mRNA-display-based selections for proteins with desired functions: a protease-substrate case study

mRNA-display is an amplification-based, iterative rounds of in vitro protein selection technique that circumvents a number of difficulties associated with yeast two-hybrid and phage display. Because of the covalent linkage between the genotype and the phenotype, mRNA-display provides a powerful means for reading and amplifying a peptide or protein sequence after it has been selected from a library with a diversity in the range of 10(12)-10(13). In this paper, we briefly review the recent progress in using mRNA-display to identify affinity reagents, binding partners, and enzyme substrates from synthetic peptide or natural proteome libraries. To facilitate the use of mRNA-display in research laboratories without previous experience, we provide a detailed analysis of the critical steps of an mRNA-display-based selection in a case study for the identification of the natural substrates of caspases, including the generation of an mRNA-displayed proteome library, removal of abundant sequences, and selection of proteins with desired functions. The advantages and technical limitations of mRNA-display as a general peptide or protein selection tool are also addressed.     

Photocleavable linkage between genotype and phenotype for rapid and efficient recovery of nucleic acids encoding affinity-selected proteins

In vitro display technologies, such as mRNA display and DNA display are powerful tools to screen peptides and proteins with desired functions from combinatorial libraries in the fields of directed protein evolution and proteomics. When screening combinatorial libraries of polypeptides (phenotype), each of which is displayed on its gene (genotype), the problem remains, how best to recover the genotype moiety whose phenotype moiety has bound to the desired target. Here, we describe the use of a photocleavable 2-nitrobenzyl linker between genotype (DNA or mRNA) and phenotype (protein) in our DNA and mRNA display systems. This technique allows rapid and efficient recovery of selected nucleic acids by simple UV irradiation at 4 degrees C for 15 min. Further, we confirmed that the photocleavable DNA display and mRNA display systems are useful for in vitro selection of epitope peptides, recombinant antibodies, and drug-receptor interactions. Thus, these improved methods should be useful in therapeutics and diagnostics, e.g., for screening high-affinity binders, such as enzyme inhibitors and recombinant antibodies from random peptide and antibody libraries, as well as for screening drug-protein interactions from cDNA libraries.     

Ribosome display: next-generation display technologies for production of antibodies in vitro

Antibodies represent an important and growing class of biologic research reagents and biopharmaceutical products. They can be used as therapeutics in a variety of diseases. With the rapid expansion of proteomic studies and biomarker discovery, there is a need for the generation of highly specific binding reagents to study the vast number of proteins encoded by the genome. Display technologies provide powerful tools for obtaining antibodies. Aside from the preservation of natural antibody repertoires, they are capable of exploiting diversity by DNA recombination to create very large libraries for selection of novel molecules. In contrast to in vivo immunization processes, display technologies allow selection of antibodies under in vitro-defined selection condition(s), resulting in enrichment of antibodies with desired properties from large populations. In addition, in vitro selection enables the isolation of antibodies against difficult antigens including self-antigens, and this can be applied to the generation of human antibodies against human targets. Display technologies can also be combined with DNA mutagenesis for antibody evolution in vitro. Some methods are amenable to automation, permitting high-throughput generation of antibodies. Ribosome display is considered as representative of the next generation of display technologies since it overcomes the limitations of cell-based display methods by using a cell-free system, offering advantages of screening larger libraries and continuously expanding new diversity during selection. Production of display-derived antibodies can be achieved by choosing one of a variety of prokaryotic and eukaryotic cell-based expression systems. In the near future, cell-free protein synthesis may be developed as an alternative for large-scale generation of antibodies.     

In vitro selection as a powerful tool for the applied evolution of proteins and peptides

New in vitro methods for the applied evolution of protein structure and function complement conventional cellular and phage-based methods. Strategies employing the direct physical linkage of genotype and phenotype, and the compartmental association of gene and product to select desired properties are discussed, and recent useful applications are described. Engineering of antibodies and other proteins, selection from cDNA libraries, and the creation of functional protein domains from completely random starting sequences illustrate the value of the in vitro approaches. Also discussed is an emerging new direction for in vitro display technology: the self-assembly of protein arrays.     

