Flotillins functionally organize the bacterial membrane

Abstract

SummaryProteins and lipids are heterogeneously distributed in biological membranes. The correct function of membrane proteins depends on spatiotemporal organization into defined membrane areas, called lipid domains or rafts. Lipid microdomains are therefore thought to assist compartmentalization of membranes. However, how lipid and protein assemblies are organized and whether proteins are actively involved in these processes remains poorly understood. We now have identified flotillins to be responsible for lateral segregation of defined membrane domains in the model organism Bacillus subtilis. We show that flotillins form large, dynamic assemblies that are able to influence membrane fluidity and prevent condensation of Laurdan stained membrane regions. Absence of flotillins in vivo leads to coalescence of distinct domains of high membrane order and, hence, loss of flotillins in the bacterial plasma-membrane reduces membrane heterogeneity. We show that flotillins interact with various proteins involved in protein secretion, cell wall metabolism, transport and membrane-related signalling processes. Importantly, maintenance of membrane heterogeneity is critical for vital cellular processes such as protein secretion.

Daptomycin inhibits cell envelope synthesis by interfering with fluid membrane microdomains

“…Transitions between the fluid lipid-daptomycin clusters (rigidified through tight clustering of fluid, short-chain lipids) and the more rigid (thicker) bulk of the cell membrane likely constitute such weak spots through which proton leakage can occur and might explain the gradual depolarization of the membrane. It should be mentioned that phase-boundary defects as a cause for membrane permeabilization have been studied only in vitro, and it remains to be seen how phase separation affects the barrier function of bacterial membranes that naturally contain lipid domains of different fluidity (38,77,78). Nonetheless, daptomycin-induced changes in bilayer organization lead to a slow increase in the passive permeability of the membrane, which may be why membrane pore formation has been considered the main function of daptomycin in former studies (16,17,31).…”

Recent progress on lipid lateral heterogeneity in plasma membranes: From rafts to submicrometric domains

“…The importance of flotillins was further highlighted by the observation that domains exhibiting high GP value in Laurdan-labeled B. subtilis (Fig. 5a) could coalesce into larger domains upon loss of flotillins [31]. However, lipid composition of these flotillin-enriched structures is not clear.…”

The membrane: transertion as an organizing principle in membrane heterogeneity

“…Reciprocally, ATP synthase and succinate dehydrogenase are at low levels in (or absent from) the mid-cell region at the onset of cell division in B. subtilis , which may reflect an association with lipid domains elsewhere that are rich in PG or other lipids rather than with lipid domains in the mid-cell region (i.e., the division site) where these lipids may be at low levels (Meredith et al, 2008). Recently, FloT and FloA, homologues of eukaryotic flotillin proteins found exclusively in lipid rafts along with proteins involved in signaling and transport have been localized to discrete microdomains in the membrane of B. subtilis (see Cardiolipin and Other Anionic Phospholipid Domains); significantly, these microdomains, which are likely to be present in many other bacteria, also contain other proteins involved in signal transduction and cell–cell communication such as the sensor kinase, KinC, and protein secretion such as SecY in B. subtilis (Donovan and Bramkamp, 2009; Lopez and Kolter, 2010; Bach and Bramkamp, 2013; Bramkamp and Lopez, 2015). Flotillins are believed to play a large part in maintaining the overall physical heterogeneity of the membrane since, in their absence, lipid-ordered domains coalesce (Bach and Bramkamp, 2013; Bramkamp and Lopez, 2015).…”

“…Recently, FloT and FloA, homologues of eukaryotic flotillin proteins found exclusively in lipid rafts along with proteins involved in signaling and transport have been localized to discrete microdomains in the membrane of B. subtilis (see Cardiolipin and Other Anionic Phospholipid Domains); significantly, these microdomains, which are likely to be present in many other bacteria, also contain other proteins involved in signal transduction and cell–cell communication such as the sensor kinase, KinC, and protein secretion such as SecY in B. subtilis (Donovan and Bramkamp, 2009; Lopez and Kolter, 2010; Bach and Bramkamp, 2013; Bramkamp and Lopez, 2015). Flotillins are believed to play a large part in maintaining the overall physical heterogeneity of the membrane since, in their absence, lipid-ordered domains coalesce (Bach and Bramkamp, 2013; Bramkamp and Lopez, 2015). Cytoplasmic membrane proteins located in the polar regions of E. coli cells include ProP, LacY, and MscS (Romantsov et al, 2010) and the MCPs (methyl-accepting chemotaxis proteins; Alley et al, 1992; Sourjik and Armitage, 2010) whilst proteins located at the sites of cell constriction in E. coli include the components of trans- envelope Tol-Pal complex, TolA, TolQ, and TolR (in the cytoplasmic membrane), the peptidoglycan-associated lipoprotein Pal, (anchored to the outer membrane), and TolB (a soluble periplasmic protein; Gerding et al, 2007).…”