Structures of LolB bound to LolA or lipoprotein resolve the final steps of bacterial lipoprotein trafficking

In Gram-negative bacteria, lipoproteins are structural elements of the outer membrane and essential components of machineries responsible for its construction and maintenance. The Lol system, responsible for the trafficking of lipoproteins from the site of maturation on the inner membrane to the outer membrane, is therefore crucial to the function of the cell envelope and a key target of efforts to find novel antimicrobials. In the final steps of this process, the outer membrane receptor, LolB accepts triacylated lipoproteins from the periplasmic chaperone LolA before inserting them into the outer membrane. Here we present a structure of LolB in complex with LolA, validated by in vivo and in vitro assays, highlighting how positively charged residues on the convex face of the LolB {beta}-barrel underpin complex formation. A protruding loop of LolB, essential for function, inserts into the LolA cavity in position to initiate the displacement of substrate lipoprotein from LolA to enable transfer to LolB. Structural resolution of a lipoprotein-bound LolB complex in combination with biophysical assays shows how a molecular latch releases the lid of the cavity to accommodate the lipoprotein acyl chains. Modelling of these structures onto computationally predicted orientations for LolB on the outer membrane provides a rationale for LolA release and lipoprotein triacyl group membrane insertion. Taken altogether, our data elucidate atomic resolution of two key intermediates and provide a greater understanding of the terminal steps of lipoprotein trafficking events at the bacterial outer membrane.