Dysregulated NUB1 and neddylation enhances rheumatoid arthritis fibroblast-like synoviocyte inflammatory responses

OBJECTIVE

Fibroblast-like synoviocytes (FLS) contribute to the pathogenesis of rheumatoid arthritis (RA), in part due to activation of the pro-inflammatory transcription factor NF-κB. Neddylation is modulated by the negative regulator of ubiquitin-like proteins-1 (NUB1). We determined whether NUB1 and neddylation are aberrant in RA FLS thereby contributing to their aggressive phenotype.

METHODS

RA or osteoarthritis (OA) FLS were obtained from arthroplasty synovia. RT-qPCR and Western blot analysis assessed gene and protein expression, respectively. NUB1 was overexpressed using an expression vector. NF-κB activation was assessed by stimulating FLS with IL-1β. Neddylation inhibitor (MLN4924) and proteasome inhibitor were used in migration and gene expression assays. MLN4924 was used in the K/BxN serum transfer arthritis model.

RESULTS

Enhanced H3K27ac and H3K27me3 peaks were observed in the NUB1 promoter in OA FLS compared with RA FLS. NUB1 was constitutively expressed by FLS but induction by IL-1β was significantly greater in OA FLS. The ratio of neddylated CUL1 to non-neddylated CUL1 was lower in OA FLS than RA FLS. NUB1 overexpression decreased NF-κB nuclear translocation and IL-6 mRNA in IL-1β-stimulated RA FLS. MLN4924 decreased CUL1 neddylation, NF-κB nuclear translocation and IL-6 mRNA in IL-1β-stimulated RA FLS. MLN4924 significantly decreased arthritis severity in K/BxN serum-transfer arthritis.

CONCLUSION

CUL1 neddylation and NUB1 induction is dysregulated in RA, which increases FLS activation. Inhibition of neddylation is an effective therapy in an animal model of arthritis. These data suggest that neddylation system contributes to the pathogenesis of RA and that regulation of neddylation could be a novel therapeutic approach.

Laser Capture Microscopy RNA Sequencing for Topological Mapping of Synovial Pathology During Rheumatoid Arthritis

OBJECTIVE

Rheumatoid arthritis (RA) is an autoimmune disease in which the joint lining or synovium becomes highly inflamed and majorly contributes to disease progression. Understanding pathogenic processes in RA synovium is critical for identifying therapeutic targets. We performed laser capture microscopy (LCM) followed by RNA sequencing (LCM-RNAseq) to study regional transcriptomes throughout RA synovium.

METHODS

Synovial lining, sublining, and vessel samples were captured by LCM from seven patients with RA and seven patients with osteoarthritis (OA). RNAseq was performed on RNA extracted from captured tissue. Principal component analysis was performed on the sample set by disease state. Differential expression analysis was performed between disease states based on log2 fold change and q value parameters. Pathway analysis was performed using the Reactome Pathway Database on differentially expressed genes among disease states. Significantly enriched pathways in each synovial region were selected based on the false discovery rate.

RESULTS

RA and OA transcriptomes were distinguishable by principal component analysis. Pairwise comparisons of synovial lining, sublining, and vessel samples between RA and OA revealed substantial differences in transcriptional patterns throughout the synovium. Hierarchical clustering of pathways based on significance revealed a pattern of association between biologic function and synovial topology. Analysis of pathways uniquely enriched in each region revealed distinct phenotypic abnormalities. As examples, RA lining samples were marked by anomalous immune cell signaling, RA sublining samples were marked by aberrant cell cycle, and RA vessel samples were marked by alterations in heme scavenging.

CONCLUSION

LCM-RNAseq confirms reported transcriptional differences between the RA synovium and the OA synovium and provides evidence supporting a relationship between synovial topology and molecular anomalies in RA.

Caspase-8 Variant G Regulates Rheumatoid Arthritis Fibroblast-Like Synoviocyte Aggressive Behavior

OBJECTIVE

Fibroblast-like synoviocytes (FLS) play a pivotal role in rheumatoid arthritis (RA) by contributing to synovial inflammation and progressive joint damage. An imprinted epigenetic state is associated with the FLS aggressive phenotype. We identified CASP8 (encoding for caspase-8) as a differentially marked gene and evaluated its pathogenic role in RA FLSs.

