A historical perspective on the multifunctional outer membrane channel protein TolC in Escherichia coli

Introduction

TolC is a versatile outer membrane channel in Escherichia coli named for its role in providing “tolerance” to colicin E1—a pore-forming E. coli toxin¹. Colicin-tolerant (tol) mutants of E. coli were first reported in 1964². By 1967, three classes—tol II, tol III, and tol VIII—were identified, with tol VIII mutants exhibiting tolerance specifically to colicin E1 while still adsorbing the molecule on the cell surface¹. At this point, it was noted that the tol VIII mutants were sensitized to various antimicrobial agents, dyes, and detergents, which was attributed to increased uptake due to alterations in permeability¹. The mutations associated with the tol VIII phenotype were mapped to a region between str and his¹, and this location was later denoted the tolC locus in 1971³. The product of tolC was identified as a 54 kDa protein in 1982⁴, and in the following year, it was determined that TolC is synthesized with an N-terminal signal sequence that is post-translationally cleaved to yield a 52 kDa protein⁵. The nucleotide and amino acid sequences of tolC were determined in 1983⁶, and shortly after, TolC was identified as an outer membrane protein⁷. During this time, it was acknowledged that tolC mutations were also associated with alterations in bacteriophage sensitivity⁸ and reductions in the outer membrane protein OmpF^9,10. Thus, tolC gained a reputation for its importance in E. coli outer membrane structure and function, in addition to antimicrobial resistance, and it was noted that the tolC mutant phenotype was pleiotropic in nature.

In the 1990s, TolC was found to have multiple roles, including the secretion of hemolysin¹¹, colicin V¹², ST_B enterotoxin¹³, and the antibacterial peptide microcin J25¹⁴. Conversely, during this time, TolC was also shown to be essential for the uptake of colicin E1¹⁵. It wasn’t until nearly 30 years after the identification of tolC that the product of this gene was hypothesized to be a component of the AcrAB efflux machinery and likely other efflux systems in E. coli, explaining why tolC mutants are hypersensitive to a broad spectrum of antimicrobial agents¹⁶ and why TolC is essential for the multiple-antibiotic-resistance (Mar) phenotype^16,17.

The first high-resolution crystal structure of TolC was reported in 2000, revealing a fold distinct from other outer membrane proteins in E. coli¹⁸. Three TolC protomers were demonstrated to assemble, forming a 140 Å-long ‘channel tunnel’ spanning the outer membrane and the periplasm, with the proximal end being sealed with coiled helices, suggesting that protein-protein interactions open the channel¹⁸. It was recognized that ‘outer membrane factors’ such as TolC enable transport across the two membranes of Gram-negative bacteria, a process that also requires a cytoplasmic membrane export system or translocase/efflux pump, providing substrate specificity and energy, coupled with a membrane fusion protein^18,19,20. In 2004, the AcrAB efflux pump and fusion protein were shown to form a strong interaction with TolC²¹. This finding was further substantiated in 2014 when electron cryo-microscopy (cryo-EM) revealed the AcrAB-TolC structure²² and in situ structures were determined by electron cryo-tomography^23,24, solidifying the role of TolC in antimicrobial drug efflux. Concurrently, it was increasingly recognized that TolC likely exports endogenously produced substrates such as the siderophore enterobactin²⁵, indole²⁶, and porphyrins²⁷, underscoring its broader role in cellular homeostasis and metabolism. Thus, TolC has been associated with a multitude of cellular functions in E. coli, highlighting its versatility and importance in bacterial physiology.

This review commemorates the 60th anniversary of the discovery of colicin-tolerant E. coli mutants—and the identification of TolC—by presenting key discoveries that highlight its multifunctional role in antimicrobial resistance, metabolism, membrane integrity, and host survival (summarized in Fig. 1). While previous reviews have offered excellent insights into the structure and function of TolC^28,29, we provide updates from the past decade and highlight areas that remain incompletely understood. Although TolC has been the primary focus of research over the years due to its well-established role in multidrug efflux and other important processes, the E. coli genome encodes several other homologous outer membrane channel proteins that remain poorly characterized. Here, we demonstrate that these homologous proteins are highly conserved across the E. coli species, suggesting their potential importance in cellular physiology or stress responses.

