An investigation into the membrane-interactive potential of the Escherichia coli KpsE C-terminus

Mechanisms governing bacterial capsular polysaccharide attachment and chain length

Capsular polysaccharides (CPSs) are high-molecular weight glycopolymers that form a capsule layer on the surface of many bacterial species. This layer serves as a crucial barrier between bacteria and their environment, protecting them from host immune responses and environmental stressors while facilitating adaptation to host niches. The capsule also affects other critical virulence factors of plant and human pathogens such as biofilm production and exchange of antimicrobial-resistance genes. Bacterial pathogens modulate several CPS properties including abundance, chain length, and cell surface retainment to optimize niche-specific fitness. CPS composition varies greatly among bacterial species due to differences in sugar units comprising the polymer. Despite the diversity in composition, three conserved CPS biosynthetic systems are common across bacterial species. Although less explored than CPS polymerization and export, the processes of chain length control and attachment are also broadly conserved among bacterial species. Here, we discuss the common strategies that bacteria use to retain CPS to their cell surface and the mechanisms by which bacteria define and control CPS chain length. Additionally, we highlight the outstanding questions related to these processes, identifying areas where future research is needed to gain better insights into these crucial CPS systems.

Structure of a full-length bacterial polysaccharide co-polymerase

Lipopolysaccharides are important components of the bacterial cell envelope that among other things act as a protective barrier against the environment and toxic molecules such as antibiotics. One of the most widely disseminated pathways of polysaccharide biosynthesis is the inner membrane bound Wzy-dependent pathway. Here we present the 3.0 Å structure of the co-polymerase component of this pathway, WzzB from E. coli solved by single-particle cryo-electron microscopy. The overall architecture is octameric and resembles a box jellyfish containing a large bell-shaped periplasmic domain with the 2-helix transmembrane domain from each protomer, positioned 32 Å apart, encircling a large empty transmembrane chamber. This structure also reveals the architecture of the transmembrane domain, including the location of key residues for the Wzz-family of proteins and the Wzy-dependent pathway present in many Gram-negative bacteria, explaining several of the previous biochemical and mutational studies and lays the foundation for future investigations.

Quaternary structure of WzzB and WzzE polysaccharide copolymerases

Bacteria have evolved cellular control mechanisms to ensure proper length specification for surface-bound polysaccharides. Members of the Polysaccharide Copolymerase (PCP) family are central to this process. PCP-1 family members are anchored to the inner membrane through two transmembrane helices and contain a large periplasm-exposed domain. PCPs are known to form homooligomers but their exact stoichiometry is controversial in view of conflicting structural and biochemical data. Several prior investigations addressing this question indicated a nonameric, hexameric, or tetrameric organization of several PCP-1 family members. In this work, we gathered additional evidence that E.coli WzzB and WzzE PCPs form octameric homo-oligomeric complexes. Detergent-solubilized PCPs were purified to homogeneity and subjected to blue native gel analysis, which indicated the presence of a predominant high-molecular product of over 500 kDa in mass. Molecular mass of WzzE and WzzB-detergent oligomers was estimated to be 550 kDA by size-exclusion coupled to multiangle laser light scattering (SEC-MALLS). Oligomeric organization of purified WzzB and WzzE was further investigated by negative stain electron microscopy and by X-ray crystallography, respectively. Analysis of EM-derived molecular envelope of WzzB indicated that the full-length protein is composed of eight protomers. Crystal structure of LDAO-solubilized WzzE was solved to 6 Å resolutions and revealed its octameric subunit stoichiometry. In summary, we identified a possible biological unit utilized for the glycan chain length determination by two PCP-1 family members. This provides an important step toward further unraveling of the mechanistic basis of chain length control of the O-antigen and the enterobacterial common antigen.

