Small siphophage binding to an open state of the LptDE outer membrane lipopolysaccharide translocon
Bacteriophages are bacterial viruses that provide alternatives to small-molecule drugs to combat infections by antibiotic-resistant bacteria. To infect a bacterial host, a phage needs to bind to the bacterial surface via receptor binding proteins (RBPs), which are critical for determining host specificity. For functionally important receptors, the RBP-receptor interaction could be exploited via phage steering, where emerging bacterial resistance due to receptor modification could make bacteria less fit or virulent. Despite this, relatively little is known about RBP-receptor interactions. Here we build on the recent discovery of coliphages that have the outer membrane (OM) lipopolysaccharide translocon LptDE as their terminal receptor and show via cryogenic electron microscopy (cryoEM) that, surprisingly, the RBP of the small siphophage Oekolampad binds to a hitherto unobserved, open state of LptDE. The open lateral gate of LptD is occupied by a {beta}-strand peptide originating from the degraded N-terminal jellyroll domain of LptD, suggesting the possibility of LptD inhibition via peptidomimetics. A structure of LptDE in complex with the superinfection exclusion (SE) protein Rtp45 of the Oekolampad-related phage Rtp shows a mechanism of SE where Rtp45-induced conformational changes in LptD resulting from steric clashes preclude RBP binding. Finally, analysis of spontaneous Oekolampad-resistant E. coli mutants identifies mutations in LptD that abolish the LptDE-RBP interaction in vitro. SDS-EDTA sensitivity assays of the mutants show no major defects in LptDE function, suggesting that phage steering via LptDE might be challenging. SignificanceThe outer membrane (OM) of Gram-negative bacteria is a protective barrier generated by lipopolysaccharide (LPS), a complex glycolipid that makes up the outer leaflet of the OM. Following its synthesis and transport from the inner membrane, LPS is inserted into the OM by LptDE, an OM protein complex essential for most Gram-negative bacteria. LPS insertion requires a hitherto unobserved, open state of LptDE. Here we report the unexpected finding that the receptor binding protein (RBP) of a bacteriophage binds to an open state of LptDE, allowing isolation and visualisation via cryogenic electron microscopy. The lateral gate of the LptDE-RBP complex is occupied by a {beta}-strand peptide, suggesting that the LptDE of pathogenic bacteria could be inhibited by peptidomimetics.