A historical perspective on the multifunctional outer membrane channel protein TolC in Escherichia coli — **Fig. 1**

Structure and assembly of TolC

Nearly 25 years ago, the αβ-barrel structure of TolC was determined, showing that this protein forms an extended homotrimer projecting into the periplasm (Fig. 2a)¹⁸. This architecture is structurally unique among E. coli outer membrane proteins (OMPs), which are generally oligomeric, with each monomer forming an individual β-barrel monomer¹⁸. The upper portion of TolC is a 12-stranded β-barrel formed by four β-strands from each protomer¹⁸. The β-barrel of trimeric autotransporters in E. coli are assembled in a similar manner³⁰, but this class of proteins lacks the unusual α-helical barrel composed of two long and four short α-helices from each TolC promoter that extends from the β-barrel into the periplasm¹⁸. Most OMPs form multimeric channel complexes, yet TolC forms a single cone-shaped channel with a large internal cavity volume of ~43,000 Å³¹⁸. An internal diameter of ~20 Å is maintained for roughly ~100 Å of the protein^18,31, and the radius reduces to ~2 Å towards the periplasmic end of the protein where the α-helices wind around each other to form a coiled-coil that seals the channel^18,32. The closed periplasmic end of TolC is maintained by strong inter-protomer salt bridge interactions and intra-monomer hydrogen bonds³³.

Fig. 2: Structural comparison between TolC and homologous proteins in *E. coli.*

While the structure of TolC is well-characterized, how the protein folds and is assembled in the outer membrane needs to be better understood. Nearly all OMPs adopt topologically homogeneous β-barrel structures³⁴, and while their folding mechanisms are not fully understood, the translocation and assembly module (TAM) and the β-barrel assembly machinery (BAM) complex (BamABCDE) function synergistically for OMP assembly in the outer membrane^35,36,37,38. The biogenesis of TolC is similar to that of other OMPs; it is synthesized in the cytoplasm with an N-terminal signal peptide that facilitates the translocation of unfolded TolC into the periplasm through the Sec translocon^39,40. Most OMPs remain unfolded in the periplasm and require periplasmic chaperones to traverse the periplasm before interacting with the BAM complex for assembly and insertion into the outer membrane⁴¹. Evolutionary studies indicate that TolC is unrelated to other β-barrel OMPs, having evolved from periplasmic efflux pump adaptor proteins and convergence on the β-barrel fold³⁴. However, others have demonstrated that outer membrane β-barrels in Gram-negative bacteria—including TolC—share a common origin and evolved from a single, ancestral ββ hairpin⁴².

Despite this, TolC is structurally distinct from other proteins assembled by the BAM complex or the TAM and appears to follow a unique assembly pathway that does not rely on known chaperones⁴³. TolC folding involves two intermediates, where the primarily α-helical TolC monomer first folds, and then the trimer assembles in the periplasm before being inserted into the outer membrane⁴⁰. The TAM complex organizes and inserts the TolC β-barrel into the outer membrane⁴⁴. The role of the BAM complex is less well understood. While this protein complex is associated with TolC biogenesis⁴⁵, the mechanism proceeds independently of the BamA periplasmic polypeptide transport-associated (POTRA) 1 domain involved in assembling most OMPs⁴¹. Several TolC homologs in other bacterial species, including Pseudomonas aeruginosa OprM, are lipoproteins⁴⁶ transported to the outer membrane with assistance from the Lol machinery⁴⁷. The E. coli LolCDE is interchangeable with the P. aeruginosa LolCDE complex and can transport OprM to the outer membrane⁴⁷; however, TolC lacks the characteristics of lipoproteins, including lipidation.

