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

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. 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.