Reversible autoinhibitory regulation of Escherichia coli metallopeptidase BepA for selective ß-barrel protein degradation.

Escherichia coli periplasmic zinc-metallopeptidase BepA normally functions by promoting maturation of LptD, a ß-barrel outer-membrane protein involved in biogenesis of lipopolysaccharides, but degrades it when its membrane assembly is hampered. These processes should be properly regulated to ensure normal biogenesis of LptD. The underlying mechanism of regulation, however, remains to be elucidated. A recently solved BepA structure has revealed unique features: In particular, the active site is buried in the protease domain and conceivably inaccessible for substrate degradation. Additionally, the His-246 residue in the loop region containing helix α9 (α9/H246 loop), which has potential flexibility and covers the active site, coordinates the zinc ion as the fourth ligand to exclude a catalytic water molecule, thereby suggesting that the crystal structure of BepA represents a latent form. To examine the roles of the α9/H246 loop in the regulation of BepA activity, we constructed BepA mutants with a His-246 mutation or a deletion of the α9/H246 loop and analyzed their activities in vivo and in vitro. These mutants exhibited an elevated protease activity and, unlike the wild-type BepA, degraded LptD that is in the normal assembly pathway. In contrast, tethering of the α9/H246 loop repressed the LptD degradation, which suggests that the flexibility of this loop is important to the exhibition of protease activity. Based on these results, we propose that the α9/H246 loop undergoes a reversible structural change that enables His-246-mediated switching (histidine switch) of its protease activity, which is important for regulated degradation of stalled/misassembled LptD.

The TPR domain of BepA is required for productive interaction with substrate proteins and the ß-barrel assembly machinery complex.

BepA (formerly YfgC) is an Escherichia coli periplasmic protein consisting of an N-terminal protease domain and a C-terminal tetratricopeptide repeat (TPR) domain. We have previously shown that BepA is a dual functional protein with chaperone-like and proteolytic activities involved in membrane assembly and proteolytic quality control of LptD, a major component of the outer membrane lipopolysaccharide translocon. Intriguingly, BepA can associate with the BAM complex: the ß-barrel assembly machinery (BAM) driving integration of ß-barrel proteins into the outer membrane. However, the molecular mechanism of BepA function and its association with the BAM complex remains unclear. Here, we determined the crystal structure of the BepA TPR domain, which revealed the presence of two subdomains formed by four TPR motifs. Systematic site-directed in vivo photo-cross-linking was used to map the protein-protein interactions mediated by the BepA TPR domain, showing that this domain interacts both with a substrate and with the BAM complex. Mutational analysis indicated that these interactions are important for the BepA functions. These results suggest that the TPR domain plays critical roles in BepA functions through interactions both with substrates and with the BAM complex. Our findings provide insights into the mechanism of biogenesis and quality control of the outer membrane.

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.

Protease homolog BepA (YfgC) promotes assembly and degradation of ß-barrel membrane proteins in Escherichia coli.

Gram-negative bacteria are equipped with quality-control systems for the outer membrane (OM) that sense and cope with defective biogenesis of its components. Accumulation of misfolded outer membrane proteins (OMPs) in Escherichia coli leads to activation of σ(E), an essential alternative σ factor that up-regulates transcription of multiple genes required to preserve OM structure and function. Disruption of bepA (formerly yfgC), a σ(E)-regulated gene encoding a putative periplasmic metalloprotease, sensitizes cells to multiple drugs, suggesting that it may be involved in maintaining OM integrity. However, the specific function of BepA remains unclear. Here, we show that BepA enhances biogenesis of LptD, an essential OMP involved in OM transport and assembly of lipopolysaccharide, by promoting rearrangement of intramolecular disulfide bonds of LptD. In addition, BepA possesses protease activity and is responsible for the degradation of incorrectly folded LptD. In the absence of periplasmic chaperone SurA, BepA also promotes degradation of BamA, the central OMP subunit of the ß-barrel assembly machinery (BAM) complex. Interestingly, defective oxidative folding of LptD caused by bepA disruption was partially suppressed by expression of protease-active site mutants of BepA, suggesting that BepA functions independently of its protease activity. We also show that BepA has genetic and physical interaction with components of the BAM complex. These findings raised the possibility that BepA maintains the integrity of OM both by promoting assembly of OMPs and by proteolytically eliminating OMPs when their correct assembly was compromised.