While the precise mechanism of TolC biogenesis is not fully understood, the data suggests that the α-helical barrel likely assembles within the periplasm, avoiding the need for chaperone assistance to traverse this region⁴⁰. The transmembrane domain of TolC is then assembled and inserted into the outer membrane by the TAM and BAM complexes through a poorly understood mechanism^38,44.

While recent studies have provided more profound insight into the TolC biogenesis and assembly pathway, future research should elucidate the detailed molecular mechanisms, including the specific interactions and sequence of events involved in the assembly of its α-helical barrel and transmembrane domain, as well as any potential accessory factors that may contribute to its efficient integration and folding.

Other outer membrane channel proteins in E. coli

Although TolC has been extensively studied, E. coli encodes three additional less well-studied homologous outer membrane channel proteins, also known as outer membrane factors: CusC (formerly YlcB), MdtP (formerly YjcP), and MdtQ (formerly YohG) (Fig. 2)⁴⁸. The cusC and mdtP genes are encoded within operons, whereas mdtQ is encoded independently (Fig. 3a). The cusC and mdtP operons also contain genes encoding proteins that either assemble or are likely to assemble with these outer membrane channels (Fig. 3a). cusC is part of the CusCFAB cation efflux system, which plays a critical role in the extrusion of copper and silver ions^49,50. The function of MdtP and MdtQ have not been elucidated; however, mdtP is encoded in an operon with a putative periplasmic adaptor protein (MdtN) and an inner membrane major facilitator superfamily efflux pump (MdtO)⁵¹. Null mutants of mdtP and mdtQ in E. coli display increased susceptibility to puromycin, acriflavine, and tetraphenylarsonium chloride, indicating they may be involved in transport⁴⁸. Intriguingly, tolC is expressed in an operon with two genes of unknown function⁵²: ygiB and ygiC (Fig. 3a). While YgiB is predicted to be an OM lipoprotein⁵², the AlphaFold predicted structure (AF-P0ADT2-F1-v4) displays low confidence, and the function of this protein remains unknown. YgiC is homologous with the C-terminal domain of E. coli GspS, which exhibits glutathionylspermidine (GSP) synthetase and GSP amidase activities⁵². Based on sequence similarity with GspS, it is predicted that YgiC is a GSP synthetase, yet the enzyme lacks this activity⁵³. YgiC has since been shown to possess peptide-spermidine ligase activity, associating the enzyme with polyamine metabolism, yet the physiological substrate remains unknown⁵⁴. Deleting these genes does not affect the localization, folding, or activity of TolC⁵²; however, they are associated with the severe growth defects exhibited by tolC mutants in minimum medium with glucose supplied as the carbon source⁵².

**Fig. 3: Phylogenetic analysis of *E. coli* outer membrane channel proteins.**

While the CusC, MdtP, and MdtQ channel proteins exhibit low (<20%, Fig. 2) amino acid sequence similarity to TolC, their known or predicted structures are homologous to TolC, as determined using the distance matrix alignment (DALI) pairwise structure comparison server (Fig. 2)^55,56. The structure of CusC has been determined⁵⁷, and superposition with TolC demonstrates that the proteins are highly similar, with an alpha-carbon position root mean square deviation (RMSD) of 2.1 Å (Fig. 2b). The structures of MdtP and MdtQ have not yet been determined, yet the superposition of their predicted structures with that of TolC reveals that they are similar to TolC (Fig. 2c, d), which requires experimental validation.