Polysaccharide co-polymerases: the enigmatic conductors of the O-antigen assembly orchestra

The O-antigen lipopolysaccharides on bacterial surface contain variable number of oligosaccharide repeat units with their length having a modal distribution specific to the bacterial strain. The polysaccharide length distribution is controlled by the proteins called polysaccharide co-polymerases (PCPs), which are embedded in the inner membrane in Gram-negative bacteria and form homo oligomers. The 3D structures of periplasmic domains of several PCPs have been determined and provided the first insights into the possible mechanism of polysaccharide length determination mechanism. Here we review the current knowledge of structure and function of these polysaccharide length regulators.

Eukaryotic expression vectors bearing genes encoding cytotoxic proteins for cancer gene therapy

Cancer gene therapy is a promising direction for the treatment of cancer patients. A primary goal of all cancer therapies is to selectively target and kill tumour cells. Such therapies are administered via different approaches, including both viral and non-viral delivery; however, both methods have advantages and disadvantages. Transcriptional targeting enables genes encoding toxic proteins to be expressed directly in cancer cells. Numerous vectors have been created with the purpose of killing cancer cells, and some have successfully suppressed malignant tumours. Data concerning the function of vectors bearing genes that encode cytotoxic proteins under the control of different promoters, including tissue/tumour specific and constitutive promoters, is summarised here. This review focuses on vectors that bear genes encoding diphtheria toxin, Pseudomonas exotoxin A, caspases, gef, streptolysin, and melittin. Data describing the efficacy of such vectors have been summarised. Notably, there are vectors that killed cancer cell lines originating from the same type of cancer with differential efficiency. Thus, there is differential inhibition of cancer cell growth dependent on the cell line. In this review, the constructs employing genes whose expression induces cell death and the efficiency with which they suppress cancer cell growth will be summarised.

Functional and structural characterization of polysaccharide co-polymerase proteins required for polymer export in ATP-binding cassette transporter-dependent capsule biosynthesis pathways

Neisseria meningitidis serogroup B and Escherichia coli K1 bacteria produce a capsular polysaccharide (CPS) that is composed of α2,8-linked polysialic acid (PSA). Biosynthesis of PSA in these bacteria occurs via an ABC (ATP-binding cassette) transporter-dependent pathway. In N. meningitidis, export of PSA to the surface of the bacterium requires two proteins that form an ABC transporter (CtrC and CtrD) and two additional proteins, CtrA and CtrB, that are proposed to form a cell envelope-spanning export complex. CtrA is a member of the outer membrane polysaccharide export (OPX) family of proteins, which are proposed to form a pore to mediate export of CPSs across the outer membrane. CtrB is an inner membrane protein belonging to the polysaccharide co-polymerase (PCP) family. PCP proteins involved in other bacterial polysaccharide assembly systems form structures that extend into the periplasm from the inner membrane. There is currently no structural information available for PCP or OPX proteins involved in an ABC transporter-dependent CPS biosynthesis pathway to support their proposed roles in polysaccharide export. Here, we report cryo-EM images of purified CtrB reconstituted into lipid bilayers. These images contained molecular top and side views of CtrB and showed that it formed a conical oligomer that extended ∼125 Å from the membrane. This structure is consistent with CtrB functioning as a component of an envelope-spanning complex. Cross-complementation of CtrA and CtrB in E. coli mutants with defects in genes encoding the corresponding PCP and OPX proteins show that PCP-OPX pairs require interactions with their cognate partners to export polysaccharide. These experiments add further support for the model of an ABC transporter-PCP-OPX multiprotein complex that functions to export CPS across the cell envelope.