METHODS

RA FLS lines were obtained from synovial tissues at arthroplasty and used at passage 5-8. Caspase-8 was silenced using small interfering RNA, and its effect was determined in cell adhesion, migration and invasion assays. Quantitative reverse transcription PCR and western blot were used to assess gene and protein expression, respectively. A caspase-8 selective inhibitor was used determine the role of enzymatic activity on FLS migration and invasion. Caspase-8 isoform transcripts and epigenetic marks in FLSs were analyzed in FLS public databases. Crystal structures of caspase-8B and G were determined.

RESULTS

Caspase-8 deficiency in RA FLSs reduced cell adhesion, migration, and invasion independent of its catalytic activity. Epigenetic and transcriptomic analyses of RA FLSs revealed that a specific caspase-8 isoform, variant G, is the dominant isoform expressed (~80% of total caspase-8) and induced by PDGF. The crystal structures of caspase-8 variant G and B were identical except for a unique unstructured 59 amino acid N-terminal domain in variant G. Selective knockdown of caspase-8G was solely responsible for the effects of caspase-8 on calpain activity and cell invasion in FLS.

CONCLUSION

Blocking caspase-8 variant G could decrease cell invasion in diseases like RA without the potential deleterious effects of nonspecific caspase-8 inhibition.

Gene Expression and Pharmacodynamic Changes in 1,760 Systemic Lupus Erythematosus Patients From Two Phase III Trials of BAFF Blockade With Tabalumab

Objective

To characterize baseline gene expression and pharmacodynamically induced changes in whole blood gene expression in 1,760 systemic lupus erythematosus (SLE) patients from 2 phase III, 52‐week, randomized, placebo‐controlled, double‐blind studies in which patients were treated with the BAFF‐blocking IgG4 monoclonal antibody tabalumab.

Methods

Patient samples were obtained from SLE patients from the ILLUMINATE‐1 and ILLUMINATE‐2 studies, and control samples were obtained from healthy donors. Blood was collected in Tempus tubes at baseline, week 16, and week 52. RNA was analyzed using Affymetrix Human Transcriptome Array 2.0 and NanoString.

Results

At baseline, expression of the interferon (IFN) response gene was elevated in patients compared with controls, with 75% of patients being positive for this IFN response gene signature. There was, however, substantial heterogeneity of IFN response gene expression and complex relationships among gene networks. The IFN response gene signature was a predictor of time to disease flare, independent of anti–double‐stranded DNA (anti‐dsDNA) antibody and C3 and C4 levels, and overall disease activity. Pharmacodynamically induced changes in gene expression following tabalumab treatment were extensive, occurring predominantly in B cell–related and immunoglobulin genes, and were consistent with other pharmacodynamic changes including anti‐dsDNA antibody, C3, and immunoglobulin levels.

Conclusion

SLE patients demonstrated increased expression of an IFN response gene signature (75% of patients had an elevated IFN response gene signature) at baseline in ILLUMINATE‐1 and ILLUMINATE‐2. Substantial heterogeneity of gene expression was detected among individual patients and in gene networks. The IFN response gene signature was an independent risk factor for future disease flares. Pharmacodynamic changes in gene expression were consistent with the mechanism of BAFF blockade by tabalumab.

PARP-14 binds specific DNA sequences to promote Th2 cell gene expression

PARP-14, a member of the poly ADP-ribose polymerase super family, promotes T helper cell 2 (Th2) differentiation by regulating interleukin-4 (IL-4) and STAT6-dependent transcription. Yet, whether PARP-14 globally impacts gene regulation has not been determined. In this report, using an RNA pol II ChIP-seq approach, we identify genes in Th2 cells that are regulated by PARP-14, and either dependent or independent of ADP-ribosyltransferase catalytic activity. Our data demonstrate that PARP-14 enhances the expression of Th2 genes as it represses the expression of Th1-associated genes. Among the relevant targets are Signal Transducer and Activator of Transcription genes required for polarizing Th1 and Th2 cells. To define a mechanism for PARP-14 function, we use an informatics approach to identify putative PARP-14 DNA binding sites. Two putative PARP-14 binding motifs are identified in multiple Th2 cytokine genes, and we demonstrate that PARP-14 interacts with each motif using in vitro binding assays. Taken together our results indicate that PARP-14 is an important factor for T helper cell differentiation and it binds to specific DNA sequences to mediate its function.