Given the poorly understood functions of TolC homologs in E. coli, we assessed the conservation of these outer membrane factors and their associated efflux systems within the core genome of E. coli (Fig. 3b). There are 14 distinct E. coli phylogroups, each exhibiting unique patterns of gene conservation⁵⁸. Similar to our previous analysis of E. coli efflux pump-encoding gene conservation⁵⁹, we performed BLASTP searches against the core genome of each phylogroup⁵⁸, revealing that the outer membrane channel proteins are highly conserved while the conservation of their cognate efflux systems is more variable (Fig. 3). mdtQ is conserved in the core genome of E. coli despite its annotation as a pseudogene⁶⁰. The conservation of these efflux-associated outer membrane channels indicates they have been maintained throughout evolution despite the genetic diversity of E. coli. The phylogenetic relationship of these outer membrane channels was also assessed using unrooted maximum likelihood trees of medoid strains (i.e., a representative strain that is the genetic center of each phylogroup) (Fig. 3c), as defined by Abram et al.⁵⁸. This analysis demonstrates that TolC is most similar to the CusC cation outer membrane channel⁶¹ and that MdtQ and MdtP are divergent and may have distinct functions.

Future research should investigate the roles of these poorly characterized outer membrane channels to understand their contributions to bacterial physiology and adaptation, focusing on environmental stress resistance, antimicrobial efflux, and virulence. Additionally, the physiological roles of YgiB and YgiC should be explored, which are encoded alongside tolC in the same operon, with particular attention to their involvement in TolC-associated pathways and polyamine metabolism and their significance in E. coli physiology.

The regulation of tolC expression

Each E. coli cell is estimated to contain approximately 1500 copies of the TolC protein²¹, with these numbers increasing in response to environmental stimuli. The tolC expression regulatory network is complex, with the identification of four tolC promoters (p1, p2, p3, and p4) (Fig. 4)⁶². Similar to the genes encoding the AcrAB efflux system, tolC is a member of the marA, soxS, and rob (marA/soxS/rob) transcriptional regulon, which is upregulated in response to environmental stressors such as superoxide stress, heavy metals, bile salts, and antibiotics^63,64. MarA, SoxS, and Rob bind to a 19 bp DNA sequence called the marbox, located in the promoters of target genes⁶⁵. The majority of MarA residues that interact with the marbox are conserved in SoxS and Rob as well, enabling these regulators to activate a similar set of target genes^66,67. This includes tolC, where the marbox is positioned between the p2 and p3 promoters⁶⁸. When MarA, SoxS, or Rob bind to this marbox, they activate tolC expression from the p3 and p4 promoters^64,68.

**Fig. 4: The *tolC* regulatory network.**

The p1 and p2 promoters are located upstream of the marbox and are controlled by other transcriptional regulons. The regulation of p1 remains unknown, and p2 is regulated by the two-component systems PhoPQ and EvgAS⁶⁹. PhoPQ is activated under low levels of divalent cations such as Mg²⁺⁷⁰, activating genes associated with cell envelope stress, including divalent cation homeostasis, acid resistance (e.g., gadE⁷¹), antimicrobial peptide resistance, and lipopolysaccharide (LPS) modification (e.g., pagP and eptB^72,73). EvgAS regulates the expression of genes involved in acid resistance, including gadE⁷⁴, and indirectly controls tolC expression by activating PhoPQ via the regulatory protein SafA, which increases PhoP phosphorylation⁷⁵. PhoP binds upstream of tolC p1, increasing expression^69,76. Other less well-understood two-component systems linked to tolC expression include EnvZ-OmpR, which is implicated in upregulating tolC in response to high environmental osmolarity^77,78 and BaeSR, which indirectly enhances tolC expression⁷⁹.

Since TolC is involved in the uptake of colicin E1 and is also a cell surface receptor for certain bacteriophages, there are several instances where the downregulation of tolC expression could be beneficial. At present, no direct transcriptional repressors of tolC have been reported; however, AcrR—the acriflavine resistance regulator—is a global repressor of the marA/soxS/rob regulon, and thus indirectly represses tolC⁸⁰, in addition to directly repressing the acrAB efflux pump-encoding genes⁸¹. Additionally, the small regulatory RNA (sRNA) SdsR, or RyeB, abundantly expressed during the stationary phase, is a post-transcriptional repressor of tolC⁸².