Association between AUTS2 haplotypes and alcohol dependence in a Japanese population.

OBJECTIVE: Recent genome-wide analysis has indicated that the autism susceptibility candidate 2 (AUTS2) gene is involved in the regulation of alcohol consumption. We hypothesised that AUTS2 might be associated with the development of alcohol dependence. Therefore, in this exploratory study, we compared the genotype and allele frequencies of the polymorphisms rs6943555 and rs9886351 in the AUTS2 gene between patients with alcohol dependence and healthy control subjects living in a Japanese provincial prefecture. We also examined whether or not the haplotypes consisting of these polymorphisms are related to alcohol dependence. METHODS: The subjects of this study consisted of 64 patients with alcohol dependence and 75 unrelated healthy people. The AUTS2 genotypes were determined by the polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) method. RESULTS: No significant differences in the genotype and allele frequencies of the polymorphisms AUTS2 rs6943555 and rs9886351 were found between alcohol dependence and control subjects. On the other hand, the frequencies of the AUTS2 haplotypes were significantly different between them, and the rs6943555 and rs9886351 A-A haplotype was associated with alcohol dependence (p=0.0187). CONCLUSION: This suggests that the rs6943555 and rs9886351 A-A haplotype might affect the vulnerability to alcohol dependence pathogenesis. Further studies are needed to confirm the reproducibility of the results of this study with increased numbers of subjects.

No Relationships between Psychosis and the Diazepam Binding Inhibitor rs2276596 (C/A) Polymorphism in Japanese: An Exploratory Study.

OBJECTIVE: Our recent study for the first time reported genotyping method of the diazepam binding inhibitor (DBI) rs2276596 polymorphism using a Polymerase Chain Reaction-Restriction Fragment Length Polymor- phism (PCR-RFLP), and revealed a significant relationships between this polymorphism and alcohol depend- ence. In this study, to facilitate elucidation of the pathogeneses of psychoses including schizophrenia and mood (affective) disorders, we investigated the relationship between the DBI rs2276596 polymorphism (C/A) and psychoses. METHOD: We analyzed the DBI genotypes using the PCR-RFLP method in healthy controls, and psychotics including schizophrenia and mood (affective) disorders (including recurrent depressive disorder and bipolar affective disorder) (ICD-10: F31, F33). RESULT: There was no significant difference in the rs2276596 genotype and allele frequencies of the DBI gene between these psychoses and healthy controls. CONCLUSION: The present data suggested that a mutated allele of the DBI was not one of the risk factors for schizophrenia and mood (affective) disorders, as for the rs2276596 polymorphism. [Original].

Activation of Toxin-Antitoxin System Toxins Suppresses Lethality Caused by the Loss of σE in Escherichia coli.

