High-throughput profiling of sequence recognition by tyrosine kinases and SH2 domains using bacterial peptide display.

Tyrosine kinases and SH2 (phosphotyrosine recognition) domains have binding specificities that depend on the amino acid sequence surrounding the target (phospho)tyrosine residue. Although the preferred recognition motifs of many kinases and SH2 domains are known, we lack a quantitative description of sequence specificity that could guide predictions about signaling pathways or be used to design sequences for biomedical applications. Here, we present a platform that combines genetically encoded peptide libraries and deep sequencing to profile sequence recognition by tyrosine kinases and SH2 domains. We screened several tyrosine kinases against a million-peptide random library and used the resulting profiles to design high-activity sequences. We also screened several kinases against a library containing thousands of human proteome-derived peptides and their naturally-occurring variants. These screens recapitulated independently measured phosphorylation rates and revealed hundreds of phosphosite-proximal mutations that impact phosphosite recognition by tyrosine kinases. We extended this platform to the analysis of SH2 domains and showed that screens could predict relative binding affinities. Finally, we expanded our method to assess the impact of non-canonical and post-translationally modified amino acids on sequence recognition. This specificity profiling platform will shed new light on phosphotyrosine signaling and could readily be adapted to other protein modification/recognition domains.

Exploring Performance Parameters of Artificial Allosteric Protein Switches.

Biological information processing networks rely on allosteric protein switches that dynamically interconvert biological signals. Construction of their artificial analogues is a central goal of synthetic biology and bioengineering. Receptor domain insertion is one of the leading methods for constructing chimeric protein switches. Here we present an in vitro expression-based platform for the analysis of chimeric protein libraries for which traditional cell survival or cytometric high throughput assays are not applicable. We utilise this platform to screen a focused library of chimeras between PQQ-glucose dehydrogenase and calmodulin. Using this approach, we identified 50 chimeras (approximately 23% of the library) that were activated by calmodulin-binding peptides. We analysed performance parameters of the active chimeras and demonstrated that their dynamic range and response times are anticorrelated, pointing to the existence of an inherent thermodynamic trade-off. We show that the structure of the ligand peptide affects both the response and activation kinetics of the biosensors suggesting that the structure of a ligand:receptor complex can influence the chimera's activation pathway. In order to understand the extent of structural changes in the reporter protein induced by the receptor domains, we have analysed one of the chimeric molecules by CD spectroscopy and hydrogen-deuterium exchange mass spectrometry. We concluded that subtle ligand-induced changes in the receptor domain propagated into the GDH domain and affected residues important for substrate and cofactor binding. Finally, we used one of the identified chimeras to construct a two-component rapamycin biosensor and demonstrated that core switch optimisation translated into improved biosensor performance.

Mapping Interactions among Cell-Free Expressed Zika Virus Proteins.

The rapid spread of arthropod-borne Zika virus poses a serious public health threat that calls for effective ways of controlling and treating viral infection. This in turn necessitates better understanding of the mechanisms of virus assembly and its interaction with the host cells. In order to facilitate such efforts, we developed a new multihost expression vector pmCellFree that allows rapid and multiplexed production of ZIKV proteins in any in vitro translation system as well as in mammalian cells. Using a combination of in vitro expression in Leishmania cell-free system and AlphaLISA interaction assay, pairwise protein interactions of all ZIKV proteins were systematically tested. We identified thirty-three intraviral binary protein interactions, of which 13 interactions are novel. These findings were further validated by expressing selected protein pairs in mammalian HEK293T cell line and assessing their interactions in the cellular lysate. The results of these interaction assays were identical to those obtained with in vitro expressed proteins. The observed novel protein-protein interactions were further validated using a pulldown assay. The unrevealed novel protein interactions may point to the previously unappreciated complexity of the ZIKV assembly process and may play an important role in the infection process. These interactions may represent new targets for antiviral drug development.

Cell-free pipeline for discovery of thermotolerant xylanases and endo-1,4-ß-glucanases.

The rapid expansion in the number of sequenced genomes and metagenomes provides an exceptional resource for mining of the enzymes with biotechnologically relevant properties. However, the majority of protein production and analysis methods are not sufficiently cost-efficient and scalable to experimentally verify the results of computational genomic mining. Here, we present a pipeline based on Leishmania tarentolae cell-free system that was used to characterize 30 putative thermostable endo-1,4-ß-glucanases and xylanases identified in public genomic databases. In order to analyse the recombinant proteins without purification, novel high-throughput assays for glucanase and xylanase activities were developed. The assays rely on solubilisation of labelled particulate substrates performed in multiwell plates. Using this approach both acidophilic and thermophilic enzymes were identified. The developed approach enables rapid discovery of new biotechnologically useful enzymes.

