A biosynthetic route for polysialylating proteins in Escherichia coli

Advances in bacterial glycoprotein engineering: A critical review of current technologies, emerging challenges, and future directions

Protein glycosylation, which involves the addition of carbohydrate chains to amino acid side chains, imparts essential properties to proteins, offering immense potential in synthetic biology applications. Despite its importance, natural glycosylation pathways present several limitations, highlighting the need for new tools to better understand glycan structures, recognition, metabolism, and biosynthesis, and to facilitate the production of biologically relevant glycoproteins. The field of bacterial glycoengineering has gained significant attention due to the ongoing discovery and study of bacterial glycosylation systems. By utilizing protein glycan coupling technology, a wide range of valuable glycoproteins for clinical and diagnostic purposes have been successfully engineered. This review outlines the recent advances in bacterial protein glycosylation from the perspective of synthetic biology and metabolic engineering, focusing on the development of new glycoprotein therapeutics and vaccines. We provide an overview of the production of high-value, customized glycoproteins using prokaryotic glycosylation platforms, with particular emphasis on four key elements: (i) glycosyltransferases, (ii) carrier proteins, (iii) glycosyl donors, and (iv) host bacteria. Optimization of these elements enables precise control over glycosylation patterns, thus enhancing the potential of the resulting products. Finally, we discuss the challenges and future prospects of leveraging synthetic biology technologies to develop microbial glyco-factories and cell-free systems for efficient glycoprotein production.

Using High-Throughput Experiments To Screen N-Glycosyltransferases with Altered Specificities

The important roles that protein glycosylation plays in modulating the activities and efficacies of protein therapeutics have motivated the development of synthetic glycosylation systems in living bacteria and in vitro. A key challenge is the lack of glycosyltransferases that can efficiently and site-specifically glycosylate desired target proteins without the need to alter primary amino acid sequences at the acceptor site. Here, we report an efficient and systematic method to screen a library of glycosyltransferases capable of modifying comprehensive sets of acceptor peptide sequences in parallel. This approach is enabled by cell-free protein synthesis and mass spectrometry of self-assembled monolayers and is used to engineer a recently discovered prokaryotic N-glycosyltransferase (NGT). We screened 26 pools of site-saturated NGT libraries to identify relevant residues that determine polypeptide specificity and then characterized 122 NGT mutants, using 1052 unique peptides and 52,894 unique reaction conditions. We define a panel of 14 NGTs that can modify 93% of all sequences within the canonical X-1-N-X+1-S/T eukaryotic glycosylation sequences as well as another panel for many noncanonical sequences (with 10 of 17 non-S/T amino acids at the X+2 position). We then successfully applied our panel of NGTs to increase the efficiency of glycosylation for three protein therapeutics. Our work promises to significantly expand the substrates amenable to in vitro and bacterial glycoengineering.

Construction of an Escherichia coli chassis for efficient biosynthesis of human-like N-linked glycoproteins

The production of N-linked glycoproteins in genetically engineered Escherichia coli holds significant potential for reducing costs, streamlining bioprocesses, and enhancing customization. However, the construction of a stable and low-cost microbial cell factory for the efficient production of humanized N-glycosylated recombinant proteins remains a formidable challenge. In this study, we developed a glyco-engineered E. coli chassis to produce N-glycosylated proteins with the human-like glycan Gal-β-1,4-GlcNAc-β-1,3-Gal-β-1,3-GlcNAc-, containing the human glycoform Gal-β-1,4-GlcNAc-β-1,3-. Our initial efforts were to replace various loci in the genome of the E. coli XL1-Blue strain with oligosaccharyltransferase PglB and the glycosyltransferases LsgCDEF to construct the E. coli chassis. In addition, we systematically optimized the promoter regions in the genome to regulate transcription levels. Subsequently, utilizing a plasmid carrying the target protein, we have successfully obtained N-glycosylated proteins with 100% tetrasaccharide modification at a yield of approximately 320 mg/L. Furthermore, we constructed the metabolic pathway for sialylation using a plasmid containing a dual-expression cassette of the target protein and CMP-sialic acid synthesis in the tetrasaccharide chassis cell, resulting in a 40% efficiency of terminal α-2,3- sialylation and a production of 65 mg/L of homogeneously sialylated glycoproteins in flasks. Our findings pave the way for further exploration of producing different linkages (α-2,3/α-2,6/α-2,8) of sialylated human-like N-glycoproteins in the periplasm of the plug-and-play E. coli chassis, laying a strong foundation for industrial-scale production.

