Proving Lipid Rafts Exist: Membrane Domains in the Prokaryote Borrelia burgdorferi Have the Same Properties as Eukaryotic Lipid Rafts

Abstract

Lipid rafts in eukaryotic cells are sphingolipid and cholesterol-rich, ordered membrane regions that have been postulated to play roles in many membrane functions, including infection. We previously demonstrated the existence of cholesterol-lipid-rich domains in membranes of the prokaryote, B. burgdorferi, the causative agent of Lyme disease [LaRocca et al. (2010) Cell Host & Microbe 8, 331–342]. Here, we show that these prokaryote membrane domains have the hallmarks of eukaryotic lipid rafts, despite lacking sphingolipids. Substitution experiments replacing cholesterol lipids with a set of sterols, ranging from strongly raft-promoting to raft-inhibiting when mixed with eukaryotic sphingolipids, showed that sterols that can support ordered domain formation are both necessary and sufficient for formation of B. burgdorferi membrane domains that can be detected by transmission electron microscopy or in living organisms by Förster resonance energy transfer (FRET). Raft-supporting sterols were also necessary and sufficient for formation of high amounts of detergent resistant membranes from B. burgdorferi. Furthermore, having saturated acyl chains was required for a biotinylated lipid to associate with the cholesterol-lipid-rich domains in B. burgdorferi, another characteristic identical to that of eukaryotic lipid rafts. Sterols supporting ordered domain formation were also necessary and sufficient to maintain B. burgdorferi membrane integrity, and thus critical to the life of the organism. These findings provide compelling evidence for the existence of lipid rafts and show that the same principles of lipid raft formation apply to prokaryotes and eukaryotes despite marked differences in their lipid compositions.

Author Summary

Specialized domains (“lipid rafts”) rich in specific membrane lipids (sphingolipids and cholesterol) have been proposed to form in the cell membranes of higher organisms, and to be of functional importance. We recently found that domains can be detected in the membranes of the bacterium that causes Lyme disease, Borrelia burgdorferi. In this report it is shown that, despite a lack of sphingolipids in B. burgdorferi, these domains have all the characteristic properties of lipid rafts, and can be detected in living B. burgdorferi. This shows that true lipid rafts can form in bacteria. In addition, it is shown that sterols having a structure that promotes lipid raft formation are necessary and sufficient for those sterols to maintain B. burgdorferi membrane integrity. This is suggestive of a role for membrane domains in B. burgdorferi membrane integrity. Therefore, interfering with lipid raft formation may have biomedical applications in combatting B. burgdorferi infections.