4 years ago

Outer membrane lipid homeostasis via retrograde phospholipid transport in Escherichia coli

Outer membrane lipid homeostasis via retrograde phospholipid transport in Escherichia coli
Shu-Sin Chng, Xiang'Er Jiang, Rahul Shrivastava
Biogenesis of the outer membrane (OM) in Gram-negative bacteria, which is essential for viability, requires the coordinated transport and assembly of proteins and lipids, including lipopolysaccharides (LPS) and phospholipids (PLs), into the membrane. While pathways for LPS and OM protein assembly are well-studied, how PLs are transported to and from the OM is not clear. Mechanisms that ensure OM stability and homeostasis are also unknown. The trans-envelope Tol-Pal complex, whose physiological role has remained elusive, is important for OM stability. Here, we establish that the Tol-Pal complex is required for PL transport and OM lipid homeostasis in Escherichia coli. Cells lacking the complex exhibit defects in lipid asymmetry and accumulate excess phospholipids (PLs) in the OM. This imbalance in OM lipids is due to defective retrograde PL transport in the absence of a functional Tol-Pal complex. Thus, cells ensure the assembly of a stable OM by maintaining an excess flux of PLs to the OM only to return the surplus to the inner membrane. Our findings also provide insights into the mechanism by which the Tol-Pal complex may promote OM invagination during cell division. This article is protected by copyright. All rights reserved. Mechanisms for bulk phospholipid transport between the inner and outer membranes of Gram-negative bacteria are not known. In Escherichia coli, the trans-envelope Tol-Pal complex is important for ensuring outer membrane stability, but its physiological role is unclear. We show that the Tol-Pal complex is required for maintaining outer membrane lipid homeostasis, likely by mediating retrograde phospholipid transport.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1111/mmi.13772

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