A lipoprotein partner for the Escherichia coli outer membrane protein TolC

This study identifies the *E. coli* lipoprotein YbjP as a structural partner that binds to and stabilizes the outer membrane protein TolC, mimicking the lipid modifications found in homologs from other bacteria, although this interaction is not strictly required for standard efflux activity.

The Gist

Imagine a bacterium like E. coli as a tiny, fortified castle. To survive, it needs a strong outer wall (the outer membrane) to keep bad things out and a sophisticated security system to pump out any toxins or antibiotics that manage to sneak in.

This security system is called a Tripartite Efflux Pump. Think of it as a three-part garbage disposal unit:

1. The Intake (AcrB/MacB): A sensor inside the castle that grabs the trash (antibiotics).
2. The Connector (AcrA/MacA): A flexible arm that bridges the gap between the inside and the wall.
3. The Exit Door (TolC): A long, hollow tunnel that sticks out of the castle wall, acting as the final chute to shoot the trash into the outside world.

For years, scientists noticed something odd about the Exit Door (TolC). In many other bacterial species, this door has a built-in "anchor"—a little fatty tail (a lipid) that sticks into the wall to hold it steady. But the E. coli version of this door is missing that anchor. It's like a door hanging on a hinge without a screw; it should be wobbly and unstable. Yet, it works perfectly fine.

The Big Discovery: The "Mystery Partner"
The researchers in this paper asked: "If the door doesn't have its own anchor, how does it stay steady?"

Using high-tech 3D cameras (Cryo-EM), they took a super-close look at the door and found a surprise guest. There was a small, previously unknown protein named YbjP clinging to the side of the TolC door.

The Analogy: The Lipoprotein "Stabilizer"
Think of YbjP as a specialized bracket or a helper strap.

• The Problem: The TolC door is missing its own fatty anchor.
• The Solution: YbjP is a protein that does have a fatty anchor. It attaches itself to the TolC door, drapes its fatty tail into the wall, and effectively acts as the missing anchor for the door.
• The Shape: YbjP wraps around the door like a supportive arm, mimicking the shape of the missing anchor that other bacteria have. It's like a friend holding a wobbly ladder steady by wrapping their arm around it and planting their feet firmly on the ground.

What Does This Helper Actually Do?
The scientists ran many tests to see what happens if you remove this helper (YbjP) from the bacteria.

• Does the door fall down? No. The door still works, and the bacteria can still pump out antibiotics just fine under normal lab conditions.
• Is it useless? Not quite. While the bacteria survive without it, they seem a bit "nervous" or less efficient when things get tough.
  • Without YbjP, the bacteria have trouble expressing other specific transporters needed to deal with certain toxins (like microcin J25) or to absorb nutrients like tryptophan.
  • It seems YbjP isn't the engine that powers the pump, but rather the mechanic that ensures the door is perfectly aligned and the whole system is ready for action when stress hits.

The Evolutionary Twist
The paper also suggests a cool evolutionary story. It looks like YbjP might have evolved specifically to help E. coli because E. coli lost its own built-in anchor. It's a case of "if you can't build it, hire a contractor." The bacteria evolved this helper protein to compensate for a missing part in their own design.

In Summary
This paper reveals that E. coli has a secret bodyguard named YbjP.

• Role: It acts as a stabilizing partner for the main exit door (TolC).
• Mechanism: It uses its own fatty tail to anchor the door to the wall, replacing a feature the door itself lacks.
• Importance: While the bacteria can survive without it in a calm lab, YbjP is crucial for keeping the bacterial "castle" stable and efficient when facing environmental stress or specific toxins.

It's a beautiful example of nature's adaptability: when a machine is missing a part, evolution often builds a clever workaround to keep the factory running.

Technical Summary

1. Problem Statement

The Escherichia coli outer membrane protein TolC serves as the essential exit duct for multi-drug efflux pumps (e.g., AcrAB-TolC) and protein secretion systems (e.g., MacAB-TolC). While many homologs of TolC in other Gram-negative bacteria possess an N-terminal lipid modification that anchors them to the outer membrane, E. coli TolC lacks this lipid moiety. The mechanism by which non-lipidated TolC is stably integrated into the outer membrane and how it interacts with its periplasmic partners remained unclear. Furthermore, the functional role of the lipoprotein YbjP (UniProt ID P75818), which was previously annotated with an unknown function (DUF3828 domain), was not understood.

