(B) Major membrane lipids: Major membrane lipids include phospholipids and glucolipids ( Nickels et al., 2017). These are flanked by regions of higher fluidity characterized as liquid-disordered (Ld). The IM contains lateral microheterogeneity in the form of functional membrane microdomains (FMMs), regions of liquid-ordered (Lo) membrane together with associated proteins such as flotillins ( Lopez and Koch, 2017). The membrane-associated lipoteichoic acid (LTA) and PG-linked wall teichoic acid (WTA) are abundant anionic polymers in the envelope ( Rajagopal and Walker, 2017). (A) The cell envelope: Bacillus subtilis is surrounded by a cell envelope comprised of a thick peptidoglycan (PG) layer and an inner membrane (IM). Membrane stress responses can modify the cell membrane, by (i) modulating the length, branching, and saturation of the fatty acid (FA) acyl chains, (ii) altering membrane lipid composition, or (iii) synthesizing proteins that modify or protect the membrane ( Table 1).įigure 1. Membrane-active compounds such as detergents, antimicrobial peptides, and pore-forming compounds often trigger stress responses that modify the lipidome and membrane proteome to confer resistance. subtilis, for example, collapsing the proton motive force activates autolysins resulting in rapid cell lysis ( Jolliffe et al., 1981). As the innermost and last line of defense, the cell membrane is critical for viability. The membrane proteome includes proteins for transport and signaling, as well as membrane synthesis, remodeling, and protection. subtilis, the major lipid species are phospholipids, glucolipids, and the lipoteichoic acids (LTA) ( Salzberg and Helmann, 2008 Nickels et al., 2017). The cell membrane is a dynamic, fluid mosaic comprising a lipid bilayer and associated proteins ( Figure 1). Here, we focus on Bacillus subtilis as a Gram-positive model for the role of CESRs in membrane homeostasis. Cell envelope stress responses (CESRs) are regulatory pathways that sense threats and mount a protective response, often involving modification of lipopolysaccharides (in Gram-negative bacteria), teichoic acids (Gram-positive bacteria), peptidoglycan, and the inner membrane ( Helmann, 2016 Radeck et al., 2017 Mitchell and Silhavy, 2019). The cell envelope is a multilayered outer barrier that protects the cell from a changing environment. Introduction: Membrane Homeostasis and Its Modulation in Response to Stress
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