Structural Insight into Phospholipid Transport by the MlaFEBD Complex from P. aeruginosa

2022

Microbiology

The outer membrane (OM) is a formidable permeability barrier that protects Gram-negative bacteria from detergents and antibiotics. It possesses exquisite lipid asymmetry, requiring the placement and retention of lipopolysaccharides (LPS) in the outer leaflet, and phospholipids (PLs) in the inner leaflet. To establish OM lipid asymmetry, LPS are transported from the inner membrane (IM) directly to the outer leaflet of the OM. In contrast, mechanisms for PL trafficking across the cell envelope are much less understood. In this review, we summarize and discuss recent advances in our understanding of PL transport, making parallel comparisons to well-established pathways for OM lipoprotein (Lol) and LPS (Lpt). Insights into putative PL transport systems highlight possible connections back to the ‘Bayer bridges’, adhesion zones between the IM and the OM that had been observed more than 50 years ago, and proposed as passages for export of OM components, including LPS and PLs.

Section: The Ompc-mla Systemmentioning

confidence: 99%

Section: The Ompc-mla Systemmentioning

confidence: 99%

Of zones, bridges and chaperones – phospholipid transport in bacterial outer membrane assembly and homeostasis

Yeow

2022

Microbiology

“…To achieve that, the OmpC-MlaA complex somehow extracts PLs from the outer leaflet of the OM, hands them over to MlaC, which in turn transfers these PLs into the IM via the MlaFEDB complex (20)(21)(22). Many recent biochemical and structural studies have provided detailed insights into ATP-dependent PL transfer steps at the IM (19,20,(23)(24)(25)(26)(27)(28)(29)(30). In particular, we now know that when PL-bound MlaC arrives at the IM, it can spontaneously transfer the lipid molecule to the binding cavity of MlaFEDB (21).…”

Section: Introductionmentioning

confidence: 99%

Molecular mechanism of phospholipid transport at the bacterial outer membrane interface

Yeow

2022

Preprint

The outer membrane (OM) of Gram-negative bacteria is an asymmetric lipid bilayer with outer leaflet lipopolysaccharides (LPS) exposed to extracellular milieu and inner leaflet phospholipids (PLs) facing the periplasm. This unique lipid asymmetry is the key to its innate drug resistance, rendering the OM impermeable to external insults, including antibiotics and bile salts. To maintain this OM barrier, the OmpC-Mla system removes mislocalized PLs from the OM outer leaflet, and transports them back to the inner membrane (IM); in the first step, the OM OmpC-MlaA complex transfers PLs to the periplasmic chaperone MlaC. This process likely occurs via a hydrophilic channel in MlaA, yet mechanistic details have remained elusive. Here, we obtain a molecular view of the architecture of the MlaA-MlaC transient complex by mapping the interaction surfaces between MlaA and MlaC in Escherichia coli. We show that electrostatic interactions are important for MlaC recruitment, and that MlaC eventually binds MlaA at the periplasmic face in a manner that juxtaposes the MlaA channel and the MlaC lipid binding cavity. We further provide evidence for conformational changes in the MlaA channel that correlate with functional states of MlaA, as well as interactions with MlaC binding and OM porins. Our work offers novel insights into the molecular mechanism for lipid transfer by the OmpC-MlaA complex for overall retrograde transport of PLs to the IM to maintain OM lipid asymmetry.

“…Consistent with this idea, an AlphaFold2 structural model of the YrbE1A/B-MceG complex (25-27), which resembles the homologous MlaFE structure from Gram-negative bacteria (RMSD=2.934 Å; SI Appendix , Fig. S2) (28), presented a smaller buried area between MceG and YrbE1A (699 Å 2 ) compared to YrbE1B (837 Å 2 ) (Fig. 3D and SI Appendix , Table S5).…”

Section: Resultsmentioning

confidence: 61%

A conserved membrane protein negatively regulates Mce1 complexes in mycobacteria

Chen

2022

Preprint

Tuberculosis (TB) remains one of the deadliest infectious diseases, posing a serious threat to global health. Mycobacterium tuberculosis, the causative agent of TB, is an intracellular pathogen that relies on various mechanisms to survive and persist within the host environment. Among the many virulence factors that contribute to pathogenesis, mycobacteria encode Mce systems, putative transporters that are important for lipid uptake. The molecular basis for Mce function(s) is poorly understood. To gain insights into the composition and architecture of mycobacterial Mce systems, we characterized the putative Mce1 complex involved in fatty acid transport. Using affinity purification, we show that the Mce1 system in Mycobacterium smegmatis is an ATP-binding cassette transporter complex comprising YrbE1A/B as heterodimeric transmembrane domains (TMDs) and MceG as nucleotide-binding domains (NBDs), associated with possible heterohexameric assemblies of MCE-domain substrate-binding proteins. We demonstrate that Msmeg_6540 and Mce1A are functionally redundant homologs, likely forming distinct Mce1 complexes that contribute to fatty acid uptake. Finally, we establish that the membrane protein Msmeg_0959, herein renamed to Mce1N, negatively regulates Mce1 function by blocking MceG access to YrbE1B. This regulatory mechanism is conserved in M. tuberculosis Mce1N (Rv0513). Our work offers molecular understanding of Mce complexes, shedding light on lipid metabolism and its regulation in mycobacteria.