Introduction of a lethal redox switch that controls the opening and closing of the hydrophobic cavity in LolA

Insights into bacterial lipoprotein trafficking from a structure of LolA bound to the LolC periplasmic domain

In Gram-negative bacteria, outer-membrane lipoproteins are essential for maintaining cellular integrity, transporting nutrients, establishing infections, and promoting the formation of biofilms. The LolCDE ABC transporter, LolA chaperone, and LolB outer-membrane receptor form an essential system for transporting newly matured lipoproteins from the outer leaflet of the cytoplasmic membrane to the innermost leaflet of the outer membrane. Here, we present a crystal structure of LolA in complex with the periplasmic domain of LolC. The structure reveals how a solvent-exposed β-hairpin loop (termed the "Hook") and trio of surface residues (the "Pad") of LolC are essential for recruiting LolA from the periplasm and priming it to receive lipoproteins. Experiments with purified LolCDE complex demonstrate that association with LolA is independent of nucleotide binding and hydrolysis, and homology models based on the MacB ABC transporter predict that LolA recruitment takes place at a periplasmic site located at least 50 Å from the inner membrane. Implications for the mechanism of lipoprotein extraction and transfer are discussed. The LolA-LolC structure provides atomic details on a key protein interaction within the Lol pathway and constitutes a vital step toward the complete molecular understanding of this important system.

Bacterial lipoproteins; biogenesis, sorting and quality control

Bacterial lipoproteins are a subset of membrane proteins localized on either leaflet of the lipid bilayer. These proteins are anchored to membranes through their N-terminal lipid moiety attached to a conserved Cys. Since the protein moiety of most lipoproteins is hydrophilic, they are expected to play various roles in a hydrophilic environment outside the cytoplasmic membrane. Gram-negative bacteria such as Escherichia coli possess an outer membrane, to which most lipoproteins are sorted. The Lol pathway plays a central role in the sorting of lipoproteins to the outer membrane after lipoprotein precursors are processed to mature forms in the cytoplasmic membrane. Most lipoproteins are anchored to the inner leaflet of the outer membrane with their protein moiety in the periplasm. However, recent studies indicated that some lipoproteins further undergo topology change in the outer membrane, and play critical roles in the biogenesis and quality control of the outer membrane. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

The Journey of Lipoproteins Through the Cell: One Birthplace, Multiple Destinations

Bacterial lipoproteins are a very diverse group of proteins characterized by the presence of an N-terminal lipid moiety that serves as a membrane anchor. Lipoproteins have a wide variety of crucial functions, ranging from envelope biogenesis to stress response. In Gram-negative bacteria, lipoproteins can be targeted to various destinations in the cell, including the periplasmic side of the cytoplasmic or outer membrane, the cell surface or the external milieu. The sorting mechanisms have been studied in detail in Escherichia coli, but exceptions to the rules established in this model bacterium exist in other bacteria. In this chapter, we will present the current knowledge on lipoprotein sorting in the cell. Our particular focus will be on the surface-exposed lipoproteins that appear to be much more common than previously assumed. We will discuss the different targeting strategies, provide numerous examples of surface-exposed lipoproteins and discuss the techniques used to assess their surface exposure.

Secretion of bacterial lipoproteins: through the cytoplasmic membrane, the periplasm and beyond