Essential factors, advanced strategies, challenges, and approaches involved for efficient expression of recombinant proteins in Escherichia coli

Producing recombinant proteins is a major accomplishment of biotechnology in the past century. Heterologous hosts, either eukaryotic or prokaryotic, are used for the production of these proteins. The utilization of microbial host systems continues to dominate as the most efficient and affordable method for biotherapeutics and food industry productions. Hence, it is crucial to analyze the limitations and advantages of microbial hosts to enhance the efficient production of recombinant proteins on a large scale. E. coli is widely used as a host for the production of recombinant proteins. Researchers have identified certain obstacles with this host, and given the growing demand for recombinant protein production, there is an immediate requirement to enhance this host. The following review discusses the elements contributing to the manifestation of recombinant protein. Subsequently, it sheds light on innovative approaches aimed at improving the expression of recombinant protein. Lastly, it delves into the obstacles and optimization methods associated with translation, mentioning both cis-optimization and trans-optimization, producing soluble recombinant protein, and engineering the metal ion transportation. In this context, a comprehensive description of the distinct features will be provided, and this knowledge could potentially enhance the expression of recombinant proteins in E. coli.

N-Glycan Engineering: Constructing the N-GlcNAc Stump

N-Glycosylation is often essential for the structure and function of proteins. However, N-glycosylated proteins from natural sources exhibit considerable heterogeneity in the appended oligosaccharides, bringing daunting challenges to corresponding basic research and therapeutic applications. To address this issue, various synthetic, enzymatic, and chemoenzymatic approaches have been elegantly designed. Utilizing the endoglycosidase-catalyzed transglycosylation method, a single N-acetylglucosamine (N-GlcNAc, analogous to a tree stump) on proteins can be converted to various homogeneous N-glycosylated forms, thereby becoming the focus of research efforts. In this concept article, we briefly introduce the methods that allow the generation of N-GlcNAc and its close analogues on proteins and peptides and highlight the current challenges and opportunities the scientific community is facing.

Strategies for efficient production of recombinant proteins in Escherichia coli: alleviating the host burden and enhancing protein activity

Escherichia coli, one of the most efficient expression hosts for recombinant proteins (RPs), is widely used in chemical, medical, food and other industries. However, conventional expression strains are unable to effectively express proteins with complex structures or toxicity. The key to solving this problem is to alleviate the host burden associated with protein overproduction and to enhance the ability to accurately fold and modify RPs at high expression levels. Here, we summarize the recently developed optimization strategies for the high-level production of RPs from the two aspects of host burden and protein activity. The aim is to maximize the ability of researchers to quickly select an appropriate optimization strategy for improving the production of RPs.

Protein Cages: From Fundamentals to Advanced Applications

Proteins that self-assemble into polyhedral shell-like structures are useful molecular containers both in nature and in the laboratory. Here we review efforts to repurpose diverse protein cages, including viral capsids, ferritins, bacterial microcompartments, and designed capsules, as vaccines, drug delivery vehicles, targeted imaging agents, nanoreactors, templates for controlled materials synthesis, building blocks for higher-order architectures, and more. A deep understanding of the principles underlying the construction, function, and evolution of natural systems has been key to tailoring selective cargo encapsulation and interactions with both biological systems and synthetic materials through protein engineering and directed evolution. The ability to adapt and design increasingly sophisticated capsid structures and functions stands to benefit the fields of catalysis, materials science, and medicine.

Production of microhomogeneous glycopeptide by a mutated NGT according FuncLib with unique sugar as substrate

N-glycosylation is one of the most important post-translational modifications of proteins. Cytoplasmic soluble N-glycosyltransferase (NGT) exists in bacteria, which is able to transfer different monosaccharide from sugar nucleotide to the NXS/T(X ≠ Pro) consensus sequence in a polypeptide. At present, the NGT enzymes reported could transfer a variety of different sugars to protein, which will lead to the heterogeneity of the sugar chain and the complexity and instability of the structure and function of glycopeptides. According to the FuncLib algorithm, we obtained mutant ApNGT-P1 from ApNGT (the NGT from Actinobacillus pleuropneumoniae) with increased substrate specificity. Compared with the wild-type ApNGT, mutant ApNGT-P1 could only utilize UDP-Glc as sugar donors. The optimum temperature of ApNGT-P1 was about 40 °C and the optimum pH was 7.5-8.0 in PBS buffer. ApNGT-P1 exhibited better tolerance for K⁺, Mn²⁺, Ca²⁺, and Mg²⁺, but was strongly inhibited by Na⁺, Cu²⁺ and Zn²⁺. The mutant can be applied to the efficient production of glycosylated peptides or proteins with uniform glucose at their glycosylation sites. Besides, this work provided a feasible pathway for further studies on the improving donor substrates selectivity of NGTs.