Bcl6 controls the Th2 inflammatory activity of regulatory T cells by repressing Gata3 function

The transcriptional repressor Bcl6 is a critical arbiter of Th cell fate, promoting the follicular Th lineage while repressing other Th cell lineages. Bcl6-deficient (Bcl6(-/-)) mice develop a spontaneous and severe Th2-type inflammatory disease, thus warranting assessment of Bcl6 in regulatory T cell (Treg) function. Bcl6(-/-) Tregs were competent at suppressing T cell proliferation in vitro and Th1-type colitogenic T cell responses in vivo. In contrast, Bcl6(-/-) Tregs strongly exacerbated lung inflammation in a model of allergic airway disease and promoted higher Th2 responses, including systemic upregulation of microRNA-21. Further, Bcl6(-/-) Tregs were selectively impaired at controlling Th2 responses, but not Th1 and Th17 responses, in mixed chimeras of Bcl6(-/-) bone marrow with Foxp3(-/-) bone marrow. Bcl6(-/-) Tregs displayed increased levels of the Th2 transcription factor Gata3 and other Th2 and Treg genes. Bcl6 potently repressed Gata3 transcriptional transactivation, providing a mechanism for the increased expression of Th2 genes by Bcl6(-/-) Tregs. Gata3 has a critical role in regulating Foxp3 expression and functional fitness of Tregs; however, the signal that regulates Gata3 and restricts its transactivation of Th2 cytokines in Tregs has remained unexplored. Our results identify Bcl6 as an essential transcription factor regulating Gata3 activity in Tregs. Thus, Bcl6 represents a crucial regulatory layer in the Treg functional program that is required for specific suppression of Gata3 and Th2 effector responses by Tregs.

Functional characterization of a genetic polymorphism in the promoter of the ESR2 gene

The ESR2 gene encodes the estrogen receptor beta protein. Several studies have shown that genetic variants in the ESR2 gene are associated with a variety of clinical phenotypes. However, very little is known about the functional significance of ESR2 genetic variants. We used a bioinformatics approach to identify regions of the ESR2 promoter that is evolutionarily conserved across the genomes of several species. We resequenced 1.6 kb of the ESR2 gene which included 0.8 kb of the promoter, 0.3 kb of exon ON, and 0.5 kb of the following intron. We identified five single-nucleotide polymorphisms (SNPs) in the ESR2 promoter and one SNP in the intron. Phase analysis indicated that the SNPs likely exist in 11 different haplotypes. Three of the SNPs (rs8008187, rs3829768, rs35036378) were predicted to alter transcription factor binding sites in the ESR2 promoter. All three were detected only in African American subjects. The rs35036378 SNP was in the TATA box and was highly conserved across species. ESR2 promoter reporter assays in LNCaP and SKBR3 cell lines showed that the variant construct containing the rs35036378 SNP allele had approximately 50% less activity relative to the wild-type construct. We conclude that the rs35036378 SNP appears to cause a reduced promoter activity of the ESR2 gene.

TranscriptomeBrowser 3.0: introducing a new compendium of molecular interactions and a new visualization tool for the study of gene regulatory networks

Background

Deciphering gene regulatory networks by in silico approaches is a crucial step in the study of the molecular perturbations that occur in diseases. The development of regulatory maps is a tedious process requiring the comprehensive integration of various evidences scattered over biological databases. Thus, the research community would greatly benefit from having a unified database storing known and predicted molecular interactions. Furthermore, given the intrinsic complexity of the data, the development of new tools offering integrated and meaningful visualizations of molecular interactions is necessary to help users drawing new hypotheses without being overwhelmed by the density of the subsequent graph.

Results

We extend the previously developed TranscriptomeBrowser database with a set of tables containing 1,594,978 human and mouse molecular interactions. The database includes: (i) predicted regulatory interactions (computed by scanning vertebrate alignments with a set of 1,213 position weight matrices), (ii) potential regulatory interactions inferred from systematic analysis of ChIP-seq experiments, (iii) regulatory interactions curated from the literature, (iv) predicted post-transcriptional regulation by micro-RNA, (v) protein kinase-substrate interactions and (vi) physical protein-protein interactions. In order to easily retrieve and efficiently analyze these interactions, we developed In-teractomeBrowser, a graph-based knowledge browser that comes as a plug-in for Transcriptome-Browser. The first objective of InteractomeBrowser is to provide a user-friendly tool to get new insight into any gene list by providing a context-specific display of putative regulatory and physical interactions. To achieve this, InteractomeBrowser relies on a "cell compartments-based layout" that makes use of a subset of the Gene Ontology to map gene products onto relevant cell compartments. This layout is particularly powerful for visual integration of heterogeneous biological information and is a productive avenue in generating new hypotheses. The second objective of InteractomeBrowser is to fill the gap between interaction databases and dynamic modeling. It is thus compatible with the network analysis software Cytoscape and with the Gene Interaction Network simulation software (GINsim). We provide examples underlying the benefits of this visualization tool for large gene set analysis related to thymocyte differentiation.