Bacterial lipoproteins are peripherally anchored membrane proteins that play a variety of roles in bacterial physiology and virulence in monoderm (single membrane-enveloped, e.g., gram-positive) and diderm (double membrane-enveloped, e.g., gram-negative) bacteria. After export of prolipoproteins through the cytoplasmic membrane, which occurs predominantly but not exclusively via the general secretory or Sec pathway, the proteins are lipid-modified at the cytoplasmic membrane in a multistep process that involves sequential modification of a cysteine residue and cleavage of the signal peptide by the signal II peptidase Lsp. In both monoderms and diderms, signal peptide processing is preceded by acylation with a diacylglycerol through preprolipoprotein diacylglycerol transferase (Lgt). In diderms but also some monoderms, lipoproteins are further modified with a third acyl chain through lipoprotein N-acyl transferase (Lnt). Fully modified lipoproteins that are destined to be anchored in the inner leaflet of the outer membrane (OM) are selected, transported and inserted by the Lol (lipoprotein outer membrane localization) pathway machinery, which consists of the inner-membrane (IM) ABC transporter-like LolCDE complex, the periplasmic LolA chaperone and the OM LolB lipoprotein receptor. Retention of lipoproteins in the cytoplasmic membrane results from Lol avoidance signals that were originally described as the "+2 rule". Surface localization of lipoproteins in diderms is rare in most bacteria, with the exception of several spirochetal species. Type 2 (T2SS) and type 5 (T5SS) secretion systems are involved in secretion of specific surface lipoproteins of γ-proteobacteria. In the model spirochete Borrelia burgdorferi, surface lipoprotein secretion does not follow established sorting rules, but remains dependent on N-terminal peptide sequences. Secretion through the outer membrane requires maintenance of lipoproteins in a translocation-competent unfolded conformation, likely through interaction with a periplasmic holding chaperone, which delivers the proteins to an outer membrane lipoprotein flippase. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Folding mechanisms of periplasmic proteins

More than one fifth of the proteins encoded by the genome of Escherichia coli are destined to the bacterial cell envelope. Over the past 20years, the mechanisms by which envelope proteins reach their three-dimensional structure have been intensively studied, leading to the discovery of an intricate network of periplasmic folding helpers whose members have distinct but complementary roles. For instance, the correct assembly of ß-barrel proteins containing disulfide bonds depends both on chaperones like SurA and Skp for transport across the periplasm and on protein folding catalysts like DsbA and DsbC for disulfide bond formation. In this review, we provide an overview of the current knowledge about the complex network of protein folding helpers present in the periplasm of E. coli and highlight the questions that remain unsolved. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Defective lipoprotein sorting induces lolA expression through the Rcs stress response phosphorelay system

The Escherichia coli LolA protein is a lipoprotein-specific chaperone that carries lipoproteins from the inner to the outer membrane. A dominant negative LolA mutant, LolA(I93C/F140C), in which both (93)Ile and (140)Phe are replaced by Cys, binds tightly to the lipoprotein-dedicated ABC transporter LolCDE complex on the inner membrane and therefore inhibits the detachment of outer membrane-specific lipoproteins from the inner membrane. We found that the expression of this mutant strongly induced lolA gene transcription. The depletion of the LolA or LolB protein also triggered lolA gene transcription, indicating that the inhibition of outer membrane lipoprotein transport triggers lolA transcription. To elucidate the mechanism, we isolated mutants that are unable to induce lolA transcription using the lacZ gene fused to the lolA promoter as a reporter and found that the Rcs phosphorelay system directly regulates lolA transcription. An outer membrane lipoprotein, RcsF, was essential for this activation, while the coactivator RcsA was dispensable. Taking the observation that an RcsF mutant localized in the inner membrane constitutively activated the Rcs phosphorelay system into consideration, the results shown here strongly suggest that correct lipoprotein sorting to the outer membrane is monitored by RcsF, which plays a key role in the Rcs stress response system.

Signal integration by the Cpx-envelope stress system

The Cpx-envelope stress system coordinates the expression and assembly of surface structures important for the virulence of Gram-negative pathogenic bacteria. It is comprised of the membrane-anchored sensor kinase CpxA, the cytosolic response regulator CpxR and the accessory protein CpxP. Characteristic of the group of two-component systems, the Cpx system responds to a broad range of stimuli including pH, salt, metals, lipids and misfolded proteins that cause perturbation in the envelope. Moreover, the Cpx system has been linked to inter-kingdom signalling and bacterial cell death. However, although signal specificity has been assumed, for most signals the mechanism of signal integration is not understood. Recent structural and functional studies provide the first insights into how CpxP inhibits CpxA and serves as sensor for misfolded pilus subunits, pH and salt. Here, we summarize and reflect on the current knowledge on signal integration by the Cpx-envelope stress system.

