Exploring the existence of lipid rafts in bacteria

Abstract

An interesting concept in the organization of cellular membranes is the proposed existence of lipid rafts. Membranes of eukaryotic cells organize signal transduction proteins into membrane rafts or lipid rafts that are enriched in particular lipids such as cholesterol and are important for the correct functionality of diverse cellular processes. The assembly of lipid rafts in eukaryotes has been considered a fundamental step during the evolution of cellular complexity, suggesting that bacteria and archaea were organisms too simple to require such a sophisticated organization of their cellular membranes. However, it was recently discovered that bacteria organize many signal transduction, protein secretion, and transport processes in functional membrane microdomains, which are equivalent to the lipid rafts of eukaryotic cells. This review contains the most significant advances during the last 4 years in understanding the structural and biological role of lipid rafts in bacteria. Furthermore, this review shows a detailed description of a number of molecular and genetic approaches related to the discovery of bacterial lipid rafts as well as an overview of the group of tentative lipid-protein and protein-protein interactions that give consistency to these sophisticated signaling platforms. Additional data suggesting that lipid rafts are widely distributed in bacteria are presented in this review. Therefore, we discuss the available techniques and optimized protocols for the purification and analysis of raft-associated proteins in various bacterial species to aid in the study of bacterial lipid rafts in other laboratories that could be interested in this topic. Overall, the discovery of lipid rafts in bacteria reveals a new level of sophistication in signal transduction and membrane organization that was unexpected for bacteria and shows that bacteria are more complex than previously appreciated.

FIG 1

Subcellular localization of flotillin in B. subtilis cells. Shown are fluorescence microscopy images of B. subtilis cells labeled with the translational fusion FloT-GFP (green fluorescent protein). The GFP signal is false colored on a glow-dark logarithmic scale. Cells also constitutively expressed CFP (cyan fluorescent protein) to facilitate the visualization of cells (false colored on a cyan-dark scale). (A) Fluorescence microscopy image of a field of cells labeled with FloT-GFP (glow-dark scale) expressed under the control of its natural promoter and the P_spac-cfp reporter, which constitutively expresses CFP (cyan-dark scale). The arrow indicates the subset of cells that are magnified in panel B. Bar, 10 μm. (B) Fluorescence microscopy detail of cells magnified from panel A. Cells are labeled with FloT-GFP and P_spac-cfp reporters. Bar, 2 μm.

FIG 2

Molecular structure of the constituent lipids of eukaryotic and bacterial lipid rafts. (A) Polycyclic terpenoids. (i) Molecular structure of cholesterol, the main constituent lipid of eukaryotic lipid rafts. (ii) Cholesterol is not present in bacterial membranes. Bacterial membranes contain other polycyclic terpenoids, which are structurally similar to cholesterol and referred to as hopanoids. (B) Noncyclic terpenoids. (i) Molecular structure of squalene, the precursor molecule of polycyclic and noncyclic terpenoids. (ii) An example of noncyclic terpenoids is carotenoids, which are widely distributed in bacteria. For instance, the carotenoid lipid staphyloxanthin is responsible for the golden coloration of the pathogen Staphylococcus aureus. (C) Molecular structure of sphingolipids (i) and cardiolipin (ii). Sphingolipids are sphingosine-based membrane lipids known to provide consistency to the lipid rafts of eukaryotic cells.

FIG 3

Taxonomic distribution of FloA and FloT in bacteria. (A) Schematic representation of the molecular structures of two different flotillin proteins, FloA and FloT, from the model organism Bacillus subtilis. MAD represents a membrane-anchoring region (whether this is a transmembrane helix or a hairpin loop has not yet been experimentally addressed). SPFH is a typical protein domain of flotillin proteins, and CC represents a coil-coiled region that localizes at the C-terminal regions of these two proteins. (B) Distribution of the FloA and FloT operons in bacteria. The first gene of the operon codes for an NfeD-like protein. The second gene is the flotillin-encoding gene. The third gene, coding for a protein of unknown function, is less well conserved and is absent from various species. Shown is the architecture of the operons from bacterial species from different phyla as a reference. The operons contain provisional gene names given by genome annotation. S. sanguinis, Streptococcus sanguinis; C. botulinum, Clostridium botulinum; n.d., not determined.

FIG 4

Cellular processes influenced by flotillin in B. subtilis. Shown is a schematic representation of a B. subtilis cell membrane, with FMMs represented in green. The pathways in transport/secretion, signaling, and proteolysis that are scaffolded by flotillins in lipid rafts are indicated.

FIG 5

Inhibition of isoprenoid biosynthesis in bacteria. Isoprenoid biosynthesis in bacteria follows the mevalonate route or the GA3P-pyruvate (Pyr) route. Statin molecules such as simvastatin or pravastatin are potent competitive inhibitors of the HMG-CoA reductase (hydroxymethylglutaryl-CoA-reductase) enzyme that participates in the mevalonate route. Furthermore, compounds such as fosfomycin or clomazone are conventionally used to inhibit the GA3P-pyruvate route, as they are potent inhibitors of DXP reductoisomerase (1-deoxy-d-xylulose 5-phosphate reductoisomerase). The condensation of several isoprenoid molecules renders polyisoprenoid molecules such as squalene, which are precursors of polycyclic and noncyclic terpenoids. Zaragozic acids are competitive inhibitors of the enzyme squalene synthase and cause an inhibition of the production of squalene in bacteria that use the mevalonate route and the GA3P-pyruvate route. The inhibitor molecules are shown in dashed green frames.