UNLABELLED: σ(E), an alternative σ factor that governs a major signaling pathway in envelope stress responses in Gram-negative bacteria, is essential for growth of Escherichia coli not only under stressful conditions, such as elevated temperature, but also under normal laboratory conditions. A mutational inactivation of the hicB gene has been reported to suppress the lethality caused by the loss of σ(E). hicB encodes the antitoxin of the HicA-HicB toxin-antitoxin (TA) system; overexpression of the HicA toxin, which exhibits mRNA interferase activity, causes cleavage of mRNAs and an arrest of cell growth, while simultaneous expression of HicB neutralizes the toxic effects of overproduced HicA. To date, however, how the loss of HicB rescues the cell lethality in the absence of σ(E) and, more specifically, whether HicA is involved in this process remain unknown. Here we showed that simultaneous disruption of hicA abolished suppression of the σ(E) essentiality in the absence of hicB, while ectopic expression of wild-type HicA, but not that of its mutant forms without mRNA interferase activity, restored the suppression. Furthermore, HicA and two other mRNA interferase toxins, HigB and YafQ, suppressed the σ(E) essentiality even in the presence of chromosomally encoded cognate antitoxins when these toxins were overexpressed individually. Interestingly, when the growth media were supplemented with low levels of antibiotics that are known to activate toxins, E. coli cells with no suppressor mutations grew independently of σ(E). Taken together, our results indicate that the activation of TA system toxins can suppress the σ(E) essentiality and affect the extracytoplasmic stress responses. IMPORTANCE: σ(E) is an alternative σ factor involved in extracytoplasmic stress responses. Unlike other alternative σ factors, σ(E) is indispensable for the survival of E. coli even under unstressed conditions, although the exact reason for its essentiality remains unknown. Toxin-antitoxin (TA) systems are widely distributed in prokaryotes and are composed of two adjacent genes, encoding a toxin that exerts harmful effects on the toxin-producing bacterium itself and an antitoxin that neutralizes the cognate toxin. Curiously, it is known that inactivation of an antitoxin rescues the σ(E) essentiality, suggesting a connection between TA systems and σ(E) function. We demonstrate here that toxin activation is necessary for this rescue and suggest the possible involvement of TA systems in extracytoplasmic stress responses.

Small-molecule inhibitors of gram-negative lipoprotein trafficking discovered by phenotypic screening.

In Gram-negative bacteria, lipoproteins are transported to the outer membrane by the Lol system. In this process, lipoproteins are released from the inner membrane by the ABC transporter LolCDE and passed to LolA, a diffusible periplasmic molecular chaperone. Lipoproteins are then transferred to the outer membrane receptor protein, LolB, for insertion in the outer membrane. Here we describe the discovery and characterization of novel pyridineimidazole compounds that inhibit this process. Escherichia coli mutants resistant to the pyridineimidazoles show no cross-resistance to other classes of antibiotics and map to either the LolC or LolE protein of the LolCDE transporter complex. The pyridineimidazoles were shown to inhibit the LolA-dependent release of the lipoprotein Lpp from E. coli spheroplasts. These results combined with bacterial cytological profiling are consistent with LolCDE-mediated disruption of lipoprotein targeting to the outer membrane as the mode of action of these pyridineimidazoles. The pyridineimidazoles are the first reported inhibitors of the LolCDE complex, a target which has never been exploited for therapeutic intervention. These compounds open the door to further interrogation of the outer membrane lipoprotein transport pathway as a target for antimicrobial therapy.

Novel antibacterial targets and compounds revealed by a high-throughput cell wall reporter assay.

UNLABELLED: A high-throughput phenotypic screen based on a Citrobacter freundii AmpC reporter expressed in Escherichia coli was executed to discover novel inhibitors of bacterial cell wall synthesis, an attractive, well-validated target for antibiotic intervention. Here we describe the discovery and characterization of sulfonyl piperazine and pyrazole compounds, each with novel mechanisms of action. E. coli mutants resistant to these compounds display no cross-resistance to antibiotics of other classes. Resistance to the sulfonyl piperazine maps to LpxH, which catalyzes the fourth step in the synthesis of lipid A, the outer membrane anchor of lipopolysaccharide (LPS). To our knowledge, this compound is the first reported inhibitor of LpxH. Resistance to the pyrazole compound mapped to mutations in either LolC or LolE, components of the essential LolCDE transporter complex, which is required for trafficking of lipoproteins to the outer membrane. Biochemical experiments with E. coli spheroplasts showed that the pyrazole compound is capable of inhibiting the release of lipoproteins from the inner membrane. Both of these compounds have significant promise as chemical probes to further interrogate the potential of these novel cell wall components for antimicrobial therapy. IMPORTANCE: The prevalence of antibacterial resistance, particularly among Gram-negative organisms, signals a need for novel antibacterial agents. A phenotypic screen using AmpC as a sensor for compounds that inhibit processes involved in Gram-negative envelope biogenesis led to the identification of two novel inhibitors with unique mechanisms of action targeting Escherichia coli outer membrane biogenesis. One compound inhibits the transport system for lipoprotein transport to the outer membrane, while the other compound inhibits synthesis of lipopolysaccharide. These results indicate that it is still possible to uncover new compounds with intrinsic antibacterial activity that inhibit novel targets related to the cell envelope, suggesting that the Gram-negative cell envelope still has untapped potential for therapeutic intervention.