2. Methodology

The authors employed a multidisciplinary approach combining structural biology, biophysics, and proteomics:

• Cryo-Electron Microscopy (Cryo-EM):
  • Reconstituted the MacA-MacB-TolC and AcrA-AcrB-TolC tripartite pumps into peptidiscs (a membrane mimetic) to preserve native-like conformations.
  • Solved high-resolution structures (2.48 Å for MacAB-TolC-YbjP and 3.17 Å for AcrAB-TolC-YbjP) to identify unexpected density near the TolC equatorial domain.
  • Utilized AlphaFold2 predictions and DALI topology searches to identify the unknown density as the lipoprotein YbjP.
• Structural Validation:
  • Performed in vitro pull-down assays using a soluble, non-lipidated mutant of YbjP (YbjPs) to confirm direct interaction with TolC.
  • Conducted Isothermal Titration Calorimetry (ITC) to measure binding affinity ($K_D \approx 4 \mu M$).
  • Executed in vivo photo-crosslinking using the non-canonical amino acid pBPA (para-benzophenone) incorporated at specific residues (N113, T110) to validate the physical proximity of YbjP and TolC within living cells.
• Bioinformatics & Evolutionary Analysis:
  • Analyzed the evolutionary history of the DUF3828 domain family (YbjP, Tai3, YqhG) and the distribution of lipidated vs. non-lipidated TolC homologs across bacterial taxa.
  • Used AlphaFold3 to predict interactions between YbjP and other TolC homologs.
• Proteomics & Phenotypic Assays:
  • Performed label-free LC-MS/MS proteomics on wild-type, $\Delta ybjP$, $\Delta tolC$, and $\Delta tolC \Delta ybjP$ strains during exponential and stationary phases.
  • Assessed antimicrobial susceptibility (MIC) and growth curves under various stress conditions (antibiotics, vanillin, EDTA).

3. Key Contributions & Results

A. Structural Discovery: YbjP as a TolC Partner

• Identification: Cryo-EM revealed that YbjP binds extensively to the periplasmic surface of the TolC trimer, specifically spanning two adjacent protomers at the equatorial domain.
• Interaction Mode: YbjP mimics the intramolecular interactions seen in lipidated TolC homologs (like Pseudomonas OprM). The N-terminal flexible linker of YbjP runs along the TolC surface, and its globular domain (composed of $\alpha$-helices and $\beta$-strands) forms a stable interface via hydrogen bonds and ionic interactions (e.g., R122-E135-R151 in YbjP interacting with D231-R227-E317 in TolC).
• Membrane Anchoring: The N-terminal lipid moiety of YbjP is embedded in the inner leaflet of the outer membrane, effectively "replacing" the missing lipid anchor of TolC. This suggests YbjP acts as an external anchor for TolC.

B. Functional Characterization

• Not Required for Efflux Activity: Despite the strong structural interaction, the deletion of ybjP ($\Delta ybjP$) did not impair the efflux activity of AcrAB-TolC or MacAB-TolC under standard laboratory conditions. MICs for various antibiotics and toxins remained unchanged compared to wild-type.
• Proteomic Impact: While global proteome changes were minor, specific alterations were observed:
  • Downregulation: The ABC transporter YojI (involved in microcin J25 efflux) and Antigen 43 (flu) were decreased in $\Delta ybjP$ strains during stationary phase.
  • Upregulation: The tryptophan permease TnaB was increased.
  • Growth Phenotypes: $\Delta ybjP$ showed a slight growth advantage in tryptophan-supplemented media but no significant defects in motility or general stress resistance (vanillin, EDTA).
• Evolutionary Context: YbjP is restricted to specific Enterobacterales. Phylogenetic analysis suggests YbjP evolved from an ancestral Tai3-like protein to specifically interact with non-lipidated TolC. However, the presence of non-lipidated TolC in organisms lacking YbjP indicates YbjP is not strictly essential for TolC biogenesis, suggesting a redundant or context-specific role.

C. Mechanistic Insights

• The study proposes that YbjP may function as a chaperone or stabilizer for TolC assembly or distribution in the membrane, particularly under stress or during rapid adaptation, rather than being a core component of the efflux machinery itself.
• The interaction is spontaneous but likely transient or weak in vivo ($K_D \sim 4 \mu M$), potentially facilitating TolC insertion or preventing aggregation.
• YbjP shares a binding interface with another stress-induced lipoprotein, SlyB, suggesting potential competition or cooperative roles in membrane stress response.

4. Significance

• First Structural Insight: This is the first study to identify and structurally characterize a protein partner that binds the lateral surface of the non-lipidated E. coli TolC.
• Mechanism of Membrane Integration: It provides a structural model for how non-lipidated outer membrane proteins can be stabilized in the outer membrane via a trans-acting lipoprotein partner, mimicking the function of intrinsic lipid anchors found in homologs.
• Functional Nuance: The findings challenge the assumption that structural partners are always essential for function. Instead, YbjP appears to modulate the expression or stability of specific transporters (like YojI and TnaB) and potentially the spatial organization of TolC, playing a role in fine-tuning bacterial adaptation to environmental stress rather than basic survival.
• Therapeutic Implications: Understanding the auxiliary proteins that stabilize efflux pumps could reveal new targets for sensitizing bacteria to antibiotics, particularly if YbjP is crucial for pump function under specific clinical stress conditions (e.g., biofilm formation or host defense).

In summary, the paper elucidates a novel structural partnership where the lipoprotein YbjP compensates for the lack of a lipid anchor on E. coli TolC, stabilizing the protein and influencing the expression of specific transporters, though it is not strictly required for basal efflux activity.