prion PrPc, regulators CD55 CD59. Multipass proteins aquaporin six helices Band 3 fourteen helices GPCR seven helices cross bilayer via peptide helices not lipid tether, distinguishing GPI-linked class.