Functional convergence of hopanoids and sterols in membrane ordering

Abstract

Liquid-ordered phases are one of two biochemically active membrane states, which until now were thought to be a unique consequence of the interactions between eukaryotic membrane lipids. The formation of a liquid-ordered phase depends crucially on the ordering properties of sterols. However, it is not known whether this capacity exists in organisms that lack sterols, such as bacteria. We show that diplopterol, the simplest bacterial hopanoid, has similar properties and that hopanoids are bacterial "sterol surrogates" with the ability to order saturated lipids and to form a liquid-ordered phase in model membranes. These observations suggest that the evolution of an ordered biochemically active liquid membrane could have evolved before the oxygenation of Earth's surface and the emergence of sterols.lipid order | polycyclic isoprenoids | bacterial membrane organization | membrane phase evolution | organic geochemistry T he capacity for sterols to modulate the ordering of lipids forms the basis for a membrane organizing principle in eukaryotes (1). The emergence of sterol-like ordering was likely a critical step in the evolution of biological membranes, allowing cells to control fluidity without compromising membrane integrity and providing a means to compartmentalize membranes into functional domains (2-4). It is not known, however, to what extent such membrane-ordering properties span the domains of life. Prokaryotes generally lack sterols; however, some bacteria produce hopanoids (5, 6), which are structurally similar (Fig. 1A) (7), and their cyclization is catalyzed by related enzymes (8). These similarities inspired the hypothesis that hopanoids are bacterial sterol surrogates (9) and led us to examine whether hopanoids might share the properties of sterols in membranes. Results and DiscussionEffects of Cholesterol and Diplopterol on the Phase Behavior and Ordering of Sphingomyelin in Model Membranes. Sterols and sphingolipids are closely associated in eukaryotic membranes, and the nature of their interactions has been extensively characterized. Therefore, we chose to test whether diplopterol behaves similarly to cholesterol in this well-defined system. Sterols interact with sphingolipids in vitro to form a liquid-ordered (L o ) phase that represents a thermodynamic intermediate between liquid-disordered (L d ) and crystalline gel phases (Fig. 1B) (10). The interactions leading to the formation of a L o phase derive from the ability of sterols to simultaneously inhibit the formation of the gel phase (by intercalating between sphingolipids and preventing their crystallization) and to order saturated acyl chains. To test whether these properties are also exhibited by hopanoids, we examined the effect of diplopterol on N-stearoyl-D-erythro-sphingosylphosphorylcholine (SM; Fig. S1), a synthetic sphingolipid.Monolayer experiments provide an approach to study the gelliquid phase transition of SM. Lipids are spread out over an airwater interface to form a monolayer and lateral pressure (measured as surface tension) is...

Hopanoids as functional analogues of cholesterol in bacterial membranes

“…Furthermore, the ΔG ex values for the diplopterollipid A and cholesterol-SM are nearly identical and negative, thus confirming that interactions between both pairs of these lipids are favorable. This finding is consistent with our previous results showing that diplopterol orders lipid A but does not order unsaturated phospholipids (15). Thus, we show that diplopterol interacts favorably only with saturated lipids and that the interactions of diplopterol with lipid A and cholesterol with SM are thermodynamically analogous.…”

“…Early investigations showed that both sterols and hopanoids exhibit the ability to condense lipids (13,14). We demonstrated that hopanoids are indeed bacterial sterol surrogates with respect to their ability to form a liquid ordered phase, and that they promote liquid-liquid phase separation in membranes (15). This observation decouples the evolution of ordered biochemically active liquid membranes from the requirement for molecular oxygen and suggests that the ability to subcompartmentalize membranes could have preceded the evolution of sterols.…”

“…In eukaryotes, sterols interact with saturated sphingolipids to form a membrane that is highly ordered. We demonstrated that hopanoids also interact with sphingolipids to form liquid ordered membranes (15). However, with a few exceptions, bacteria do not produce sphingolipids (32).…”

“…It is clear that hopanoids in vitro exhibit an ability to order synthetic membranes in a sterol-like manner (15). However, it is not known whether hopanoids impart molecular order within bacterial membranes in vivo, and with which lipids they interact to achieve such order.…”

Cholesterol and nicotinic acetylcholine receptor: An intimate nanometer-scale spatial relationship spanning the billion year time-scale

“…Using a combination of deuterium NMR spectroscopy on multilamellar lipid-sterol systems in combination with Monte Carlo simulations of microscopic models of lipid-sterol interactions, Mouritsen and coworkers [88] concluded that the evolution in the molecular chemistry from lanosterol to cholesterol is manifested in the increase in the ability of the sterols to promote and stabilize lipid order, in the form of the Lo phase in model phospholipid-sterol membranes. However, experimental work using the simplest of bacterial sterol surrogates, the hopanoid diplopterol, demonstrated that this property was also present in bacteria early in evolution [117,118]. The second requirement, i.e.…”