Transport and coordination of membrane lipids during bacterial cell envelope biogenesis

Abstract

The bacterial cell wall prevents osmotic lysis and specifies cell shape. Synthesis of this essential structure is one of the most enduring antibiotic targets. How bacteria build and maintain their cell walls have been intensely studied since the discovery of penicillin over 70 years ago. Many of the molecular mechanisms that underly cell wall biogenesis have been uncovered. However, the mechanisms by which bacteria coordinate the assembly of the cell wall with other cell envelope layers remain poorly understood. Here I focus on a key lipid, undecaprenyl phosphate, that needs to be coordinated for assembly of the cell envelope. Undecaprenyl phosphate (UndP) is an essential bacterial lipid that transports sugars and glycopolymers across the membrane. UndP is maintained at low levels and how cells distribute this limited resource among competing pathways is unknown. I discovered that a membrane sensor monitors the levels of UndP in Bacillus subtilis membranes and modulates the activity of a transcription factor (Sigma-M) that prioritizes UndP for cell wall synthesis. As part of my analysis of this pathway, I discovered three genes induced by Sigma-M that appear to liberate the lipid carrier from non-essential surface polymer synthesis pathways. After transport of UndP-linked surface polymers, UndP must be recycled and reused. The identity of the UndP transporter that recycles the lipid carrier has remained elusive. Working in Staphylococcus aureus and B. subtilis, I identified and characterized two broadly conserved families of UndP transporters. These proteins were among the last unknown enzymes in most cell surface glycopolymer assembly pathways and represents an attractive target for future antibiotic discovery efforts. One of the UndP transporters that I discovered is a member of the DedA superfamily. In follow up work, I showed that a distinct DedA family member is a phosphatidylethanolamine transporter. My data suggest that bacterial DedA proteins are lipid transporters with distinct substrate specificities.