Performance benchmarking of four cell-free protein expression systems.

Over the last half century, a range of cell-free protein expression systems based on pro- and eukaryotic organisms have been developed and have found a range of applications, from structural biology to directed protein evolution. While it is generally accepted that significant differences in performance among systems exist, there is a paucity of systematic experimental studies supporting this notion. Here, we took advantage of the species-independent translation initiation sequence to express and characterize 87 N-terminally GFP-tagged human cytosolic proteins of different sizes in E. coli, wheat germ (WGE), HeLa, and Leishmania-based (LTE) cell-free systems. Using a combination of single-molecule fluorescence spectroscopy, SDS-PAGE, and Western blot analysis, we assessed the expression yields, the fraction of full-length translation product, and aggregation propensity for each of these systems. Our results demonstrate that the E. coli system has the highest expression yields. However, we observe that high expression levels are accompanied by production of truncated species-particularly pronounced in the case of proteins larger than 70 kDa. Furthermore, proteins produced in the E. coli system display high aggregation propensity, with only 10% of tested proteins being produced in predominantly monodispersed form. The WGE system was the most productive among eukaryotic systems tested. Finally, HeLa and LTE show comparable protein yields that are considerably lower than the ones achieved in the E. coli and WGE systems. The protein products produced in the HeLa system display slightly higher integrity, whereas the LTE-produced proteins have the lowest aggregation propensity among the systems analyzed. The high quality of HeLa- and LTE-produced proteins enable their analysis without purification and make them suitable for analysis of multi-domain eukaryotic proteins.

Gateway-compatible vectors for high-throughput protein expression in pro- and eukaryotic cell-free systems.

Although numerous techniques for protein expression and production are available the pace of genome sequencing outstrips our ability to analyze the encoded proteins. To address this bottleneck, we have established a system for parallelized cloning, DNA production and cell-free expression of large numbers of proteins. This system is based on a suite of pCellFree Gateway destination vectors that utilize a Species Independent Translation Initiation Sequence (SITS) that mediates recombinant protein expression in any in vitro translation system. These vectors introduce C or N terminal EGFP and mCherry fluorescent and affinity tags, enabling direct analysis and purification of the expressed proteins. To maximize throughput and minimize the cost of protein production we combined Gateway cloning with Rolling Circle DNA Amplification. We demonstrate that as little as 0.1 ng of plasmid DNA is sufficient for template amplification and production of recombinant human protein in Leishmania tarentolae and Escherichia coli cell-free expression systems. Our experiments indicate that this approach can be applied to large gene libraries as it can be reliably performed in multi-well plates. The resulting protein expression pipeline provides a valuable new tool for applications of the post genomic era.

Generation and characterization of a Leishmania tarentolae strain for site-directed in vivo biotinylation of recombinant proteins.

Leishmania tarentolae is a non-human-pathogenic Leishmania species of growing interest in biotechnology, as it is well-suited for the expression of human recombinant proteins. For many applications it is desirable to express recombinant proteins with a tag allowing easy purification and detection. Hence, we adopted a scheme to express recombinant proteins with a His6-tag and, additionally, to site-specifically in vivo biotinylate them for detection. Biotinylation is a relatively rare modification of endogenous proteins that allows easy detection with negligible cross-reactivity. Here, we established a genetically engineered L. tarentolae strain constitutively expressing the codon-optimized biotin-protein ligase from Escherichia coli (BirA). We thoroughly analyzed the strain for functionality using 2-D polyacrylamide-gel electrophoresis (PAGE), mass spectrometry, and transmission electron microscopy (TEM). We could demonstrate that neither metabolic changes (growth rate) nor structural abnormalities (TEM) occurred. To our knowledge, we show the first 2-D PAGE analyses of L. tarentolae. Our results demonstrate the great benefit of the established L. tarentolae in vivo biotinylation strain for production of dual-tagged recombinant proteins. Additionally, 2-D PAGE and TEM results give insights into the biology of L. tarentolae, helping to better understand Leishmania species. Finally, we envisage that the system is transferable to human-pathogenic species.