Construction of a multihybrid display system: flagellar filaments carrying two foreign adhesive peptides

An overview on display systems (phage, bacterial, and yeast display) for production of anticancer antibodies; advantages and disadvantages

Antibodies as ideal therapeutic and diagnostic molecules are among the top-selling drugs providing considerable efficacy in disease treatment, especially in cancer therapy. Limitations of the hybridoma technology as routine antibody generation method in conjunction with numerous developments in molecular biology led to the development of alternative approaches for the streamlined identification of most effective antibodies. In this regard, display selection technologies such as phage display, bacterial display, and yeast display have been widely promoted over the past three decades as ideal alternatives to traditional methods. The display of antibodies on phages is probably the most widespread of these methods, although surface display on bacteria or yeast have been employed successfully, as well. These methods using various sizes of combinatorial antibody libraries and different selection strategies possessing benefits in screening potency, generating, and isolation of high affinity antibodies with low risk of immunogenicity. Knowing the basics of each method assists in the design and retrieval process of antibodies suitable for different diseases, including cancer. In this review, we aim to outline the basics of each library construction and its display method, screening and selection steps. The advantages and disadvantages in comparison to alternative methods, and their applications in antibody engineering will be explained. Finally, we will review approved or non-approved therapeutic antibodies developed by employing these methods, which may serve as therapeutic antibodies in cancer therapy.

Discovery of a proteolytic flagellin family in diverse bacterial phyla that assembles enzymatically active flagella

Bacterial flagella are cell locomotion and occasional adhesion organelles composed primarily of the polymeric protein flagellin, but to date have not been associated with any enzymatic function. Here, we report the bioinformatics-driven discovery of a class of enzymatic flagellins that assemble to form proteolytically active flagella. Originating by a metallopeptidase insertion into the central flagellin hypervariable region, this flagellin family has expanded to at least 74 bacterial species. In the pathogen, Clostridium haemolyticum, metallopeptidase-containing flagellin (which we termed flagellinolysin) is the second most abundant protein in the flagella and is localized to the extracellular flagellar surface. Purified flagellar filaments and recombinant flagellin exhibit proteolytic activity, cleaving nearly 1000 different peptides. With ~ 20,000 flagellin copies per  ~ 10-μm flagella this assembles the largest proteolytic complex known. Flagellum-mediated extracellular proteolysis expands our understanding of the functional plasticity of bacterial flagella, revealing this family as enzymatic biopolymers that mediate interactions with diverse peptide substrates.

So far no enzymatic activity has been attributed to flagellin, the major component of bacterial flagella. Here the authors use bioinformatic analysis and identify a metallopeptidase insertion in flagellins from 74 bacterial species and show that recombinant flagellin and flagellar filaments have proteolytic activity.

Bifunctional nanotube scaffolds for diverse ligands are purified simply from Escherichia coli strains coexpressing two functionalized flagellar genes

We functionalized Escherichia coli FliC flagellin proteins to form tailored nanotubes binding single types or pairs of ligands, including divalent cations, fluorescent antibodies, or biotin-avidin-linked moieties such as ferritins. The ratio of each tag in bifunctionalized flagella could be toggled extending their sophistication as nanoscaffolds. Tobacco Etch Virus (TEV) protease site-containing FliCs were cleaved by the cognate protease without filament disintegration, potentiating their use as removable nanolithography masks to deposit attached ligands by protease cleavage.

Simultaneous display of multiple foreign peptides in the FliD capping and FliC filament proteins of the Escherichia coli flagellum

The bacterial flagellum is composed of more than 20 different proteins. The filament, which constitutes the major extracellular part of the flagellum, is built up of approximately 20,000 FliC molecules that assemble at the growing distal end of the filament. A capping structure composed of five FliD molecules located at the tip of the filament promotes polymerization of FliC. Lack of FliD leads to release of the subunits into the growth medium. We show here that FliD can be successfully used in bacterial surface display. We tested various insertion sites in the capping protein, and the optimal region for display was at the variable region in FliD. Deletion and/or insertion at other sites resulted in decreased formation of flagella. We further developed the technique into a multihybrid display system in which three foreign peptides are simultaneously expressed within the same flagellum, i.e., D repeats of FnBPA from Staphylococcus aureus at the tip and fragments of YadA from Yersinia enterocolitica as well as SlpA from Lactobacillus crispatus along the filament. This technology can have biotechnological applications, e.g., in simultaneous delivery of several effector molecules.