A periplasmic LolA derivative with a lethal disulfide bond activates the Cpx stress response system

LolA accommodates the acyl chains of lipoproteins in its hydrophobic cavity and shuttles between the inner and outer membranes through the hydrophilic periplasm to place lipoproteins in the outer membrane. The LolA(I93C/F140C) derivative, in which Cys replaces Ile at position 93 and Phe at position 140, strongly inhibited growth in the absence of a reducing agent because of the lethal intramolecular disulfide bond between the two Cys residues. Expression of I93C/F140C was found to activate the Cpx two-component system, which responds to cell envelope stress. The inhibition of growth by I93C/F140C was partly suppressed by overproduction of LolCDE, which is an ATP-binding cassette transporter and mediates the transfer of lipoproteins from the inner membrane to LolA. A substantial portion of the oxidized form, but not the reduced one, of I93C/F140C expressed on LolCDE overproduction was recovered in the membrane fraction, whereas wild-type LolA was localized in the periplasm even when LolCDE was overproduced. Moreover, LolCDE overproduction stabilized I93C/F140C and therefore caused an increase in its level. Taken together, these results indicate that oxidized I93C/F140C stably binds to LolCDE, which causes strong envelope stress.

Structural investigation of the interaction between LolA and LolB using NMR

Lipoproteins that play critical roles in various cellular functions of Gram-negative bacteria are localized in the cells inner and outer membranes. Lol proteins (LolA, LolB, LolC, LolD, and LolE) are involved in the transportation of outer membrane-directed lipoproteins from the inner to the outer membrane. LolA is a periplasmic chaperone that transports lipoproteins, and LolB is an outer membrane receptor that accepts lipoproteins. To clarify the structural basis for the lipoprotein transfer from LolA to LolB, we examined the interaction between LolA and mLolB, a soluble mutant of LolB, using solution NMR spectroscopy. We determined the interaction mode between LolA and mLolB with conformational changes of LolA. Based upon the observations, we propose that the LolA.LolB complex forms a tunnel-like structure, where the hydrophobic insides of LolA and LolB are connected, which enables lipoproteins to transfer from LolA to LolB.

Model of mouth-to-mouth transfer of bacterial lipoproteins through inner membrane LolC, periplasmic LolA, and outer membrane LolB

Outer membrane-specific lipoproteins in Escherichia coli are released from the inner membrane by an ATP-binding cassette transporter, the LolCDE complex, which causes the formation of a soluble complex with a periplasmic molecular chaperone, LolA. LolA then transports lipoproteins to the outer membrane where an outer membrane receptor, LolB, incorporates lipoproteins into the outer membrane. The molecular mechanisms underlying the Lol-dependent lipoprotein sorting have been clarified in detail. However, it remained unclear how Lol factors interact with each other to conduct very efficient lipoprotein transfer in the periplasm where ATP is not available. To address this issue, a photo-reactive phenylalanine analogue, p-benzoyl-phenylalanine, was introduced at various positions of LolA and LolB, of which the overall structures are very similar and comprise an incomplete beta-barrel with a hydrophobic cavity inside. Cells expressing LolA or LolB derivatives containing the above analogue were irradiated with UV for in vivo photo-cross-linking. These analyses revealed a hot area in the same region of LolA and LolB, through which LolA and LolB interact with each other. This area is located at the entrance of the hydrophobic cavity. Moreover, this area in LolA is involved in the interaction with a membrane subunit, LolC, whereas no cross-linking occurs between LolA and the other membrane subunit, LolE, or ATP-binding subunit LolD, despite the structural similarity between LolC and LolE. The hydrophobic cavities of LolA and LolB were both found to bind lipoproteins inside. These results indicate that the transfer of lipoproteins through Lol proteins occurs in a mouth-to-mouth manner.