Roles of the protruding loop of factor B essential for the localization of lipoproteins (LolB) in the anchoring of bacterial triacylated proteins to the outer membrane.

The Lol system comprising five Lol proteins, LolA through LolE, sorts Escherichia coli lipoproteins to outer membranes. The LolCDE complex, an ATP binding cassette transporter in inner membranes, releases outer membrane-specific lipoproteins in an ATP-dependent manner, causing formation of the LolA-lipoprotein complex in the periplasm. LolA transports lipoproteins through the periplasm to LolB on outer membranes. LolB is itself a lipoprotein anchored to outer membranes, although the membrane anchor is functionally dispensable. LolB then localizes lipoproteins to outer membranes through largely unknown mechanisms. The crystal structure of LolB is similar to that of LolA, and it possesses a hydrophobic cavity that accommodates acyl chains of lipoproteins. To elucidate the molecular function of LolB, a periplasmic version of LolB, mLolB, was mutagenized at various conserved residues. Despite the lack of acyl chains, most defective mutants were insoluble. However, a derivative with glutamate in place of leucine 68 was soluble and unable to localize lipoproteins to outer membranes. This leucine is present in a loop protruding from mLolB into an aqueous environment, and no analogous loop is present in LolA. Thus, leucine 68 was replaced with other residues. Replacement by acidic, but not hydrophobic, residues generated for the first time mLolB derivatives that can accept but cannot localize lipoproteins to outer membranes. Moreover, deletion of the leucine with neighboring residues impaired the lipoprotein receptor activity. Based on these observations, the roles of the protruding loop of LolB in the last step of lipoprotein sorting are discussed.

Psychological Evaluation of Animal-assisted Intervention (AAI) Programs Involving Visiting Dogs and Cats for Alcohol Dependents: A Pilot Study.

The purpose of this study was to develop an evaluation method for animal-assisted intervention (AAI) programs involving Mood Check List-Short form.2 (MCL-S.2) and the State-Trait Anxiety Inventory (STAI) for psychiatric daycare of Japanese alcohol. dependents. A total of 36 alcohol dependents completed the study and questionnaires assessing their state. A single session of AAI reduced both subjective and physiological measures of state anxiety (A-State); and this program induced a significant reduction in the anxiety after an AAI program session with the dogs and cats involved in the intervention (p = 0.001). The Wilcoxon t-test showed that there were also significant differences in the "anxiety", "pleasantness", and "relaxation". scores for MCL-S.2 among the alcohol dependents, before and after AAI; a significantly decreased "anxiety" score (p = 0.006), and increased "pleasantness" (p = 0.002) and "relaxation" (p=0.012) scores for MCL-S.2 after AAI. The results of this study indicated that alcohol dependents who experienced a group AAI session-program exhibited significant improvements in their feeling; decreased anxiety, and increased pleasantness and relaxation.

[Association between GSK3ß polymorphisms and the smoking habits in young Japanese].

Schizophrenia and bipolar disorder show high comorbidity with smoking dependence. Several previous studies reported that glycogen synthase kinase 3ß (GSK3ß), which is widely expressed in the brain including the dopamine projection areas such as the amygdala, nucleus accumbens and hippocampus, may play a role in neuropsychiatric disorders and dopamine- and serotonin-mediated behavior. In this study, we have analyzed the association of three single nucleotide polymorphisms (SNPs) within GSK3ß gene (rs3755557, rs334558, rs6438552) with the smoking habits and age at smoking initiation in a sample of 384 young adult Japanese, which included 172 smokers and 212 non-smokers. As a result, rs334558 was significantly associated with smoking habits in genotype frequency and allelic frequency (P < 0.05). Furthermore, higher haplotype 3 (T-T-T) and haplotype 5 (A-T-C) frequencies were observed in non-smokers than smokers (P < 0.05). Three functional polymorphisms examined in this study reportedly increase transcriptional activity when they have a high-activation allele such as the A allele of -1727A/T (rs3755557), the T allele of -50T/C (rs334558) or T allele of -157T/C (rs6438552). Thus, it was suggested in this study that changes in GSK3ß activity may have an impact on smoking habits.

