Differential sorting and fate of endocytosed GPI-anchored proteins

Abstract

In this paper, we studied the fate of endocytosed glycosylphosphatidyl inositol anchored proteins (GPIAPs) in mammalian cells, using aerolysin, a bacterial toxin that binds to the GPI anchor, as a probe. We ®nd that GPI-APs are transported down the endocytic pathway to reducing late endosomes in BHK cells, using biochemical, morphological and functional approaches. We also ®nd that this transport correlates with the association to raft-like membranes and thus that lipid rafts are present in late endosomes (in addition to the Golgi and the plasma membrane). In marked contrast, endocytosed GPI-APs reach the recycling endosome in CHO cells and this transport correlates with a decreased raft association. GPI-APs are, however, diverted from the recycling endosome and routed to late endosomes in CHO cells, when their raft association is increased by clustering seven or less GPI-APs with an aerolysin mutant. We conclude that the different endocytic routes followed by GPI-APs in different cell types depend on the residence time of GPI-APs in lipid rafts, and hence that raft partitioning regulates GPI-APs sorting in the endocytic pathway.

Bacterial protein toxins and lipids: role in toxin targeting and activity

“…Certain bacterial toxins which are active intracellularly, such as Helicobacter pylori VacA (vacuolating toxin) (Ricci et al, 2000) and clostridial NTs (see below), recognize either GPI-anchored proteins or other molecules, but are associated with GPI-anchored proteins as receptors, and enter a specific intracellular pathway. In addition, toxins interacting with GPI-anchored proteins on the cell surface, such as aerolysin, have been used as tools to monitor the sorting of endocytic routes (Fivaz et al, 2002).…”

“…VacA does not directly recognize GPI-anchored proteins as receptors, but its tight association with lipid rafts through a yet unidentified receptor might be responsible for the sorting into the degradative pathway (Gauthier et al, 2005). Similarly, in other cells types, such as BHK (baby-hamster kidney) cells, GPI-anchored proteins, monitored with aerolysin as probe, are transported to late endosomes instead of accumulating in the recycling compartment (Fivaz et al, 2002). Lipid composition of microdomains might be the signal for sorting of endocytosis routes.…”

“…Indeed, lipid rafts with long and saturated fatty acids are preferentially targeted to late endosomes/lysosomes, whereas those with short and unsaturated fatty acid chains are preferentially transported to the endocytic/recycling endosome compartment. Lipid shape and fluidity preference, according to length and degree of saturation of acyl chains, appear to be critical in lipid sorting that probably results from preferential partitioning in more fluid or more rigid regions of the bilayer (Mukherjee et al, 1999;Fivaz et al, 2002).…”

Arf6-independent GPI-anchored Protein-enriched Early Endosomal Compartments Fuse with Sorting Endosomes via a Rab5/Phosphatidylinositol-3′-Kinase–dependent Machinery

“…There also seem to be one or more clathrin-independent pathways where dynamin is required, for example, the pathway for internalization of the interleukin (IL)-2 receptor subunits (Lamaze et al, 2001). In addition, there are a growing number of dynamin-independent endocytic processes that serve as internalization routes for a number of cell surface proteins, including lipid-linked proteins, such as GPI-anchored proteins (Lamaze and Schmid, 1995;Conner and Schmid, 2003;Nichols, 2003;Mayor and Riezman, 2004;Naslavsky et al, 2004;.GPI-anchored proteins are internalized via a dynamin, caveolin, and clathrin-independent pathway that is also responsible for the entry of a sizable fraction of the fluid phase in a number of cell types (Fivaz et al, 2002;Sabharanjak et al, 2002;Guha et al, 2003;. GPI-anchored proteins and the fluid phase enter the cell via tubular invaginations at the cell surface, into compartments termed GPI-anchored protein-enriched early endosomal compartments (GEECs) .…”

“…Other instances of such protein-specific modulation are binding of a complex of the urokinase-type plasminogen activator (uPA) and its GPIanchored uPA receptor to the ␣-2 macroglobulin receptor/ low-density lipoprotein receptor-related protein (Nykjaer et al, 1992;Conese et al, 1995); and GPI-anchored CD14, which interacts with bacterial lipopolysaccharide (LPS) bound to the plasma-LPS-binding protein (Poussin et al, 1998;Triantafilou et al, 2001). In the absence of lateral associations of the type described above, or cross-linking and internalization through the caveolar/raft-dependent pathway (Nabi and Le, 2003), GPI-anchored proteins seem to be selectively internalized through a dynamin-independent pathway (Fivaz et al, 2002;Sabharanjak et al, 2002;Guha et al, 2003). It has been recently shown that the nanoscale organization of GPI-anchored proteins on the plasma membrane also influences their endocytic pathways (Sharma et al, 2004).…”