Conclusions

The InteractomeBrowser plugin is a powerful tool to get quick access to a knowledge database that includes both predicted and validated molecular interactions. InteractomeBrowser is available through the TranscriptomeBrowser framework and can be found at: http://tagc.univ-mrs.fr/tbrowser/. Our database is updated on a regular basis.

Regulating Il9 transcription in T helper cells

T helper (Th) cells are crucial for the development of immunity to infections and inflammatory disease. The acquisition of specific cytokine-secreting profiles, primed by the cytokine microenvironment, is required for effector function of Th cells. The most recent addition to the growing list of effector subsets are Th9 cells that secrete IL-9. In this review, we propose a model for the transcriptional regulation of the Il9 gene in IL-9-expressing T cells and the relatedness of this subset to other Th phenotypes. We suggest that transcription factors restricted to certain Th subsets and common among several subsets might play a role in the plasticity of Th9 cells.

Motif prediction to distinguish LPS-stimulated pro-inflammatory vs. antibacterial macrophage genes

BACKGROUND

Innate immunity is the first line of defence offered by host cells to infections. Macrophage cells involved in innate immunity are stimulated by lipopolysaccharide (LPS), found on bacterial cell surface, to express a complex array of gene products. Persistent LPS stimulation makes a macrophage tolerant to LPS with down regulation of inflammatory genes ("pro-inflammatory") while continually expressing genes to fight the bacterial infection ("antibacterial"). Interactions of transcription factors (TF) at their cognate TF binding sites (TFBS) on the expressed genes are important in transcriptional regulatory networks that control these pro-inflammatory and antibacterial expression paradigms involved in LPS stimulation.

RESULTS

We used differential expression patterns in a public domain microarray data set from LPS-stimulated macrophages to identify 228 pro-inflammatory and 18 antibacterial genes. Employing three different motif search tools, we predicted respectively four and one statistically significant TF-TFBS interactions from the pro-inflammatory and antibacterial gene sets. The biological literature was utilized to identify target genes for the four pro-inflammatory profile TFs predicted from the three tools, and 18 of these target genes were observed to follow the pro-inflammatory expression pattern in the original microarray data.

CONCLUSIONS

Our analysis distinguished pro-inflammatory vs. antibacterial transcriptomic signatures that classified their respective gene expression patterns and the corresponding TF-TFBS interactions in LPS-stimulated macrophages. By doing so, this study has attempted to characterize the temporal differences in gene expression associated with LPS tolerance, a major immune phenomenon implicated in various pathological disorders.

Temporal induction pattern of STAT4 target genes defines potential for Th1 lineage-specific programming

STAT4 is a critical component in the development of inflammatory adaptive immune responses. It has been extensively characterized as a lineage-determining factor in Th1 development. However, the genetic program activated by STAT4 that results in an inflammatory cell type is not well defined. In this report, we use DNA isolated from STAT4-chromatin immunoprecipitation to perform chromatin immunoprecipitation-on-chip analysis of over 28,000 mouse gene promoters to identify STAT4 targets. We demonstrate that STAT4 binds multiple gene-sets that program distinct components of the Th1 lineage. Although many STAT4 target genes display STAT4-dependent IL-12-inducible expression, other genes displayed IL-12-induced histone modifications but lack induction, possibly due to high relative basal expression. In the subset of genes that STAT4 programs for expression in Th1 cells, IL-12-induced mRNA levels remain increased for a longer time than mRNA from genes that are not programmed. This suggests that STAT4 binding to target genes, while critical, is not the only determinant for STAT4-dependent gene programming during Th1 differentiation.