[Association between norepinephrine transporter gene polymorphism and alcohol dependence in Japanese].

Several studies have suggested that the norepinephrine transporter (NET) may play an important role in the pathogenesis of alcohol dependence. Therefore, in this study, we investigated whether the NET gene polymorphism is a susceptibility factor for alcohol dependence in 64 alcoholics and 73 healthy controls. In addition, we examined whether the combination of the NET and serotonin transporter genotypes are associated with alcohol dependence. The NET (1287G/A, -182T/C, and -3081A/T) and serotonin transporter (5-HTT3'UTR) genotypes were determined by the polymerase chain reaction (PCR)--restriction fragment length polymorphism (RFLP) method. No significant differences in genotype and allele frequencies of the NET and serotonin transporter gene polymorphisms were found between alcoholics and controls. The haplotype frequencies of the NET gene polymorphisms were not also significantly different between them. Furthermore, the combination of the NET and serotonin transporter genotypes had not significant effects on alcohol dependence. The present study suggests that the polymorphisms of 1287G/A, -182T/C and -3081A/T in NET gene are not.risk factors in alcohol dependence.

Relationship between alcohol dependence and the DBI rs2276596 (C/A) polymorphism in Japanese.

To facilitate elucidation of the pathogenesis of alcohol dependence, we investigated the relationship between a genetic variant of diazepam biding inhibitor (DBI) C/A polymorphism (rs2276596) and alcohol dependence. We determined the DBI genotypes using a novel method involving PCR-RFLP in healthy controls and alcoholics with a diagnosis of alcohol dependence by ICD-10 (F10.20). There was a significant difference in the rs2276596 polymorphism C/A allele frequency of the DBI gene (P < 0.0001) between alcoholics and healthy controls. The present data suggested that a mutant allele of the DBI was one of the risk factors for alcohol dependence as for the rs2276596 polymorphism.

Dissection of an ABC transporter LolCDE function analyzed by photo-crosslinking.

The envelope of Escherichia coli contains approximately 100 different species of lipoproteins, most of which are localized to the inner leaflet of the outer membrane. The localization of lipoprotein (Lol) system, consisting of five Lol proteins, is responsible for the trafficking of lipoproteins to the outer membrane. LolCDE binds to lipoproteins destined for the outer membrane and transfers them to the periplasmic chaperone LolA. Although the cryo-EM structures of E. coli LolCDE have been reported, the mechanisms by which outer membrane lipoproteins are transferred to LolA remain elusive. In this study, we investigated the interaction between LolCDE and lipoproteins using site-specific photo-crosslinking. We introduced a photo-crosslinkable amino acid into different locations across the four helices which form the central lipoprotein-binding cavity, and identified domains that crosslink with peptidoglycan-associated lipoprotein (Pal) in vivo. Using one of the derivatives containing the photo-crosslinkable amino acid, we developed an in vitro system to analyze the binding of lipoproteins to LolCDE. Our results indicate that compound 2, a LolCDE inhibitor, does not inhibit the binding of lipoproteins to LolCDE, but rather promotes the dissociation of bound lipoproteins from LolCDE.

[The relationship between BDNF gene polymorphisms and alcoholics in Japan].

As a help of the mechanism elucidation of alcoholism, we studied the relationship between brain-derived neurotrophic factor (BDNF) rs6265, 270 C/T (ID number has not yet been determined), and rs10835210 gene polymorphisms, which are reported to be related to bipolar disorder, and alcoholics. We genotyped the three polymorphisms in the BDNF gene using polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) in 65 alcoholics and 71 healthy controls. In this study, there was no significant difference in the frequency of rs6265 and 270 C/T polymorphisms between alcoholics and controls (P > 0.05). However, there was a significant difference in the genotype frequency of rs10835210 polymorphism between alcoholics and controls (P < 0.05), in which the CA heterozygote genotype and A allele frequency was higher in alcoholics than in the controls. It suggests the possibility that the BDNF rs10835210 gene polymorphism affects the etiology of alcoholism.

