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

<div><p>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, <i>B. burgdorferi</i>, the causative agent of Lyme disease [LaRocca <i>et al.</i> (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 <i>necessary</i> and <i>sufficient</i> for formation of <i>B. burgdorferi</i> membrane domains that can be detected by transmission electron microscopy or in <i>living</i> 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 <i>B. burgdorferi</i>. Furthermore, having saturated acyl chains was required for a biotinylated lipid to associate with the cholesterol-lipid-rich domains in <i>B. burgdorferi</i>, another characteristic identical to that of eukaryotic lipid rafts. Sterols supporting ordered domain formation were also necessary and sufficient to maintain <i>B. burgdorferi</i> 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.</p></div