Comparison of mRNA-display-based selections using synthetic peptide and natural protein libraries

mRNA display is a genotype-phenotype conjugation method that allows the amplification-based, iterative rounds of in vitro selection to be applied to peptides and proteins. Compared to prior protein selection techniques, mRNA display can be used to select functional sequences from both long natural protein and short combinatorial peptide libraries with much higher complexities. To investigate the basic features and problems of using mRNA display in studying conditional protein-protein interactions, we compared the target-binding selections against calmodulin (CaM) using both a natural protein library and a combinatorial peptide library. The selections were efficient in both cases and required only two rounds to isolate numerous Ca2+/CaM-binding natural proteins and synthetic peptides with a wide range of affinities. Many known and novel CaM-binding proteins were identified from the natural human protein library. More than 2000 CaM-binding peptides were selected from the combinatorial peptide library. Unlike sequences from prior CaM-binding selections that correlated poorly with naturally occurring proteins, synthetic peptides homologous to the Ca2+/CaM-binding motifs in natural proteins were isolated. Interestingly, a large number of synthetic peptides that lack the conventional CaM-binding secondary structures bound to CaM tightly and specifically, suggesting the presence of other interaction modes between CaM and its downstream binding targets. Our results indicate that mRNA display is an ideal approach to the identification of Ca2+-dependent protein-protein interactions, which are important in the regulation of numerous signaling pathways.     

Engineered affinity proteins—Generation and applications

The use of combinatorial protein engineering to design proteins with novel binding specificities and desired properties has evolved into a powerful technology, resulting in the recent advances in protein library selection strategies and the emerge of a variety of new engineered affinity proteins. The need for different protein library selection methods is due to that each target protein pose different challenges in terms of its availability and inherent properties. At present, alternative engineered affinity proteins are starting to complement and even challenge the classical immunoglobulins in different applications in biotechnology and potentially also for in vivo use as imaging agents or as biotherapeutics. This review article covers the generation and use of affinity proteins generated through combinatorial protein engineering. The most commonly used selection techniques for isolation of desired variants from large protein libraries are described. Different antibody derivatives, as well as a variety of the most validated engineered protein scaffolds, are discussed. In addition, we provide an overview of some of the major present and future applications for these engineered affinity proteins in biotechnology and medicine.     

Ribosome display: next-generation display technologies for production of antibodies in vitro

Antibodies represent an important and growing class of biologic research reagents and biopharmaceutical products. They can be used as therapeutics in a variety of diseases. With the rapid expansion of proteomic studies and biomarker discovery, there is a need for the generation of highly specific binding reagents to study the vast number of proteins encoded by the genome. Display technologies provide powerful tools for obtaining antibodies. Aside from the preservation of natural antibody repertoires, they are capable of exploiting diversity by DNA recombination to create very large libraries for selection of novel molecules. In contrast to in vivo immunization processes, display technologies allow selection of antibodies under in vitro-defined selection condition(s), resulting in enrichment of antibodies with desired properties from large populations. In addition, in vitro selection enables the isolation of antibodies against difficult antigens including self-antigens, and this can be applied to the generation of human antibodies against human targets. Display technologies can also be combined with DNA mutagenesis for antibody evolution in vitro. Some methods are amenable to automation, permitting high-throughput generation of antibodies. Ribosome display is considered as representative of the next generation of display technologies since it overcomes the limitations of cell-based display methods by using a cell-free system, offering advantages of screening larger libraries and continuously expanding new diversity during selection. Production of display-derived antibodies can be achieved by choosing one of a variety of prokaryotic and eukaryotic cell-based expression systems. In the near future, cell-free protein synthesis may be developed as an alternative for large-scale generation of antibodies.     

Biotin-tagged cDNA expression libraries displayed on lambda phage: a new tool for the selection of natural protein ligands

cDNA expression libraries displayed on lambda phage have been successfully employed to identify partners involved in antibody–antigen, protein– protein and DNA–protein interactions and represent a novel approach to functional genomics. However, as in all other cDNA expression libraries based on fusion to a carrier polypeptide, a major issue of this system is the absence of control over the translation frame of the cDNA. As a consequence, a large number of clones will contain lambda D/cDNA fusions, resulting in the foreign sequence being translated on alternative reading frames. Thus, many phage will not display natural proteins, but could be selected, as they mimic the binding properties of the real ligand, and will hence interfere with the selection outcome. Here we describe a novel lambda vector for display of exogenous peptides at the C-terminus of the capsid D protein. In this vector, translation of fusion peptides in the correct reading frame allows efficient in vivo biotinylation of the chimeric phage during amplification. Using this vector system we constructed three libraries from human hepatoma cells, mouse hepatocytic MMH cells and from human brain. Clones containing open reading frames (ORFs) were rapidly selected by streptavidin affinity chromatography, leading to biological repertoires highly enriched in natural polypeptides. We compared the selection outcome of two independent experiments performed using an anti-GAP-43 monoclonal antibody on the human brain cDNA library before and after ORF enrichment. A significant increase in the efficiency of identification of natural target peptides with very little background of false-positive clones was observed in the latter case.     

