An octameric PqiC toroid stabilises the outer-membrane interaction of the PqiABC transport system

• Published in: EMBO reports 2024-01, Vol.25 (1), p.82-101
• Main Authors: Cooper, Benjamin F, Ratkevičiūtė, Giedrė, Clifton, Luke A, Johnston, Hannah, Holyfield, Rachel, Hardy, David J, Caulton, Simon G, Chatterton, William, Sridhar, Pooja, Wotherspoon, Peter, Hughes, Gareth W, Hall, Stephen CL, Lovering, Andrew L, Knowles, Timothy J
• Format: Article
• Language: English
• Subjects: Biological Transport; Biomedical and Life Sciences; EMBO20; EMBO23; EMBO40; Escherichia coli - genetics; Escherichia coli - metabolism; Escherichia coli Proteins - chemistry; Life Sciences; Lipoproteins - metabolism; Membrane Proteins - metabolism
• ISSN: 1469-3178; 1469-221X; 1469-3178
• DOI: 10.1038/s44319-023-00014-4

The E. coli Paraquat Inducible (Pqi) Pathway is a putative Gram-negative phospholipid transport system. The pathway comprises three components: an integral inner membrane protein (PqiA), a periplasmic spanning MCE family protein (PqiB) and an outer membrane lipoprotein (PqiC). Interactions between all complex components, including stoichiometry, remain uncharacterised; nevertheless, once assembled into their quaternary complex, the trio of Pqi proteins are anticipated to provide a continuous channel between the inner and outer membranes of diderms. Here, we present X-ray structures of both the native and a truncated, soluble construct of the PqiC lipoprotein, providing insight into its biological assembly, and utilise neutron reflectometry to characterise the nature of the PqiB-PqiC-membrane interaction. Finally, we employ phenotypic complementation assays to probe specific PqiC residues, which imply the interaction between PqiB and PqiC is less intimate than previously anticipated. Synopsis The PqiC lipoprotein, a component of the putative envelope spanning Pqi lipid transport system, forms an octameric toroid which stabilises the interaction between the Gram-negative outer membrane and the remainder of the Pqi complex enabling its function. The lipoprotein PqiC assembles an octameric toroid which embeds partially into the inner leaflet of the Gram-negative outer membrane. The PqiC toroid receives the C-terminus of PqiB, stabilising its interaction with the Gram-negative outer membrane. The trio of Pqi proteins assemble a complex which spans the Gram-negative cell envelope. The PqiC lipoprotein, a component of the putative envelope spanning Pqi lipid transport system, forms an octameric toroid which stabilises the interaction between the Gram-negative outer membrane and the remainder of the Pqi complex enabling its function.