After biosynthesis, bacterial lipopolysaccharides (LPS) are transiently anchored to the outer leaflet of the inner membrane (IM). The ATP-binding cassette (ABC) transporter LptBFG extracts LPS molecules from the IM and transports them to the outer membrane. Here we report the crystal structure of nucleotide-free LptBFG from Pseudomonas aeruginosa. The structure reveals that lipopolysaccharide transport proteins LptF and LptG each contain a transmembrane domain (TMD), a periplasmic β-jellyroll-like domain and a coupling helix that interacts with LptB on the cytoplasmic side. The LptF and LptG TMDs form a large outward-facing V-shaped cavity in the IM. Mutational analyses suggest that LPS may enter the central cavity laterally, via the interface of the TMD domains of LptF and LptG, and is expelled into the β-jellyroll-like domains upon ATP binding and hydrolysis by LptB. These studies suggest a mechanism for LPS extraction by LptBFG that is distinct from those of classical ABC transporters that transport substrates across the IM.

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Engineering functional vascular networks in vitro is critical for tissue engineering and a variety of applications. There is still a general lack of straightforward approaches for recapitulating specific structures and functions of vasculature. This report describes a microfluidic method that utilizes fibrillogenesis of collagen and a liquid mold to engineer three-dimensional vascular networks in hydrogel. The well-controlled vascular network demonstrates both mechanical stability for perfusing solutions and biocompatibility for cell adhesion and coverage. This technique enables the mimicry of passive diffusion in a nephron one of the main routes transferring soluble organic molecules. This approach could be used for in vitro modelling of mass transfer-involved physiology in vasculature-rich tissues and organs for regeneration and drug screening.