Abstract
Most mammalian cells have in their plasma membrane at least two types of lipid microdomains, non-invaginated lipid rafts and caveolae. Glycosylphosphatidylinositol (GPI)-anchored proteins constitute a class of proteins that are enriched in rafts but not caveolae at steady state. We have analyzed the effects of abolishing GPI biosynthesis on rafts, caveolae, and cholesterol levels. GPI-deficient cells were obtained by screening for resistance to the pore-forming toxin aerolysin, which uses this class of proteins as receptors. Despite the absence of GPI-anchored proteins, mutant cells still contained lipid rafts, indicating that GPI-anchored proteins are not crucial structural elements of these domains. Interestingly, the caveolae-specific membrane proteins, caveolin-1 and 2, were up-regulated in GPI-deficient cells, in contrast to flotillin-1 and GM1, which were expressed at normal levels. Additionally, the number of surface caveolae was increased. This effect was specific since recovery of GPI biosynthesis by gene recomplementation restored caveolin expression and the number of surface caveolae to wild type levels. The inverse correlation between the expression of GPI-anchored proteins and caveolin-1 was confirmed by the observation that overexpression of caveolin-1 in wild type cells led to a decrease in the expression of GPI-anchored proteins. In cells lacking caveolae, the absence of GPI-anchored proteins caused an increase in cholesterol levels, suggesting a possible role of GPI-anchored proteins in cholesterol homeostasis, which in some cells, such as Chinese hamster ovary cells, can be compensated by caveolin up-regulation.
Highlights
According to the current view, the plasma membrane of mammalian cells is not a homogeneous sea of lipids but is composed of different domains with different lipid and protein compositions and different physical states
The proposed functions of caveolae are similar to those attributed to rafts. They are believed to be implicated in signal transduction, based on the observation that caveolin-1 interacts with a number of signaling molecules [14], and in cholesterol regulation [4, 7]
Similar observations were made on paroxysmal nocturnal hemoglobinuria erythrocytes, which were found to have 2.37 times more cholesterol than control erythrocytes [68]
Summary
According to the current view, the plasma membrane of mammalian cells is not a homogeneous sea of lipids but is composed of different domains with different lipid and protein compositions and different physical states. Even though this might be an overestimation, considering the small size of elementary raft units, a given raft can only contain a limited number of these proteins It is not clear at present whether each raft contains a random sample of raft components or whether some rafts contain a specific set of proteins or lipids, i.e. whether there are different types of rafts. A class of raft proteins that has attracted much attention over the last decade is that of GPI1-anchored proteins (GPIAPs) These lipid-anchored polypeptides are mainly present at the plasma membrane where they are found in rafts, i.e. outside of caveolae, at steady state [19], even though they may enter caveolar pits upon cross-linking with antibodies [20]. In COS-7 cells, GPI-anchored proteins were found to cycle between the plasma membrane and the Golgi apparatus, trafficking was again found to be cholesteroldependent [23], further highlighting the functional interactions between GPI-APs and cholesterol
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