The mature form of urokinase plasminogen activator receptor (uPAR) is attached to the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor (Ploug et al. 1991). As nascent uPAR polypeptide moves into the lumen of the endoplasmic reticulum, it is attacked by a transamidase complex that cleaves the uPAR polypeptide after residue 305, releasing the carboxyterminal peptide of uPAR and replacing it with an acylated GPI moiety. In a second step, the GPI moiety is deacylated, yielding a uPAR-GPI conjugate that can be efficiently transported to the Golgi apparatus.

Glycosylphosphatidyl inositol (GPI) acts as a membrane anchor for many cell surface proteins. GPI is synthesized in the endoplasmic reticulum. In humans, a single pathway consisting of nine reactions appears to be responsible for the synthesis of the major GPI species involved in membrane protein anchoring. This pathway is shown in the figure. Two additional reactions, not shown, allow the synthesis of variant forms of GPI, one with an additional mannose residue and one with an additional ethanolamine (Tauron et al. 2004; Shishioh et al. 2005). These variant GPI molecules may be used for tissue-specific protein modifications, or may function independently.

The steps of GPI synthesis were first identified by isolating large numbers of mutant cell lines that had lost the ability to express GPI anchored proteins on their surfaces. Somatic cell hybrid analyses of these lines allowed the definition of complementation groups corresponding to distinct mutated genes, and cDNAs corresponding to normal forms of these genes were identified on the basis of their abilities to restore normal cell surface protein expression in mutant cells. Co-precipitation experiments with tagged cloned proteins have allowed the identification of additional proteins involved in GPI anchor biosynthesis.

Glycosylphosphatidyl inositol (GPI) acts as a membrane anchor for many cell surface proteins. GPI is synthesized in the endoplasmic reticulum. In humans, a single pathway consisting of eleven reactions appears to be responsible for the synthesis of the major GPI species involved in membrane protein anchoring.

As a nascent protein fated to become GPI-anchored moves into the lumen of the endoplasmic reticulum, it is attacked by a transamidase complex that cleaves it near its carboxy terminus and attaches an acylated GPI moiety. The GPI moiety is deacylated, yielding a protein-GPI conjugate that can be efficiently transported to the Golgi apparatus.

LBP delivers LPS from bacteria (or bacterial membrane fragments) to CD14 on the surfaces of phagocytes, where it is recognised by the MD2:TLR4 complex . Thus, LBP is an opsonin and CD14 is an opsonic receptor for complexes of LPS (or LPS-containing particles such as bacteria) and LBP. CD14 exists as two isoforms. CD14 can be either secreted into the extracellular compartment, or it can be anchored to the plasma membrane via its GPI module.

TMED2 and TMED10 are members of the p24 family of proteins that bind GPI anchored proteins in the ER and mediate their incorporation into COPII vesicles (Bonnon et al, 2010; reviewed in Strating et al, 2009). p24 proteins function as heteromeric complexes and interact with components of the COPII coat (Jenne et al, 2002; Dominguez et al, 1998; reviewed in Strating et al, 2009). In complex with LMAN1 and SURF4, they also play a role in maintaining the structure of the ERGIC (Mitrovic et al, 2008).

In the endoplasmic reticulum, the precursor form of urokinase plasminogen activator receptor is transamidated at residue 305, replacing the 30 carboxyterminal residues of the protein with an acylated glucosylphosphatidylinositol (acyl-GPI) moiety. The released carboxyterminal propeptide has no known function. The reaction is catalyzed by GPI transamidase, a complex of at least five proteins associated with the lumenal surface of the endoplasmic reticulum membrane (Yu et al. 1997; Ohishi et al. 2001; Hong et al. 2003).

Some proteins function at the cell's surface, attached to the plasma membrane via GPI (glycosylphosphatidylinositol) anchors and include enzymes, receptors, cell adhesion molecules and antigens. These GPI-anchored proteins participate in many important cellular functions including immune recognition, complement regulation and intracellular signaling. Phosphatidylinositol-glycan-specific phospholipase D (GPLD1) (Schofield et al. 2000) is a secreted protein that specifically cleaves GPI-anchored proteins by cleaving the linkage between the phosphate and inositol in GPI (Davitz et al. 1987, Low & Prasad 1988). In addition, it also localises to the ER where it can cleave GPI anchor intermediates transiting to the plasma membrane (not shown here). GPLD1 may play a role in the regulation of GPI-anchored proteins on (intra)cellular membranes.

At the beginning of this reaction, 1 molecule of 'GPI-anchored form of CD14', and 1 molecule of 'LBP complexed with bacterial LPS' are present. At the end of this reaction, 1 molecule of 'LPS complexed with GPI-anchored CD14', and 1 molecule of 'LBP' are present.

GPI-anchor transamidase is an enzyme that in humans is encoded by the PIGK gene

Glycoprotein Ib-IX-V Receptor Complex

Glycosyl-phosphatidylinositol