In summary, tolC expression is governed by a complex network of regulators, with some acting directly on one of the four tolC promoters and others acting indirectly. Figure 4 provides a schematic overview of our current understanding of tolC regulation. Although an OmpR binding site is upstream of p1⁷⁷, the specific regulators controlling expression from this promoter remain unidentified, highlighting that undiscovered factors or conditions may modulate activity. Future research should explore environmental cues that trigger p1 activation, providing a more comprehensive view of the regulatory landscape of tolC.

The diverse functions of TolC

The complex regulatory network governing tolC expression reflects the multifunctional nature of this outer membrane channel, enabling adaptation and survival of E. coli in different environmental conditions. TolC is implicated in a wide range of physiological functions, which we expand upon below, and the inactivation of tolC is associated with pleiotropic phenotypes. It is well established that TolC is a fundamental component of the efflux machinery and is thus an essential antimicrobial resistance factor (Fig. 5)^16,83. The protein is also a component of type I secretion systems¹¹, enabling delivery of virulence factors, functions as a receptor for bacteriophage TLS⁸⁴, and is exploited for entry of the colicin E1 toxin^85,86 (Fig. 5). However, several tolC mutant phenotypes remain poorly understood, such as decreased membrane integrity^9,10,87, altered cell envelope mechanics⁸⁷, and reduced acid survival⁸⁸. Insights from analyzing tolC regulatory pathways suggest that some of these phenotypes may result from the activation of two-component systems and transcriptional regulators, and thus their target genes, in response to the inactivation of tolC (Fig. 4).

Antimicrobial resistance

Arguably, the most prominent role of this outer membrane channel protein is intrinsic antimicrobial resistance through the formation of tripartite complexes with resistance-nodulation-cell division (RND), ATP-binding cassette (ABC), and major facilitator superfamily (MFS) efflux pumps in E. coli⁸⁹ (Fig. 5). These multicomponent complexes collectively provide broad-spectrum resistance to antimicrobial agents^83,90. Additionally, as a component of the efflux machinery, TolC indirectly facilitates the acquisition of horizontally acquired resistance mechanisms in the presence of growth inhibitors, which may play a role in the dissemination of drug resistance⁹¹ and contributes to persister cell formation in E. coli in the presence of antimicrobial agents⁹².

TolC is considered non-selective, with antimicrobial substrate specificity originating from the efflux pumps and their periplasmic adaptor proteins^93,94. However, the internal surface of the periplasmic end of TolC’s α-helical barrel is electronegative, which may hinder the transport of specific substrates through the channel¹⁸, and several residues in the α-helical barrel are associated with substrate specificity⁹³. TolC forms cell envelope-spanning complexes with at least eight different efflux pumps and their corresponding periplasmic adaptor proteins in E. coli: AcrAB, AcrAD, AcrEF, MdtABC, MdtEF, MacAB, EmrAB, and EmrKY⁸⁸ (Fig. 5). Structures of TolC in complex with AcrAB^22,23,24, MacAB⁹⁵, and EmrAB⁹⁶ have provided valuable insights into its association with these pumps, but without corresponding structures for the remaining complexes, the molecular mechanisms of TolC’s interactions with a broader array of pumps remain speculative. This gap underscores the need for further structural studies to clarify how TolC facilitates multidrug resistance across diverse efflux systems.

TolC adopts an open conformation to facilitate the efflux of substrates across the outer membrane; when the protein is not in a complex with inner membrane components, the closed periplasmic end of TolC impedes small molecules from entering its channel and thus must be opened for transport³³. The in situ structure of AcrAB-TolC reveals that the peptidoglycan layer interfaces between AcrA and TolC²⁴. The β-barrel opening of TolC at the outer leaflet of the outer membrane is more than 10 Å wide, facilitating the easy passage of substrates²⁴. The AcrA periplasmic adaptor protein, anchored to the inner membrane, makes extensive contact with the AcrB efflux pump and contacts the peptidoglycan and TolC, communicating AcrB conformational changes to the outer membrane channel²⁴. To open TolC, conformational changes in AcrB, induced by drug binding, are relayed to TolC through the interface of TolC and AcrA, which leads to the twisting of AcrA and the opening of the channel²⁴. The small protein AcrZ also interacts with AcrB in the inner membrane influencing substrate specificity⁹⁷.