[Haplotype analysis of serotonin 2A receptor gene polymorphisms and smoking behavior].

In this study, the relationship between the haplotypes consisting of single nucleotide polymorphisms (SNPs) of serotonin 2A receptor (5HT2AR) gene (HTR2A) 102T/C (rs6313) and -1438A/G (rs6311) and smoking behavior was studied among 101 smokers and 99 non-smokers. It was shown that the genotypic and allelic frequencies of these polymorphisms were not associated with the smoking behavior. However, according to haplotype analysis, higher haplotype 1 ((-1438G) G-(102)T) frequency was observed in smokers than in non-smokers (P < 0.05). Pairwise D' and gamma2 values between the two SNPs in this study were 0.916 and 0.805, respectively. The two SNPs thus showed strong linkage disequilibrium with each other. This study suggests that 5-HT2AR gene haplotype (G-T) may be related to smoking behavior.

An unusual case of fatal hypothermia involving topical diphenhydramine.

PURPOSE: Diphenhydramine is an antihistamine drug widely used to alleviate symptoms caused by allergies and the common cold. Diphenhydramine-involved fatalities have been reported in the past but usually involving overdose by ingestion. We report a peculiar case of fatal hypothermia during non-winter season involving topical diphenhydramine. METHODS: A 23-year-old male with no known preexisting medical conditions was found dead in the bathroom of his apartment with a small amount of running water on his back. Postmortem examinations and toxicological analysis on blood and urine were performed. RESULTS: Color difference was apparent between the right and left cardiac blood. Wischnewski spots were observed in the gastric mucosa. Histological examination revealed no obvious findings that could attribute to serious cardiovascular events. Drug screening by gas chromatograph-tandem mass spectrometry (GC/MS/MS) detected diphenhydramine in blood and urine. Further quantification revealed the postmortem concentrations to be 0.44 µg/mL in blood and 2500 µg/mL in urine. CONCLUSIONS: The cause of death was determined to be hypothermia. Diphenhydramine-induced drowsiness and possible intrinsic cardiac factor may have led to prolonged impaired consciousness, preventing his ability to escape from the running cold water leading to hypothermia and death.

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.

Overexpression of LolCDE allows deletion of the Escherichia coli gene encoding apolipoprotein N-acyltransferase.

Bacterial lipoproteins represent a subset of membrane-associated proteins that are covalently modified with lipids at the N-terminal cysteine. The final step of lipoprotein modification, N-acylation of apolipoproteins, is mediated by apolipoprotein N-acyltransferase (Lnt). Examinations with reconstituted proteoliposomes and a conditional mutant previously indicated that N-acylation of lipoproteins is required for their efficient release from the inner membrane catalyzed by LolA and LolCDE, the lipoprotein-specific chaperone and ABC transporter, respectively. Because Lnt is essential for Escherichia coli, a mutant lacking Lnt activity has not been isolated. However, we report here that lnt-null strains can be constructed when LolCDE is overproduced in strains lacking either the major outer membrane lipoprotein Lpp or transpeptidases that cross-link Lpp with peptidoglycan. Lipoproteins purified from the lnt-null strain exhibited increased mobility on SDS-PAGE compared to those from wild-type cells and could be sequenced by Edman degradation, indicating that lipoproteins in this mutant exist as apolipoproteins that lack N-acylation. Overexpression of Lpp in the lnt-null strain resulted in the accumulation of apoLpp in the inner membrane and caused growth arrest. In contrast to the release of mature Lpp in the presence of LolA and LolCDE, that of apoLpp from the inner membrane was significantly retarded. Furthermore, the amount of lipoproteins copurified with LolCDE was significantly reduced in the lnt-null strain. These results indicate that the affinity of LolCDE for apolipoprotein is very low, and therefore, overexpression of LolCDE is required for its release and sorting to the outer membrane.