Effect of the Structure of Lipids Favoring Disordered Domain Formation on the Stability of Cholesterol-Containing Ordered Domains (Lipid Rafts): Identification of Multiple Raft-Stabilization Mechanisms

Abstract

Despite the importance of lipid rafts, commonly defined as liquid-ordered domains rich in cholesterol and in lipids with high gel-to-fluid melting temperatures (T(m)), the rules for raft formation in membranes are not completely understood. Here, a fluorescence-quenching strategy was used to define how lipids with low T(m), which tend to form disordered fluid domains at physiological temperatures, can stabilize ordered domain formation by cholesterol and high-T(m) lipids (either sphingomyelin or dipalmitoylphosphatidylcholine). In bilayers containing mixtures of low-T(m) phosphatidylcholines, cholesterol, and high-T(m) lipid, the thermal stability of ordered domains decreased with the acyl-chain structure of low-T(m) lipids in the following order: diarachadonyl > diphytanoyl > 1-palmitoyl 2-docosahexenoyl = 1,2 dioleoyl = dimyristoleoyl = 1-palmitoyl, 2-oleoyl (PO). This shows that low-T(m) lipids with two acyl chains having very poor tight-packing propensities can stabilize ordered domain formation by high-T(m) lipids and cholesterol. The effect of headgroup structure was also studied. We found that even in the absence of high-T(m) lipids, mixtures of cholesterol with PO phosphatidylethanolamine (POPE) and PO phosphatidylserine (POPS) or with brain PE and brain PS showed a (borderline) tendency to form ordered domains. Because these lipids are abundant in the inner (cytofacial) leaflet of mammalian membranes, this raises the possibility that PE and PS could participate in inner-leaflet raft formation or stabilization. In bilayers containing ternary mixtures of PO lipids, cholesterol, and high-T(m) lipids, the thermal stability of ordered domains decreased with the polar headgroup structure of PO lipids in the order PE > PS > phosphatidylcholine (PC). Analogous experiments using diphytanoyl acyl chain lipids in place of PO acyl chain lipids showed that the stabilization of ordered lipid domains by acyl chain and headgroup structure was not additive. This implies that it is likely that there are two largely mutually exclusive mechanisms by which low-T(m) lipids can stabilize ordered domain formation by high-T(m) lipids and cholesterol: 1), by having structures resulting in immiscibility of low-T(m) and high-T(m) lipids, and 2), by having structures allowing them to pack tightly within ordered domains to a significant degree.

Lipid Rafts Alter the Stability and Activity of the Cholera Toxin A1 Subunit

“…To simulate the composition of a lipid raft (38 -41), we mixed sphingomyelin and cholesterol with zwitterionic PE and anionic PS ( Table 1). The latter two lipids are highly enriched in the inner leaflet of a lipid raft (42)(43)(44)(45). Acyl chain selection was based upon the intestinal lipid composition.…”

Salamander retina phospholipids and their localization by MALDI imaging mass spectrometry at cellular size resolution

“…The phototransduction process involving conformational changes of rhodopsin is largely facilitated by a suffi ciently fl exible lipid bilayer, and the lipid bilayer fl uidity is mostly dependent on the composition of phospholipids' fatty acyl groups. Phospholipids composing PUFAs like DHA, with low gel-to-fl uid properties ( 26 ), provide sufficient fl uidity of the lipid bilayer and play signifi cant roles in the phototransduction process ( 8 ). Thus, the specifi c presence of lipids containing PUFAs in the OS and IS layers indicates that these lipids form the fl exible lipid bilayers and their different compositions might indicate their cell and cell part specifi city in the OS and IS layers.…”

Live-Cell TIRF Imaging of Molecular Assembly and Plasma Membrane Topography

“…The complexity of the membrane lipid composition has suggested the coexistence of domains characterized by different dynamic properties in the membrane plane as sites for preferential partitioning of proteins and solutes, for modulating membrane activity, and for diffusion along the plane and through the bilayer. [24][25][26][27][28][29][30][31][32][33] Several invasive methods that use membrane isolation can be used to establish the exact lipid composition of membranes. However, when these methods are used to study membrane domains, they can be subject to localization error because lipids can migrate between different cellular compartments during the membrane isolation.…”