A systems biology approach on oral epithelial responses to periodontal pathogens (45.32)

Periodontitis, a chronic inflammatory disease, characterized by destruction of soft and hard tissues supporting the teeth affects ~20% in the US. The most widely implicated species in periodontitis are Actinobacillus actinomycetemcomitans (Aa), Porphyromonas gingivalis (Pg), and Fusobacterium nucleatum (Fn). We employ a systems biology approach with bioinformatic, genetic and biochemical methods to study differential responses of oral epithelial cells to specific pathogens. Using public domain microarray data, we compared gene expression profiles of cells infected with 4 different bacteria viz., Aa, Pg, Fn and Streptococcus gordonii (Sg). Various subsets of genes were differentially expressed (two-tailed T test) in these strains and clustered into 3 GO classifications: apoptosis, detection of external stimuli and cytokine activity. We have confirmed gene expression of some TLR genes (real time PCR) and cytokine genes (ELISA). Interestingly, apoptosis related genes were upregulated in cells stimulated with an oral commensal strain (Sg), but downregulated in cells exposed to pathogens (Pg/Aa). Also, many pro-inflammatory cytokines were upregulated in cells stimulated with Pg/Aa as compared to cells stimulated with Sg. In sum, our studies suggest that select periodontal pathogens enhance the survival of infected cells that secrete cytokines and mediate pathology.

Stat4 isoforms differentially regulate inflammation and demyelination in experimental allergic encephalomyelitis

Experimental allergic encephalomyelitis (EAE) is a T cell-mediated autoimmune disease model of multiple sclerosis. Signal transducer and activator of transcription 4 (Stat4) is a transcription factor activated by IL-12 and IL-23, two cytokines known to play important roles in the pathogenesis of EAE by inducing T cells to secrete IFN-gamma and IL-17, respectively. We and others have previously shown that therapeutic intervention or targeted disruption of Stat4 was effective in ameliorating EAE. Recently, a splice variant of Stat4 termed Stat4beta has been characterized that lacks 44 amino acids at the C terminus of the full-length Stat4alpha. In this study we examined whether T cells expressing either isoform could affect the pathogenesis of EAE. We found that transgenic mice expressing Stat4beta on a Stat4-deficient background develop an exacerbated EAE compared with wild-type mice following immunization with myelin oligodendrocyte glycoprotein peptide 35-55, while Stat4alpha transgenic mice have greatly attenuated disease. The differential development of EAE in transgenic mice correlates with increased IFN-gamma and IL-17 in Stat4beta-expressing cells in situ, contrasting increased IL-10 production by Stat4alpha-expressing cells. This study demonstrates that Stat4 isoforms differentially regulate inflammatory cytokines in association with distinct effects on the onset and severity of EAE.

LymphTF-DB: a database of transcription factors involved in lymphocyte development

B and T cells develop following a similar early stepwise progression to later stages where multiple developmental options are available. These developmental regimes necessitate differential gene expression regulated by a large number of transcription factors (TFs). The resultant burgeoning amount of information has opened a knowledge gap between TF activities during lymphocyte development and a researcher's experiments. We have created the LymphTF database (DB) to fill this gap. This DB holds interactions between individual TFs and their specific targets at a given developmental time. By storing such interactions as a function of developmental progression, we hope to advance the elucidation of regulatory networks that guide lymphocyte development. Besides queries for TF-target gene interactions in developmental stages, the DB provides a graphical representation of downloadable target gene regulatory sequences with locations of the transcriptional start sites and TF-binding sites. The LymphTF-DB can be accessed freely on the web at http://www.iupui.edu/~tfinterx/.

Intrinsic disorder in transcription factors

Intrinsic disorder (ID) is highly abundant in eukaryotes, which reflect the greater need for disorder-associated signaling and transcriptional regulation in nucleated cells. Although several well-characterized examples of intrinsically disordered proteins in transcriptional regulation have been reported, no systematic analysis has been reported so far. To test for the general prevalence of intrinsic disorder in transcriptional regulation, we used the predictor of natural disorder regions (PONDR) to analyze the abundance of intrinsic disorder in three transcription factor datasets and two control sets. This analysis revealed that from 94.13 to 82.63% of transcription factors possess extended regions of intrinsic disorder, relative to 54.51 and 18.64% of the proteins in two control datasets, which indicates the significant prevalence of intrinsic disorder in transcription factors. This propensity of transcription factors to intrinsic disorder was confirmed by cumulative distribution function analysis and charge-hydropathy plots. The amino acid composition analysis showed that all three transcription factor datasets were substantially depleted in order-promoting residues and significantly enriched in disorder-promoting residues. Our analysis of the distribution of disorder within the transcription factor datasets revealed that (a) the AT-hooks and basic regions of transcription factor DNA-binding domains are highly disordered; (b) the degree of disorder in transcription factor activation regions is much higher than that in DNA-binding domains; (c) the degree of disorder is significantly higher in eukaryotic transcription factors than in prokaryotic transcription factors; and (d) the level of alpha-MoRF (molecular recognition feature) prediction is much higher in transcription factors. Overall, our data reflected the fact that eukaryotes with well-developed gene transcription machinery require transcription factor flexibility to be more efficient.