Cellular and Molecular Engineering of Glycan Sialylation in Heterologous Systems

Glycans have been shown to play a key role in many biological processes, such as signal transduction, immunogenicity, and disease progression. Among the various glycosylation modifications found on cell surfaces and in biomolecules, sialylation is especially important, because sialic acids are typically found at the terminus of glycans and have unique negatively charged moieties associated with cellular and molecular interactions. Sialic acids are also crucial for glycosylated biopharmaceutics, where they promote stability and activity. In this regard, heterogenous sialylation may produce variability in efficacy and limit therapeutic applications. Homogenous sialylation may be achieved through cellular and molecular engineering, both of which have gained traction in recent years. In this paper, we describe the engineering of intracellular glycosylation pathways through targeted disruption and the introduction of carbohydrate active enzyme genes. The focus of this review is on sialic acid-related genes and efforts to achieve homogenous, humanlike sialylation in model hosts. We also discuss the molecular engineering of sialyltransferases and their application in chemoenzymatic sialylation and sialic acid visualization on cell surfaces. The integration of these complementary engineering strategies will be useful for glycoscience to explore the biological significance of sialic acids on cell surfaces as well as the future development of advanced biopharmaceuticals.

Building better biobetters: From fundamentals to industrial application

Biological drugs or biopharmaceuticals off patent open a large market for biosimilars and biobetters, follow-on biologics. Biobetters, in particular, are new drugs designed from existing ones with improved properties such as higher selectivity, stability, half-life and/or lower toxicity/immunogenicity. Glycosylation is one of the most used strategies to improve biological drugs, nonetheless bioconjugation is an additional alternative and refers to the covalent attachment of polymers to biological drugs. Extensive research on novel polymers is underway, nonetheless PEGylation is still the best alternative with the longest clinical track record. Innovative trends based on genetic engineering techniques such as fusion proteins and PASylation are also promising. In this review, all these alternatives wereexplored as well as current market trends, legislation and future perspectives.

Exploring a combined Escherichia coli-based glycosylation and in vitro transglycosylation approach for expression of glycosylated interferon alpha

The conventional use of E. coli system for protein expression is limited to non-glycosylated proteins. While yeast, insect and mammalian systems are available to produce heterologous glycoproteins, developing an engineered E. coli-based glycosylation platform will provide a faster, more economical, and more convenient alternative. In this work, we present a two-step approach for production of a homogeneously glycosylated eukaryotic protein using the E. coli expression system. Human interferon α-2b (IFNα) is used as a model protein to illustrate this glycosylation scheme. In the first step, the N-glycosyltransferase from Actinobacillus pleuropneumoniae (ApNGT) is co-expressed for in vivo transfer of a glucose residue to IFNα at an NX(S/T) N-glycosylation sequon. Several E. coli systems were examined to evaluate the efficiency of IFNα N-glucosylation. In the second step, the N-glucosylated protein is efficiently elaborated with biantennary sialylated complex-type N-glycan using an in vitro chemoenzymatic method. The N-glycosylated IFNα product was found to be biologically active and displayed significantly improved proteolytic stability. This work presents a feasible E. coli-based glycosylation machinery for producing therapeutic eukaryotic glycoproteins.

Cell-free systems for accelerating glycoprotein expression and biomanufacturing

Protein glycosylation, the enzymatic modification of amino acid sidechains with sugar moieties, plays critical roles in cellular function, human health, and biotechnology. However, studying and producing defined glycoproteins remains challenging. Cell-free glycoprotein synthesis systems, in which protein synthesis and glycosylation are performed in crude cell extracts, offer new approaches to address these challenges. Here, we review versatile, state-of-the-art systems for biomanufacturing glycoproteins in prokaryotic and eukaryotic cell-free systems with natural and synthetic N-linked glycosylation pathways. We discuss existing challenges and future opportunities in the use of cell-free systems for the design, manufacture, and study of glycoprotein biomedicines.