The factors driving the assembly of these tripartite complexes are intricate and still need to be fully understood. TolC exhibits differing affinities for the periplasmic adaptor proteins, which are dynamic interactions impacted by oligomerization and pH⁹⁸. Given that TolC can assemble membrane-spanning complexes with a variety of structurally distinct inner membrane efflux pumps, their periplasmic adaptor proteins, and type I secretion systems, future research should examine the environmental signals and mechanisms driving the recruitment and formation of these multi-component complexes. This investigation may reveal whether these assemblies have unifying structural or functional features or if TolC exhibits flexibility in its interactions with different inner membrane components.

TolC-mediated export of endogenously produced physiological substrates

In addition to exporting exogenous antimicrobial compounds as a defense strategy, TolC also facilitates the extrusion of several endogenously produced compounds, including virulence factors, as described in the section below. During microbial competition, TolC facilitates the efflux of antimicrobial peptides such as colicin V¹² and microcin J25¹⁴. TolC associates with the ABC transporter CvaAB to export colicin V^12,99 and the ABC transporter YojHI to export microcin J25¹⁰⁰. TolC is also involved in the export of metabolites in conjunction with inner membrane drug efflux pumps⁵⁹. We have demonstrated that the E. coli drug efflux network is highly conserved in E. coli, indicating that these proteins contribute to important physiological functions⁵⁹. Perhaps the most well-known example is enterobactin, a high-affinity siderophore synthesized and secreted by E. coli for environmental iron acquisition¹⁰¹. The large size of enterobactin restricts it from diffusing through porins, indicating that a transporter is required for transport across the outer membrane²⁵. The inactivation of tolC impairs E. coli growth under iron-limiting conditions²⁵, which is attributed to the periplasmic accumulation of enterobactin that TolC ordinarily exports with assistance from inner membrane efflux pumps^101,102. TolC is also linked to the export of several other metabolites, including porphyrins²⁷ and reactive nitrogen species¹⁰³. Untargeted metabolomics supports a role for TolC in central metabolism^104,105, and future studies should experimentally validate the export of identified metabolites. Further details pertaining to the drug efflux pump-mediated export of metabolites can be found in our review article on the physiological roles and conservation of E. coli drug efflux pumps⁵⁹.

TolC and cell envelope integrity

TolC has long been associated with cell envelope integrity and function in E. coli. Early studies demonstrated that tolC mutants exhibit reduced levels of the outer membrane protein OmpF^9,10, which is now known to be due to upregulation of the mar-sox-rob regulon, and particularly Rob, the primary regulator involved in increased MicF expression in tolC mutants¹⁰⁶. MicF is a small regulatory RNA that binds upstream of ompF mRNAs, inhibiting translation¹⁰⁶. Since several antibiotics enter the cell through porins such as OmpF (reviewed in¹⁰⁷), the cell may respond to loss-of-efflux by reducing the uptake of toxic compounds. Another early observation was that tolC mutants exhibited heightened sensitivity to various antimicrobial agents, initially attributed to increased outer membrane permeability¹. Although it is now understood that the primary role of TolC in antimicrobial resistance involves its participation in drug efflux systems, inactivating tolC leads to pleiotropic effects on the cell beyond drug efflux alone. While we acknowledge that several phenotypes, particularly growth defects, are evident in an iron-limited minimal medium, we have demonstrated that the inactivation of tolC induces cell surface structural changes and a reduction in cell envelope mechanics under nutrient-rich conditions⁸⁷. A decrease in cell envelope mechanical strength and surface morphological changes indicate outer membrane defects in tolC deficient strains since this structure is the major contributor to E. coli stiffness and strength¹⁰⁸. Supporting defects in outer membrane composition, we also demonstrated that the inactivation of tolC sensitized E. coli to perturbations in LPS transport, and we identified a synthetic sick interaction between tolC and msbA, the gene encoding the translocase essential for transporting LPS across the inner membrane⁸⁷. Synthetic sick interactions occur when the combination of two genetic or chemical perturbations leads to greater fitness defects than either perturbation alone¹⁰⁹. These interactions highlight the intricate relationships between genes and can provide insights into cellular pathways and mechanisms.