Structure and Assembly of Escherichia coli Capsules

The capsule is a cell surface structure composed of long-chain polysaccharides that envelops many isolates of Escherichia coli. It protects the cell against host defenses or physical environmental stresses, such as desiccation. The component capsular polysaccharides (CPSs) are major surface antigens in E. coli. They are named K antigens (after the German word Kapsel). Due to variations in CPS structures, more than 80 serologically unique K antigens exist in E. coli. Despite the hypervariability in CPS structures, only two capsule-assembly strategies exist in E. coli. These have led to the assignment of group 1 and group 2 capsules, and many of the key elements of the corresponding assembly pathways have been resolved. Structural features, as well as genetic and regulatory variations, give rise to additional groups 3 and 4. These employ the same biosynthesis processes described in groups 2 and 1, respectively. Each isolate possesses a distinctive set of cytosolic and inner-membrane enzymes, which generate a precise CPS structure, defining a given K serotype. Once synthesized, a multiprotein complex is needed to translocate the nascent CPS across the Gram-negative cell envelope to the outer surface of the outer membrane, where the capsule structure is assembled. While the translocation machineries for group 1 and group 2 CPSs are fundamentally different from one another, they possess no specificity for a given CPS structure. Each is conserved in all isolates producing capsules belonging to a particular group.

Pivotal roles of the outer membrane polysaccharide export and polysaccharide copolymerase protein families in export of extracellular polysaccharides in gram-negative bacteria

Many bacteria export extracellular polysaccharides (EPS) and capsular polysaccharides (CPS). These polymers exhibit remarkably diverse structures and play important roles in the biology of free-living, commensal, and pathogenic bacteria. EPS and CPS production represents a major challenge because these high-molecular-weight hydrophilic polymers must be assembled and exported in a process spanning the envelope, without compromising the essential barrier properties of the envelope. Emerging evidence points to the existence of molecular scaffolds that perform these critical polymer-trafficking functions. Two major pathways with different polymer biosynthesis strategies are involved in the assembly of most EPS/CPS: the Wzy-dependent and ATP-binding cassette (ABC) transporter-dependent pathways. They converge in an outer membrane export step mediated by a member of the outer membrane auxiliary (OMA) protein family. OMA proteins form outer membrane efflux channels for the polymers, and here we propose the revised name outer membrane polysaccharide export (OPX) proteins. Proteins in the polysaccharide copolymerase (PCP) family have been implicated in several aspects of polymer biogenesis, but there is unequivocal evidence for some systems that PCP and OPX proteins interact to form a trans-envelope scaffold for polymer export. Understanding of the precise functions of the OPX and PCP proteins has been advanced by recent findings from biochemistry and structural biology approaches and by parallel studies of other macromolecular trafficking events. Phylogenetic analyses reported here also contribute important new insight into the distribution, structural relationships, and function of the OPX and PCP proteins. This review is intended as an update on progress in this important area of microbial cell biology.

Biosynthesis and Assembly of Capsular Polysaccharides inEscherichia coli

Capsules are protective structures on the surfaces of many bacteria. The remarkable structural diversity in capsular polysaccharides is illustrated by almost 80 capsular serotypes in Escherichia coli. Despite this variation, the range of strategies used for capsule biosynthesis and assembly is limited, and E. coli isolates provide critical prototypes for other bacterial species. Related pathways are also used for synthesis and export of other bacterial glycoconjugates and some enzymes/processes have counterparts in eukaryotes. In gram-negative bacteria, it is proposed that biosynthesis and translocation of capsular polysaccharides to the cell surface are temporally and spatially coupled by multiprotein complexes that span the cell envelope. These systems have an impact on both a general understanding of membrane trafficking in bacteria and on bacterial pathogenesis.

The cell surface expression of group 2 capsular polysaccharides in Escherichia coli: the role of KpsD, RhsA and a multi-protein complex at the pole of the cell

The export of large negatively charged capsular polysaccharides across the outer membrane represents a significant challenge to Gram negative bacteria. In the case of Escherichia coli group 2 capsular polysaccharides, the mechanism of export across the outer membrane was unknown, with no identified candidate outer membrane proteins. In this paper we demonstrate that the KpsD protein, previously believed to be a periplasmic protein, is an outer membrane protein involved in the export of group 2 capsular polysaccharides across the outer membrane. We demonstrate that KpsD and KpsE are located at the poles of the cell and that polysaccharide biosynthesis and export occurs at these polar sites. By in vivo chemical cross-linking and MALDI-TOF-MS analysis we demonstrate the presence of a multi-protein biosynthetic/export complex in which cytoplasmic proteins involved in polysaccharide biosynthesis could be cross-linked to proteins involved in export across the inner and outer membranes. In addition, we show that the RhsA protein, of previously unknown function, could be cross-linked to the complex and that a rhsA mutation reduces K5 biosynthesis suggesting a role for RhsA in coupling biosynthesis and export.

