Ordered Membrane Domain-Forming Properties of the Lipids of Borrelia burgdorferi

Abstract

Co-existing disordered and ordered (raft) membrane domains exist in Borrelia burgdorferi, the causative agent of Lyme disease. However, although B. burgdorferi contains cholesterol lipids, it lacks sphingolipids-a crucial component of rafts in eukaryotes. To define the principles of ordered lipid domain formation in Borrelia, the domain forming properties of vesicles composed of its three major lipids, acylated cholesteryl galactoside (ACGal), monogalactosyl diacyglycerol (MGalD), and phosphatidylcholine (PC) and/or their mixtures were studied. Anisotropy and fluorescence resonance energy transfer measurements were used to assay membrane order and ordered-domain formation. ACGal had the highest potential to form ordered domains. Interestingly, mixtures of ACGal with B. burgdorferi PC formed ordered domains more readily than mixtures of ACGal with MGalD. This appears to reflect the relatively high level of saturation observed for B. burgdorferi PC, as vesicles containing ACGal and PC, but in which the unsaturated lipid dioleoyl PC was substituted for Borrelia PC, failed to form ordered domains. In addition, the properties of ACGal were compared to those of cholesterol. Depending on what other lipids were present, ordered-domain formation in the presence of ACGal was greater than or equal to that in the presence of cholesterol. Giant unilamellar vesicles formed from ACGal-containing mixtures showed rounded domain shapes similar to those in analogous vesicles containing cholesterol, indicative of liquid-ordered state rather than solid-like gel-state domain formation. Over all, principles of ordered-domain formation in B. burgdorferi appear to be very similar to those in eukaryotes, with saturated PC taking the place of sphingolipids, but with ACGal being the main lipid component inducing ordered-domain formation.