TCR-gamma genes are rearranged but not transcribed in IL-7R alpha-deficient mice

IL-7, a cytokine produced by bone marrow and thymic stroma, is a growth factor for B and T lymphocytes very early in their development. The IL-7R is a heterodimer of an alpha-chain that specifically binds IL-7 and the common gamma-chain, gamma(c), which is also a component of the receptors for IL-2, IL-4, IL-9, and IL-15. IL-7 has also been hypothesized to play a role in the differentiation of gammadelta T cells, which is supported by the recent findings that mice deficient in the alpha-chain of the IL-7R (IL-7R alpha -/-) or IL-7 (IL-7 -/-) have a complete absence of gammadelta T cells, but not alphabeta T cells. We show in this work that Vgamma4 and Vgamma6 TCR genes are rearranged, and sterile Vgamma4 and Vgamma6 TCR-gamma transcripts are expressed in IL-7R alpha -/- thymocytes, but these TCR-gamma genes, and Vgamma5, are not transcribed in thymocytes from IL-7R alpha -/- mice. RAG-1 and RAG-2 genes are transcriptionally active in fetal and adult IL-7R alpha -/- thymocytes. The IL-7-inducible transcription factor, STAT5, is not active in the fetal thymus of IL-7R alpha -/- compared with IL-7R alpha +/+ mice. These data point to a specific role for IL-7/IL-7R signaling in regulating the transcriptional activity, possibly mediated by STAT5, of the rearranged TCR-gamma complex during development of gammadelta T cells, and point to mechanistic differences in the regulation of rearrangement of Vgamma4 and Vgamma6 genes vs Vgamma5.

TCR-gamma genes are rearranged but not transcribed in IL-7R alpha-deficient mice

IL-7, a cytokine produced by bone marrow and thymic stroma, is a growth factor for B and T lymphocytes very early in their development. The IL-7R is a heterodimer of an alpha-chain that specifically binds IL-7 and the common gamma-chain, gamma(c), which is also a component of the receptors for IL-2, IL-4, IL-9, and IL-15. IL-7 has also been hypothesized to play a role in the differentiation of gammadelta T cells, which is supported by the recent findings that mice deficient in the alpha-chain of the IL-7R (IL-7R alpha -/-) or IL-7 (IL-7 -/-) have a complete absence of gammadelta T cells, but not alphabeta T cells. We show in this work that Vgamma4 and Vgamma6 TCR genes are rearranged, and sterile Vgamma4 and Vgamma6 TCR-gamma transcripts are expressed in IL-7R alpha -/- thymocytes, but these TCR-gamma genes, and Vgamma5, are not transcribed in thymocytes from IL-7R alpha -/- mice. RAG-1 and RAG-2 genes are transcriptionally active in fetal and adult IL-7R alpha -/- thymocytes. The IL-7-inducible transcription factor, STAT5, is not active in the fetal thymus of IL-7R alpha -/- compared with IL-7R alpha +/+ mice. These data point to a specific role for IL-7/IL-7R signaling in regulating the transcriptional activity, possibly mediated by STAT5, of the rearranged TCR-gamma complex during development of gammadelta T cells, and point to mechanistic differences in the regulation of rearrangement of Vgamma4 and Vgamma6 genes vs Vgamma5.

The product of the F sex factor traT surface exclusion gene is a lipoprotein

The product of the Escherichia coli sex factor F traT gene (TraTp), an outer membrane protein of Mr = 25,000, is covalently modified in vivo by the addition of glycerol and fatty acids. Consistent with this result, and as would be expected for a bacterial lipoprotein, the novel amino acid glycerylcysteine can be detected in purified TraTp. Being a secreted protein, TraTp is made from a signal sequence containing precursor, and glycerol and fatty acids can be detected in both the precursor and mature (processed) species of TraTp. The peptide antibiotic globomycin inhibits the cleavage of the pro-TraTp signal sequence, but not the glycerol and fatty acid modification. Diglyceride modification of the Cys residue at the site of signal sequence cleavage probably precedes and is a prerequisite for processing of the TraTp signal sequence. Thus, TraTp appears to be a typical E. coli lipoprotein, having a pathway for modification and processing that is similar to that of Braun's lipoprotein (the major outer membrane lipoprotein).