We hypothesize that tolC mutants exhibit changes in the lipid composition of the cell envelope, including alterations to the LPS structure, as well as potential disruptions to the asymmetric organization of the outer membrane. Indeed, tolC mutants are resistant to the LPS-specific phage U3¹¹⁰, and there are anecdotal references to alterations in the properties of LPS in tolC mutants¹¹¹. For example, tolC mutants exhibit many of the characteristics of ‘deep-rough mutants’ associated with changes to the core oligosaccharide region of LPS¹¹¹. The LPS core, particularly the heptose (Hep) core region, is critical for outer membrane stability¹¹², with the phosphoryl moieties enabling divalent cross-linking of adjacent LPS molecules¹¹³. TolC was proposed to play a role in the maturation-assembly of LPS by converting the inner core HepI pyrophosphorylethanolamine (2-aminoethanol; PPEtN, or PPEA) to phosphate after the export of LPS¹¹¹. Blockage of the HepI-phosphate in a tolC mutant could explain the decreases in membrane integrity and cell envelope mechanics⁸⁷ since the HepI phosphate contributes to membrane stability¹¹². However, the data supporting a role for TolC in the conversion of HepI-PPEtN remains unavailable, with existing references relying on unpublished data¹¹¹. Additionally, MacA, the periplasmic adaptor protein of the MacAB-TolC efflux pump, binds rough LPS with high affinity; however, it has not been demonstrated that LPS is transported by this efflux assembly¹¹⁴.

In summary, while several lines of evidence link TolC to cell envelope integrity and LPS in E. coli, much remains to be understood about the precise mechanisms underlying these associations. We note that tolC regulators also control genes associated with outer membrane symmetry, and we hypothesize that some of the phenotypes may be regulatory in response to the loss of tolC (Fig. 4).

Virulence, pathogenicity, and host interactions

A unifying feature of TolC’s diverse functions is its critical role in host survival. This multi-functional outer membrane protein is essential for the efflux of antimicrobial compounds used to treat infections and for enabling bacterial cells to adapt to hostile environments within the host. As a critical component of the type I secretion system (T1SS) (Fig. 5), TolC enables the secretion of virulence factors that enhance the ability of pathogenic E. coli strains to colonize, invade, and persist within host tissues. Hemolytic E. coli activity was first described in 1960, which was later attributed to the cytolytic α-hemolysin (HlyA) T1SS comprised of HlyB, HlyD, and TolC (reviewed in¹¹⁵). Akin to the tripartite drug efflux machinery, this secretion system spans the entire cell envelope and features an inner membrane ATP-binding cassette (ABC) transporter (HlyB), periplasmic adaptor protein (HlyD), and outer membrane factor (TolC). To date, the structure of an assembled T1SS has not been reported, hindered by the transient nature of these systems¹¹⁶, and should be the focus of future research to expand our understanding of TolC’s role in the secretion process, revealing how TolC interacts with other inner membrane transporters and periplasmic adaptor proteins, and shedding light on the molecular mechanisms underlying substrate translocation across the bacterial envelope. Additionally, the mechanisms underlying T1SS assembly are not well understood, yet the interaction of the substrate with the inner membrane transporter is considered to trigger assembly, with conformational changes in the periplasmic adaptor protein recruiting TolC¹¹⁷.

