Different GPI-attachment signals affect the oligomerisation of GPI-anchored proteins and their apical sorting.

To understand the mechanism involved in the apical sorting of glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) we fused to the C-terminus of GFP the GPI-anchor-attachment signal of the folate receptor (FR) or of the prion protein (PrP), two native GPI-anchored proteins that are sorted apically or basolaterally, respectively, in MDCK cells. We investigated the behaviour of the resulting fusion proteins GFP-FR and GFP-PrP by analysing three parameters: their association with DRMs, their oligomerisation and their apical sorting. Strikingly, we found that different GPI-attachment signals differently modulate the ability of the resulting GFP-fusion protein to oligomerise and to be apically sorted. This is probably owing to differences in the GPI anchor and/or in the surrounding lipid microenvironment. Accordingly, we show that addition of cholesterol to the cells is necessary and sufficient to drive the oligomerisation and consequent apical sorting of GFP-PrP, which under control conditions does not oligomerise and is basolaterally sorted.