Cell-Free Synthetic Glycobiology: Designing and Engineering Glycomolecules Outside of Living Cells

Glycans and glycosylated biomolecules are directly involved in almost every biological process as well as the etiology of most major diseases. Hence, glycoscience knowledge is essential to efforts aimed at addressing fundamental challenges in understanding and improving human health, protecting the environment and enhancing energy security, and developing renewable and sustainable resources that can serve as the source of next-generation materials. While much progress has been made, there remains an urgent need for new tools that can overexpress structurally uniform glycans and glycoconjugates in the quantities needed for characterization and that can be used to mechanistically dissect the enzymatic reactions and multi-enzyme assembly lines that promote their construction. To address this technology gap, cell-free synthetic glycobiology has emerged as a simplified and highly modular framework to investigate, prototype, and engineer pathways for glycan biosynthesis and biomolecule glycosylation outside the confines of living cells. From nucleotide sugars to complex glycoproteins, we summarize here recent efforts that harness the power of cell-free approaches to design, build, test, and utilize glyco-enzyme reaction networks that produce desired glycomolecules in a predictable and controllable manner. We also highlight novel cell-free methods for shedding light on poorly understood aspects of diverse glycosylation processes and engineering these processes toward desired outcomes. Taken together, cell-free synthetic glycobiology represents a promising set of tools and techniques for accelerating basic glycoscience research (e.g., deciphering the "glycan code") and its application (e.g., biomanufacturing high-value glycomolecules on demand).

Synthetic Glycobiology: Parts, Systems, and Applications

Protein glycosylation, the attachment of sugars to amino acid side chains, can endow proteins with a wide variety of properties of great interest to the engineering biology community. However, natural glycosylation systems are limited in the diversity of glycoproteins they can synthesize, the scale at which they can be harnessed for biotechnology, and the homogeneity of glycoprotein structures they can produce. Here we provide an overview of the emerging field of synthetic glycobiology, the application of synthetic biology tools and design principles to better understand and engineer glycosylation. Specifically, we focus on how the biosynthetic and analytical tools of synthetic biology have been used to redesign glycosylation systems to obtain defined glycosylation structures on proteins for diverse applications in medicine, materials, and diagnostics. We review the key biological parts available to synthetic biologists interested in engineering glycoproteins to solve compelling problems in glycoscience, describe recent efforts to construct synthetic glycoprotein synthesis systems, and outline exemplary applications as well as new opportunities in this emerging space.

An Engineered Pathway for Production of Terminally Sialylated N-glycoproteins in the Periplasm of Escherichia coli

Terminally sialylated N-glycoproteins are of great interest in therapeutic applications. Due to the inability of prokaryotes to carry out this post-translational modification, they are currently predominantly produced in eukaryotic host cells. In this study, we report a synthetic pathway to produce a terminally sialylated N-glycoprotein in the periplasm of Escherichia coli, mimicking the sialylated moiety (Neu5Ac-α-2,6-Gal-β-1,4-GlcNAc-) of human glycans. A sialylated pentasaccharide, Neu5Ac-α-2,6-Gal-β-1,4-GlcNAc-β-1,3-Gal-β-1,3-GlcNAc-, was synthesized through the activity of co-expressed glycosyltransferases LsgCDEF from Haemophilus influenzae, Campylobacter jejuni NeuBCA enzymes, and Photobacterium leiognathi α-2,6-sialyltransferase in an engineered E. coli strain which produces CMP-Neu5Ac. C. jejuni oligosaccharyltransferase PglB was used to transfer the terminally sialylated glycan onto a glyco-recognition sequence in the tenth type III cell adhesion module of human fibronectin. Sialylation of the target protein was confirmed by lectin blotting and mass spectrometry. This proof-of-concept study demonstrates the successful production of terminally sialylated, homogeneous N-glycoproteins with α-2,6-linkages in the periplasm of E. coli and will facilitate the construction of E. coli strains capable of producing terminally sialylated N-glycoproteins in high yield.

Sequential Glycosylation of Proteins with Substrate-Specific N-Glycosyltransferases

Protein glycosylation is a common post-translational modification that influences the functions and properties of proteins. Despite advances in methods to produce defined glycoproteins by chemoenzymatic elaboration of monosaccharides, the understanding and engineering of glycoproteins remain challenging, in part, due to the difficulty of site-specifically controlling glycosylation at each of several positions within a protein. Here, we address this limitation by discovering and exploiting the unique, conditionally orthogonal peptide acceptor specificities of N-glycosyltransferases (NGTs). We used cell-free protein synthesis and mass spectrometry of self-assembled monolayers to rapidly screen 41 putative NGTs and rigorously characterize the unique acceptor sequence preferences of four NGT variants using 1254 acceptor peptides and 8306 reaction conditions. We then used the optimized NGT-acceptor sequence pairs to sequentially install monosaccharides at four sites within one target protein. This strategy to site-specifically control the installation of N-linked monosaccharides for elaboration to a variety of functional N-glycans overcomes a major limitation in synthesizing defined glycoproteins for research and therapeutic applications.