As a component of T1SSs, TolC has a clear role in virulence, secreting different toxins like HlyA and heat-stable (ST) enterotoxins, which are responsible for extraintestinal and diarrheal E. coli diseases, respectively^11,118,119. In addition to cytotoxicity, HlyA alters the host’s inflammatory responses, suppressing cytokine production in epithelial cells in vitro¹²⁰ and stimulating interleukin-1β release from cultured human monocytes¹²¹. By mediating the export of HlyA, TolC is associated with modulating the innate immune response during infections. This has been further demonstrated by reduced expression of interleukin-1β mRNA in epithelial cells incubated with extraintestinal pathogenic E. coli (ExPEC) lacking tolC¹²².

Mutants of Enteroaggregative E. coli lacking tolC also show reduced aggregative adherence¹²³, and ExPEC tolC mutants demonstrate attenuated epithelial cell invasion in vitro¹²². TolC-associated adhesion and invasion have also been demonstrated in Salmonella enterica, with tolC null mutants displaying reduced adhesion to epithelial cells and an impaired ability to invade macrophages^124,125. TolC has also been linked to biofilm production in several species of Enterobacteriaceae, including Klebsiella pneumoniae, S. enterica Typhimurium, and E. coli^123,126,127. The exact role of TolC in bacterial aggregation, adherence, and biofilm formation remains to be determined but likely involves the efflux abilities of this protein, as several inner membrane pumps that associate with TolC have also been associated with E. coli biofilm formation¹²⁸. TolC and its associated efflux systems have also been shown to influence the formation of E. coli persister cells, enabling survival following exposure to environmental stressors such as antimicrobials⁹², leading to recurrent infections¹²⁹.

TolC is also implicated in acid resistance, a critical adaptive mechanism that enables E. coli to survive the host’s gastrointestinal tract, particularly during the transition through the stomach, where gastric acid poses a significant barrier to bacterial survival. E. coli can survive at pH 2.0 for hours due to acid resistance mechanisms¹³⁰. Several studies demonstrate that the fitness of tolC-deficient E. coli strains is impaired under acidic conditions, including during extreme acid survival^88,90, the latter of which is associated with the glutamate-dependent (Gad) acid resistance system. The genes encoding the glutamate decarboxylase system (GadA, GadB) are not expressed in tolC mutants under acid conditions⁸⁸. TolC-mediated survival under acidic conditions is associated with the efflux pumps EmrB and MdtB, which form complexes with this outer membrane channel⁸⁸. Acidic environments also stimulate the recruitment of TolC into efflux assemblies⁹⁸. Finally, the two-component EvgAS system, which upregulates tolC (Fig. 4), also upregulates the Gad system (reviewed in¹³¹). Thus, tolC plays an important role in E. coli acid survival, a phenomenon vital for the bacterium’s life cycle; however, the underlying mechanisms associated with tolC-dependent acid resistance remain poorly understood.

In summary, the multifunctional nature of TolC promotes E. coli virulence and survival under diverse environmental stressors in the host^122,124, making it a critical factor for bacterial adaptability and a potential target for novel antimicrobial strategies.

Concluding remarks

TolC has emerged as an important contributor to E. coli physiology, with myriad functions attributed to this versatile outer membrane channel. From its role in multidrug resistance, membrane integrity, acid survival, and function in T1SSs, TolC has proven to be a remarkably multifaceted protein. Yet, despite decades of research, many functions remain understudied. While the structure of TolC within the diderm environment is well characterized, the processes governing its folding and insertion into the outer membrane still need to be fully understood. Additionally, while we now have increased insight into how this outer membrane channel complexes with several inner membrane efflux pumps and their periplasmic adaptor proteins, the molecular details associated with its interaction with other efflux systems and T1SSs remain unknown, in addition to the environmental cues stimulating recruitment and assembly into these complexes. Several studies associate TolC with E. coli membrane integrity and LPS composition in E. coli, yet further research is needed to elucidate the specific mechanisms underlying these associations. TolC also makes multifaceted contributions to virulence-associated processes in E. coli, mediating survival against various environmental host stressors. As a result, it is expected that E. coli strains lacking tolC would show reduced survival within the host environment. However, this hypothesis requires further experimental validation and investigation into the molecular mechanisms underpinning tolC-associated acid survival.