Abstract
The coexistence of different lipid phases is well-known in vitro, but evidence for their presence and function in cellular membranes remains scarce. Using a combination of fluorescent lipid probes, we observe segregation of domains that suggests the coexistence of liquid and gel phases in the membrane of Streptococcus pneumoniae, where they are localized to minimize bending stress in the ellipsoid geometry defined by the cell wall. Gel phase lipids with high bending rigidity would be spontaneously organized at the equator where curvature is minimal, thus marking the future division site, while liquid phase membrane maps onto the oblong hemispheres. In addition, the membrane-bound cell wall precursor with its particular dynamic acyl chain localizes at the division site where the membrane is highly curved. We propose a complete “chicken-and-egg” model where cell geometry determines the localization of lipid phases that positions the cell division machinery, which in turn alters the localization of lamellar phases by assembling the cell wall with a specific geometry.
Highlights
Membrane curvatures can induce partitioning of lipid phases
Liquid disordered phase has been observed in higher-curvature regions compared to ordered phase which preferentially segregates into more planar regions, suggesting a possible interplay between cell geometry and lipid domain organization (Baumgart et al, 2003)
Bacterial cell division, where the membrane supports the assembly of the cell-wall while being subject to large geometrical constrains induced by the cell-wall itself, is a perfect system to investigate possible functions of the lipid bilayer
Summary
Membrane curvatures can induce partitioning of lipid phases. Bacterial cell division, where the membrane supports the assembly of the cell-wall while being subject to large geometrical constrains induced by the cell-wall itself, is a perfect system to investigate possible functions of the lipid bilayer. The main conserved element of the division process is the polymerization of FtsZ as a cytoplasmic circumferential Z-ring, where it scaffolds other division proteins (Jacq et al, 2015; Haeusser and Margolin, 2016). Correct positioning of the Z-ring depends on various factors (Monahan et al, 2014; Garcia et al, 2016; Haeusser and Margolin, 2016; Kretschmer and Schwille, 2016), but the underlying physicochemical cues are not known. In rod-shaped bacteria, it has been proposed that the heterogeneous distribution of specific phospholipids and regions enriched in anionic head groups may contribute to the choice of the division site (Mileykovskaya and Dowhan, 2005), the mechanisms driving membrane heterogeneity are unknown
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