Structure of bacterial phospholipid transporter MlaFEDB with substrate bound

Abstract

In double-membraned bacteria, phospholipid transport across the cell envelope is critical to maintain the outer membrane barrier, which plays a key role in virulence and antibiotic resistance. An MCE transport system called Mla has been implicated in phospholipid trafficking and outer membrane integrity, and includes an ABC transporter, MlaFEDB. The transmembrane subunit, MlaE, has minimal sequence similarity to other transporters, and the structure of the entire inner-membrane MlaFEDB complex remains unknown. Here we report the cryo-EM structure of MlaFEDB at 3.05 Å resolution, revealing distant relationships to the LPS and MacAB transporters, as well as the eukaryotic ABCA/ABCG families. A continuous transport pathway extends from the MlaE substrate-binding site, through the channel of MlaD, and into the periplasm. Unexpectedly, two phospholipids are bound to MlaFEDB, suggesting that multiple lipid substrates may be transported each cycle. Our structure provides mechanistic insight into substrate recognition and transport by MlaFEDB.

Structure and lipid dynamics in the maintenance of lipid asymmetry inner membrane complex of A. baumannii

“…It should be emphasized that the structural studies reported here favour an anteretrograde direction for lipid transport by structure similarity, as supported by our previous biochemical data in A. baumannii 21 as well as other studies on E. coli 12 , 19 , 21 . Nonetheless, other assays point towards a retrograde directionality 20 , 23 , and the current structures do not conclusively preclude this.…”

“…We also emphasize that the detergent binding pockets observed in our MlaBDEF ab structures are also confirmed in the MlaBDEF ec structures, notably the lipids found between the MlaD TM and the N-terminal helix of MlaE 19 , and the detergent present in the central cavity 19 – 21 . For the latter, different exact detergent positions are observed depending on the state and study, but they are largely consistent with the lipid positions we have obtained in the MD simulations, with the charged group buried at the MlaE–MlaD interface.…”

“…While this manuscript was under review, three independent groups released the structure of the E. coli MlaBDEF complex (MlaBDEF ec ), in a range of nucleotide states and with various solubilization approaches 19 – 21 .…”

Phospholipid transporter shifts into reverse

“…3 ) and 3.05 Å (ref. 4 ), respectively, in the presence of a membrane scaffolding protein to produce lipid nanodiscs. These findings largely recapitulate the structure reported by Tang et al, which was solved at 3.3 Å resolution using a mixture of detergent and phospholipids.…”

“…First described by Waller in 1850 (hence also known as Wallerian degeneration) 1 , groundbreaking research over the past ten years has established that the protein SARM1 has a critical role in axon degeneration 2,3 and contributes to various neurological and neurodegenerative diseases (reviewed in refs. [4][5][6] ). SARM1 has nicotinamide adenine dinucleotide (NAD + ) hydrolase (NADase) activity, and its destruction of NAD + triggers axon degeneration 7,8 .…”