Molecular basis of antigenic polymorphism of EspA filaments: development of a peptide display technology

Like many Gram-negative pathogens, enteropathogenic (EPEC) and enterohaemorrhagic Escherichia coli (EHEC) use a macromolecular type III secretion system (TTSS) to inject effector proteins into eukaryotic host cells. The membrane-associated needle complex (NC) of the TTSS, which shows broad similarity to the flagellar basal body, is conserved amongst bacterial pathogens. However, the extracellular part of the TTSS of EPEC and EHEC is unique, in that it has a hollow, approximately 12 nm in diameter, filamentous extension to the NC. EspA filaments are homo-polymers made of the translocator protein EspA. The three-dimensional structure of EspA filaments is comparable to that of flagella; the helical symmetry and packing of the subunits forming both filamentous structures are very similar. Like flagella, EspA filaments show antigenic polymorphism as EspA from different EPEC and EHEC clones show no immunological cross-reactivity. In this study, we determined the molecular basis of the antigenic polymorphism of EspA filaments and identified a surface-exposed hypervariable domain that contains the immunodominant EspA epitope. By exchanging the hypervariable domains of EspA(EPEC) and EspA(EHEC) we swapped the antigenic specificity of the EspA filaments. As for the flagellin D3 domain, which is known to tolerate insertions of natural and artificial amino acid sequences, we have inserted short peptides into the surface-exposed, hypervariable domain of EspA. We demonstrated that the inserted peptides are presented on the surface of the recombinant EspA filaments forming a new immunodominant epitope. Accordingly, EspA filaments have a potential to be developed into a novel epitope display system.

Cell-Surface display of heterologous proteins: From high-throughput screening to environmental applications

A variety of expression systems for the display of either short peptides or fully folded proteins on E.coli and, to a lesser extent, on Gram-positive bacteria have been developed. The expression of proteins on the surface of microbial cells has proved extremely important for numerous applications ranging from combinatorial library screening and protein engineering, to whole cell biocatalysts and adsorbants for bioremediation purposes.

Display of proteins on bacteria

Display of heterologous proteins on the surface of microorganisms, enabled by means of recombinant DNA technology, has become an increasingly used strategy in various applications in microbiology, biotechnology and vaccinology. Gram-negative, Gram-positive bacteria, viruses and phages are all being investigated in such applications. This review will focus on the bacterial display systems and applications. Live bacterial vaccine delivery vehicles are being developed through the surface display of foreign antigens on the bacterial surfaces. In this field, 'second generation' vaccine delivery vehicles are at present being generated by the addition of mucosal targeting signals, through co-display of adhesins, in order to achieve targeting of the live bacteria to immunoreactive sites to thereby increase immune responses. Engineered bacteria are further being evaluated as novel microbial biocatalysts with heterologous enzymes immobilized as surface exposed on the bacterial cell surface. A discussion has started whether bacteria can find use as new types of whole-cell diagnostic devices since single-chain antibodies and other type of tailor-made binding proteins can be displayed on bacteria. Bacteria with increased binding capacity for certain metal ions can be created and potential environmental or biosensor applications for such recombinant bacteria as biosorbents are being discussed. Certain bacteria have also been employed for display of various poly-peptide libraries for use as devices in in vitro selection applications. Through various selection principles, individual clones with desired properties can be selected from such libraries. This article explains the basic principles of the different bacterial display systems, and discusses current uses and possible future trends of these emerging technologies.