噬菌体展示技术及其在肿瘤研究中的应用

噬菌体表面展示技术是一项特异性多肽或蛋白的筛选技术,它将随机序列的多肽或蛋白片段与噬菌体衣壳蛋白融合表达而呈现于病毒表面,被展示的多肽能保持相对独立的空间结构,使其能够与配体作用而达到模仿性筛选特异性分子表位,从而提供了高通量高效率的筛选系统。近年来噬菌体展示技术已广泛应用于肿瘤抗原抗体库的建立、单克隆抗体制备、多肽筛选、疫苗研制、肿瘤相关抗原筛选和抗原表位研究、药物设计、癌症检测和诊断、基因治疗及细胞信号转导研究等。就近年来噬菌体展示技术在肿瘤相关研究中的运用作以综述。     

Advantages of mRNA display selections over other selection techniques for investigation of protein-protein interactions

mRNA display is a genotype-phenotype conjugation method that allows for amplification-based, iterative rounds of in vitro selection to be applied to peptides and proteins. mRNA display can be used to display both long natural protein and short synthetic peptide libraries with unusually high diversities for the investigation of protein-protein interactions. Here, we summarize the advantages of mRNA display by comparing it with other widely used peptide or protein-selection techniques, and discuss various applications of this technique in studying protein-protein interactions.     

mRNA display: ligand discovery,interaction analysis and beyond

In vitro peptide and protein selection using mRNA display enables the discovery and directed evolution of new molecules from combinatorial libraries. These selected molecules can serve as tools to control and understand biological processes, enhance our understanding of molecular interactions and potentially treat disease in therapeutic applications. In mRNA display, mRNA molecules are covalently attached to the peptide or protein they encode. These mRNA-protein fusions enable in vitro selection of peptide and protein libraries of >10(13) different sequences. mRNA display has been used to discover novel peptide and protein ligands for RNA, small molecules and proteins, as well as to define cellular interaction partners of proteins and drugs. In addition, several unique applications are possible with mRNA display, including self-assembling protein chips and library construction with unnatural amino acids and chemically modified peptides.     

mRNA display for the selection and evolution of enzymes from in vitro-translated protein libraries

The mRNA display technology enables the in vitro selection and directed evolution of functional proteins from libraries of more than 10(12) different mutants in a single test tube. The size of these libraries is well beyond the limit of screening technologies and of most in vivo and in vitro selection methods. The mRNA display technology has been used to select peptides and proteins that bind to a specific ligand, as well as novel enzymes. This protocol details the procedure to produce mRNA-displayed proteins (3 d) and to subject them to a selection and evolution of enzymes for bond-forming reactions (4-10 weeks). This method is demonstrated by the generation of new RNA ligase enzymes.     

Flow cytometric screening of cDNA expression libraries for fluorescent proteins

Fluorescence-activated cell sorting (FACS) was applied for quantitative screening of cDNA expression libraries in bacteria for rare fluorescent protein encoding cDNAs. Rare fluorescent cells, observed at a frequency of 1 in 200,000 bacteria in a cDNA expression library constructed from Astrangia lajollaensis, were detected, enriched, and purified by sorting, yielding three distinct green fluorescent proteins. Two of the isolated fluorescent proteins were found to be 2.5-fold brighter in whole cell fluorescence than the widely used and already optimized EGFP variant and possessed a novel cysteine-containing chromophore. FACS can possess significant advantages in the screening of cDNA libraries in bacteria, since desired genes may occur at low frequencies and possess unexpected properties. This strategy provides a high-throughput, quantitative approach for isolating fluorescent proteins from a more diverse range of organisms and should be extendable to proteins that are not intrinsically fluorescent with the use of available fluorescent indicators.     

Ribosome display: cell-free protein display technology.

Ribosome display is a cell-free system for the in vitro selection of proteins and peptides from large libraries. It uses the principle of coupling individual nascent proteins (phenotypes) to their corresponding mRNA (genotypes), through the formation of stable protein-ribosome-mRNA (PRM) complexes. This permits the simultaneous isolation of a functional nascent protein, through affinity for a ligand, together with the encoding mRNA, which is then converted and amplified as DNA for further manipulation, including repeated cycles or protein expression. Ribosome display has a number of advantages over cell-based systems such as phage display; in particular, it can display very large libraries without the restriction of bacterial transformation. It is also suitable for generating toxic, proteolytically sensitive and unstable proteins, and allows the incorporation of modified amino acids at defined positions. In combination with polymerase chain reaction (PCR)-based methods, mutations can be introduced efficiently into the selected DNA pool in subsequent cycles, leading to continuous DNA diversification and protein selection (in vitro protein evolution). Both prokaryotic and eukaryotic ribosome display systems have been developed and each has its own distinctive features. In this paper, ribosome display systems and their application in selection and evolution of proteins are reviewed.     

