Fig. 6.
Proposed model for differences between a lipid ordered versus disordered bilayer. The cholesterol-induced tighter ordering of phospholipid head groups is in our model supposed to cause the detergence resistance rather than the hydrophobic fluidness of hydrogen carbon chains attached to the phospholipids. The wider distance of phospholipids in the Ld phase in our model allows intercalation of the detergent and this is the solubilization of Ld ordered lipids. As is measured in this study, under a threshold concentration of MBCD, lowered cholesterol levels from DRM do not disrupt the DRM and we expect that despite cholesterol removal the phospholipid head groups remain tightly packed. Above a threshold concentration of MBCD, the whole DRM becomes disrupted as phospholipids become disorganized in phospholipid head group packing. For MDCK, this level was reached at >10 mM MBCD while sperm DRMs became disrupted at 2–5 mM (van Gestel et al. 2005b). This difference may be explained by the lower abundance of cholesterol in the DRM of sperm when compared to MDCK, presumably making them more sensitive for cholesterol depletion. In this simplified model, we have not included information on the DRM accumulation of sphingolipids - including the ceramide based sulfatides of MDCK cells and the exclusion of the alkylacyl based seminolipid of sperm; two glycolipids with the same head group (see Fig. 2). This phenomenon shows that the cholesterol induced tighter packing of phospholipid head groups does have an impact on attraction and repulsion of glycolipids. Likewise, the specific attraction of specific membrane proteins and gangliosides is not included in this model