YdbL directly modulates YdbH-YnbE bridge formation to maintain

Gram-negative bacteria pose a threat to global healthcare mainly because their outer membrane (OM) provides an intrinsic barrier to many antimicrobials. Key to this barrier function is the asymmetric structure of the OM, with phospholipids constituting the inner leaflet and lipopolysaccharides, the outer leaflet. Although the mechanism of phospholipid transport between the inner membrane (IM) and OM remains poorly understood, recent studies implicate TamB, YhdP, and YdbH as functionally redundant proteins mediating this process in IMPORTANCE: The mechanism underlying phospholipid transport between the inner and outer membranes of gram-negative bacteria remains enigmatic. Bacterial bridge-like protein systems such as the YdbH-YnbE complex resemble proteins found at membrane contact sites between eukaryotic organelles. These proteins are proposed to mediate intermembrane phospholipid transport, which is essential for growth of the outer membrane (OM). Here, we define the role of YdbL, a periplasmic protein that specifically modulates the YdbH-YnbE system. YdbL directly interacts with YnbE and controls the formation of the YdbH-YnbE complex. Additionally, we reveal that YdbL is selectively degraded by the periplasmic protease DegP. We propose a regulatory model that connects the YdbH-YnbE complex assembly and controls the levels of YdbL, providing new insight into OM homeostasis in gram-negative bacteria.