A new generation of protein display scaffolds for molecular recognition

Engineered antibodies and their fragments are invaluable tools for a vast range of biotechnological and pharmaceutical applications. However, they are facing increasing competition from a new generation of protein display scaffolds, specifically selected for binding virtually any target. Some of them have already entered clinical trials. Most of these nonimmunoglobulin proteins are involved in natural binding events and have amazingly diverse origins, frameworks, and functions, including even intrinsic enzyme activity. In many respects, they are superior over antibody-derived affinity molecules and offer an ever-extending arsenal of tools for, e.g., affinity purification, protein microarray technology, bioimaging, enzyme inhibition, and potential drug delivery. As excellent supporting frameworks for the presentation of polypeptide libraries, they can be subjected to powerful in vitro or in vivo selection and evolution strategies, enabling the isolation of high-affinity binding reagents. This article reviews the generation of these novel binding reagents, describing validated and advanced alternative scaffolds as well as the most recent nonimmunoglobulin libraries. Characteristics of these protein scaffolds in terms of structural stability, tolerance to multiple substitutions, ease of expression, and subsequent applications as specific targeting molecules are discussed. Furthermore, this review shows the close linkage between these novel protein tools and the constantly developing display, selection, and evolution strategies using phage display, ribosome display, mRNA display, cell surface display, or IVC (in vitro compartmentalization). Here, we predict the important role of these novel binding reagents as a toolkit for biotechnological and biomedical applications.     

Ribosome display: selecting and evolving proteins in vitro that specifically bind to a target

Ribosome display is an in vitro selection and evolution technology for proteins and peptides from large libraries. As it is performed entirely in vitro, there are two main advantages over other selection technologies. First, the diversity of the library is not limited by the transformation efficiency of bacterial cells, but only by the number of ribosomes and different mRNA molecules present in the test tube. Second, random mutations can be introduced easily after each selection round, as no library must be transformed after any diversification step. This allows facile directed evolution of binding proteins over several generations. A prerequisite for the selection of proteins from libraries is the coupling of genotype (RNA, DNA) and phenotype (protein). In ribosome display, this link is accomplished during in vitro translation by stabilizing the complex consisting of the ribosome, the mRNA and the nascent, correctly folded polypeptide. The DNA library coding for a particular library of binding proteins is genetically fused to a spacer sequence lacking a stop codon. This spacer sequence, when translated, is still attached to the peptidyl tRNA and occupies the ribosomal tunnel, and thus allows the protein of interest to protrude out of the ribosome and fold. The ribosomal complexes are allowed to bind to surface-immobilized target. Whereas non-bound complexes are washed away, mRNA of the complexes displaying a binding polypeptide can be recovered, and thus, the genetic information of the binding polypeptides is available for analysis. Here we describe a step-by-step procedure to perform ribosome display selection using an Escherichia coli S30 extract for in vitro translation, based on the work originally described and further refined in our laboratory. A protocol that makes use of eukaryotic in vitro translation systems for ribosome display is also included in this issue.     

Automation of yeast two-hybrid screening

We have developed an automated format for screening yeast two-hybrid libraries for protein-protein interactions. The format consists of a liquid array in which pooled library subsets of yeast, expressing up to 1000 different cDNAs, are mated to a yeast strain of the opposite mating type, expressing a protein of interest. Interactors are detected by a liquid assay for beta-galacsidase following prototrophic selection. The method is demonstrated by the detection of interactions between two encoded yeast RNA polymerase subunits in simulated libraries of varied complexity. To demonstrate its utility for large scale screening of complex cDNA libraries, two nuclear receptor ligand-binding domains were screened through two cDNA libraries arrayed in pooled subsets. Screening these libraries yielded clones which had previously been identified in traditional yeast two hybrid screens, as well as several new putative interacting proteins. The formatting of the cDNA library into pooled subsets lends itself to functional subtraction of the promiscuous positive class of interactor from the library. Also, the liquid arrayed format enables electronic handling of the data derived from interaction screening, which, together with the automated handling of samples, should promote large-scale proteome analysis.