Speaker Dr Georgia Isom, Group Leader, Sir William Dunn School of Pathology, University of Oxford

Title Bacterial lipid transport through envelope-spanning tunnels and bridges

Host Julien Bergeron

 

Abstract Antimicrobial resistant bacteria pose a significant threat to human health. Gram-negative bacteria are particularly difficult to treat with antimicrobials, partly due to the nature of the cell envelope, which consists of an inner and outer membrane, separated by an aqueous periplasm. In particular, the outer membrane of Gram-negative bacteria acts as a physical barrier against antimicrobials. Therefore, understanding the mechanisms behind outer membrane biogenesis is essential to understand how we can disrupt this barrier. In order to build their outer membrane, bacteria must first transport hydrophobic molecules, including phospholipids, across the periplasm.

Recently, two proteins have emerged as candidate phospholipid transporters, LetB and YhdP, that could fulfil this role. First, using cryo-EM, we show that LetB consists of 7 stacked dynamic rings that form a continuous, hydrophobic tunnel. Second, using a combination of negative stain EM and AlphaFold, we show that YhdP is formed of repeated beta sheets that fold into a long bridge lined with hydrophobic residues. Furthermore, YhdP and LetB are both large enough to span the entirely of the periplasm, directly connecting the inner and outer membranes. Using site specific crosslinking, we have successfully captured putative phospholipids at locations within the tunnel of LetB and bridge of YhdP.

Overall, our data supports a model that LetB and YhdP form tunnels and bridges connecting the inner and outer membranes, creating a hydrophobic pathway for the translocation of phospholipids across the periplasm.