Glypiation

Similar to the precursor glycan used for N-glycosylation, GPI anchor biosynthesis begins on the cytoplasmic leaflet of the ER and is completed on the luminal side. During this process, 3-4 Man and various other sugars (e.g., GlcNAc, Gal) are built onto a phosphatidylinositol (PI) molecule embedded in the membrane using sugars donated from sugar nucleotides and dolichol-P-mannose outside and inside the ER, respectively. Additionally, 2-3 phosphoethanolamine (EtN-P) linker residues are donated from phosphatidylethanolamine in the ER lumen to facilitate binding of the anchor to proteins.^{[9][10][11][12][13]}

Proteins destined to be glypiated have two signal sequences:

• An N-terminal signal sequence that directs co-translational transport into the ER
• A C-terminal signal sequence that is recognized by a GPI transamidase (GPIT)^[8]

GPIT does not have a consensus sequence but instead recognizes a C-terminal sequence motif that enables it to covalently attach a GPI anchor to an amino acid in the sequence. This C-terminal sequence is embedded in the ER membrane immediately after translation, and the protein is then cleaved from the sequence and attached to a preformed GPI anchor.^[14][15]

In silico prediction of glypiation sites can be performed by:

• GPI-SOM: Identification of GPI-anchor signals by a Kohonen Self Organizing Map Archived 2023-06-25 at the Wayback Machine
• PredGPI: a GPI-anchor predictor Archived 2014-08-13 at the Wayback Machine
• big-PI Predictor - GPI Modification Site Prediction Archived 2020-07-21 at the Wayback Machine
• FragAnchor: GPI-Anchored Protein Prediction Tandem System (NN+HMM)
• MemType-2L
• NetGPI