Inhibitory effect of recombinant adenovirus carrying melittin gene on hepatocellular carcinoma

OBJECTIVES

To search for a new clinical application of melittin (Mel): treating hepatocellular carcinoma with Mel gene.

METHODS

Recombinant adenoviruses carrying the Mel gene and alpha-fetoprotein (AFP) promoter (Ad-rAFP-Mel) were constructed through a bacterial homologous recombinant system. The efficiency of adenovirus-mediated gene transfer and the inhibitory effect of Ad-rAFP-Mel on the proliferation of hepatocarcinoma cells were determined by X-gal stain and MTT assay, respectively. The tumorigenicity of hepatocarcinoma cells transfected by Ad-rAFP-Mel and the antitumor effect of Ad-rAFP-Mel on transplanted tumor in nude mice were detected in vivo.

RESULTS

The Mel mRNA was transcribed in BEL-7402 hepatocellular carcinoma cells transducted by Ad-rAFP-Mel. The efficiency of adenovirus-mediated gene transferred to BEL-7402 cells was 100% when the multiplicity of infection of Ad-rAFP-Mel was 10 in vitro, and was also high in vivo. The inhibitive rates of Ad-rAFP-Mel and Ad-rAFP for BEL7402 cells were 66.2 +/- 2.7% and 2.9 +/- 2.3% (t=30.83, P=6.6 x 10(-6)) by MTT assay. The inhibitive rates of Ad-CMV-Mel for BEL7402, SMMC7721 and L02 cells were 58.9 +/- 9.6%, 65.9 +/- 3.8% and 31.7 +/- 1.2%, respectively, and of Ad-rAFP-Mel were 66.2 +/- 2.7%, 16.1 +/- 6.6% and 7.5 +/- 3.3%, respectively (t=1.27, P=0.27; t=11.31, P=3.5 x 10(-4); and t=12.12, P=2.7 x 10(-4) versus the Ad-CMV-Mel group in the same cells). The tumorigenicity rates of hepatocarcinoma cells transfected by Ad-rAFP-Mel were decreased. A significant antineoplastic effect was detected on transplanted tumor in nude mice by intratumoral injection of Ad-rAFP-Mel.

CONCLUSIONS

Ad-rAFP-Mel can inhibit specifically proliferation of AFP-producing human hepatocarcinoma cells in vitro and in vivo. This suggests that animal toxin gene can be used as an antitumor gene.

Molecular characterization of a membrane-bound cGMP dependent protein kinase from the silk moth Bombyx mori

The cGMP signalling pathway has been suggested to be involved in the signal transduction of various physiological functions in insects; olfaction, antidiuresis and eclosion. However, the cGMP signalling mechanism has remained elusive. We isolated two cDNAs of the cGMP dependent protein kinase, designated BmPKG-Ialpha and BmPKG-Ibeta. The deduced amino acid sequences indicate that both BmPKG-Ialpha and BmPKG-Ibeta appear to consist of an amino terminal region, a cGMP binding domain and a protein kinase domain. Transcripts of BmPKG-Ialpha and BmPKG-Ibeta were detected in various tissues: flight muscles, antennae, midgut, legs, head, thoracic ganglia and Malphighian tubules. Recombinant BmPKG-Ialpha bound to lipid membranes, while BmPKG-Ialpha with a deleted amino terminal region failed to bind to lipid membranes.