Free GPIs

Phosphatidylinositol (PI), a major component of membrane lipids, is formed by displacement of CMP from CMD-dialylglycerol by n/i/o-inositol.186 It is also converted into a variety of less abundant phosphory-lated derivatives that engage in signaling activities (see Fig. 11-9). In addition, PI is a component of the glycosylphosphatidylinositol (GPI) membrane anchors for suface proteins (Fig. 8-13). Free GPI anchors, lacking bound proteins, are also present in membranes. 

Fig. 6.1 Scheme representing the structure and hydrolysis of glycosyl-phosphatidylinositol (GPI) (adapted from Refs. [1] and [7]). Free GPI and GPI-anchored proteins are represented, showing their similarity in structure. The points for phospholipase C (PLC) and phospholipase D (PLD) hydrolysis are indicated. The glycan component of free GPI is... 

Another open issue in GPI hydrolysis by phospholipases is how intracellular receptor activation couples to the hydrolysis of an extracellular plasma membrane lipid. There are many hypotheses that have not been fully explored, some of which have an experimental basis - for example, there are evidences of fhe presence of both free-GPI and GPI-PLD in intracellular vesicles [27, 30]. Alfhough intracellular GPI-PLD has not been reported to hydrolyse GPI, an intracellular re-... 

There are other examples of remodeling acyl residues on glycerol. In the insect form of T. brucei, a partial remodeling occurs, whereby the sn-2 fatty acid is removed to form a lyso GPI that is then attached to proteins [59]. In L. mexicana free GPIs, fatty acid remodeling involves myristate replacing a longer sn-2 fatty acid... 

Many cells also contain significant amounts of free GPIs, not linked to proteins or glyco-polymers [12]. The function of these molecules is unclear, although there exist controversial proposals that they may be the source of phospho-oligosaccharides involved in insulin signal transduction [13, 14]. 

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