PT  - JOURNAL ARTICLE
AU  - Rahul Shrivastava
AU  - Xiang’Er Jiang
AU  - Shu-Sin Chng
TI  - A trans-envelope complex maintains outer membrane lipid homeostasis via retrograde phospholipid transport in <em>Escherichia coli</em>
AID  - 10.1101/109884
DP  - 2017 Jan 01
TA  - bioRxiv
PG  - 109884
4099  - http://biorxiv.org/content/early/2017/03/01/109884.short
4100  - http://biorxiv.org/content/early/2017/03/01/109884.full
AB  - The outer membrane (OM) is essential for viability in Gram-negative bacteria. It also serves as a formidable permeability barrier against external insults, such as antibiotics and bile salts, thus allowing cells to survive in harsh environments. Biogenesis of the OM 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, and for proper OM invagination during cell division. Here, we establish the function of the Tol-Pal complex in 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 via transport mediated by the Tol-Pal complex. Our findings also provide insights into the mechanism by which the Tol-Pal complex promotes OM invagination during cell division.Significance statement Biological membranes define cellular boundaries, allow compartmentalization, and represent a prerequisite for life; yet, our understanding of membrane biogenesis and stability remain rudimentary. In Gram-negative bacteria, the outer membrane prevents entry of toxic substances, conferring intrinsic resistance against many antibiotics. How the outer membrane is assembled, and stably maintained, are not well understood. In this study, we established the role of a trans-envelope protein complex in outer membrane lipid homeostasis, and demonstrated that this complex is functionally important for phospholipid transport. Our work provides fundamental understanding of lipid trafficking within the Gram-negative double-membrane envelope in the context of membrane assembly and homeostasis. Furthermore, it highlights the importance of exploiting lipid transport processes as targets for the development of future antibiotics.Classification Biological Sciences – Microbiology and Biochemistry