GPI-Anchored Proteins: Biophysical Behaviour and Cleavage by Pi-Specific Phospholipases

Glycosylphosphatidylinositol (GPI) anchors appear to confer unique biophysical properties on the proteins to which they are covalently linked. Model membrane systems provide a powerful tool to explore the effects of bilayer properties on the behaviour of GPI-anchored proteins. Such studies have typically been carried out after reconstitution/insertion of purified GPI-anchored proteins into symmetric or asymmetric lipid bilayer vesicles, supported lipid bilayers, or lipid monolayers. Biophysical studies using atomic force microscopy and Langmuir isotherms have revealed quantitative details of the interactions between GPI-anchored proteins and model membranes. Proteins carrying GPI anchors are believed to be targeted to detergent-resistant cholesterol/ sphingolipid-rich lipid rafts in both intact cells and model membranes, and the special properties of these microenvironments may also modulate their functional activity. GPI-anchored proteins are likely closely associated with the bilayer surface, so that the biophysical properties of the membrane, including curvature and lipid fluidity, modulate their conformation and activity. The GPI anchor can be cleaved by both endogenous and exogenous phosphatidylinositol (PI)-specific phospholipases C and D, from sources such as bacteria, protozoa and mammalian tissues. The release of GPI-anchored proteins in soluble form by phospholipases may play a key role in regulating their surface expression and activity. The GPI anchor appears to impose structural restraints, and its removal may alter the conformation, antigenicity and enzymatic activity of the protein. PI-specific phospholipases must interact closely with the membrane surface to cleave GPI anchors, and their activity is also greatly influenced by membrane biophysical properties.