Glycoengineering bioconjugate vaccines, therapeutics, and diagnostics in E. coli

The first, general glycosylation pathway in bacteria, the N-linked glycosylation system of Campylobacter jejuni, was discovered two decades ago. Since then, many diverse prokaryotic glycosylation systems have been characterized, including O-linked glycosylation systems that have no homologous counterparts in eukaryotic organisms. Shortly after these discoveries, glycosylation pathways were recombinantly introduced into E. coli creating the field of bacterial glycoengineering. Bacterial glycoengineering is an emerging biotechnological tool that harnesses prokaryotic glycosylation systems for the generation of recombinantly glycosylated proteins using E. coli as a host. Over the last decade, as our understanding of prokaryotic glycosylation systems has advanced, so too has the glycoengineering toolbox. Currently, glycoengineering utilizes two broad approaches to recombinantly glycosylate proteins, both of which can generate N- or O-linkages: oligosaccharyltransferase (OTase)-dependent and OTase-independent. This review discusses the applications of these bacterial glycoengineering techniques as they relate to the development of glycoconjugate vaccines, therapeutic proteins, and diagnostics.

High-Throughput Synthesis and Analysis of Intact Glycoproteins Using SAMDI-MS

High-throughput quantification of the post-translational modification of many individual protein samples is challenging with current label-based methods. This paper demonstrates an efficient method that addresses this gap by combining Escherichia coli-based cell-free protein synthesis (CFPS) and self-assembled monolayers for matrix-assisted laser desorption/ionization mass spectrometry (SAMDI-MS) to analyze intact proteins. This high-throughput approach begins with polyhistidine-tagged protein substrates expressed from linear DNA templates by CFPS. Here, we synthesized an 87-member library of the E. coli Immunity Protein 7 (Im7) containing an acceptor sequence optimized for glycosylation by the Actinobacillus pleuropneumoniae N-glycosyltransferase (NGT) at every possible position along the protein backbone. These protein substrates were individually treated with NGT and then selectively immobilized to self-assembled monolayers presenting nickel-nitrilotriacetic acid (Ni-NTA) complexes before final analysis by SAMDI-MS to quantify the conversion of substrate to glycoprotein. This method offers new opportunities for rapid synthesis and quantitative evaluation of intact glycoproteins.

A cell-free biosynthesis platform for modular construction of protein glycosylation pathways

Glycosylation plays important roles in cellular function and endows protein therapeutics with beneficial properties. However, constructing biosynthetic pathways to study and engineer precise glycan structures on proteins remains a bottleneck. Here, we report a modular, versatile cell-free platform for glycosylation pathway assembly by rapid in vitro mixing and expression (GlycoPRIME). In GlycoPRIME, glycosylation pathways are assembled by mixing-and-matching cell-free synthesized glycosyltransferases that can elaborate a glucose primer installed onto protein targets by an N-glycosyltransferase. We demonstrate GlycoPRIME by constructing 37 putative protein glycosylation pathways, creating 23 unique glycan motifs, 18 of which have not yet been synthesized on proteins. We use selected pathways to synthesize a protein vaccine candidate with an α-galactose adjuvant motif in a one-pot cell-free system and human antibody constant regions with minimal sialic acid motifs in glycoengineered Escherichia coli. We anticipate that these methods and pathways will facilitate glycoscience and make possible new glycoengineering applications.

Constructing biosynthetic pathways to study and engineer glycoprotein structures is difficult. Here, the authors use GlycoPRIME, a cell-free workflow for mixing-and-matching glycosylation enzymes, to evaluate 37 putative glycosylation pathways and discover routes to 18 new glycoprotein structures

Cytoplasmic glycoengineering enables biosynthesis of nanoscale glycoprotein assemblies

Glycosylation of proteins profoundly impacts their physical and biological properties. Yet our ability to engineer novel glycoprotein structures remains limited. Established bacterial glycoengineering platforms require secretion of the acceptor protein to the periplasmic space and preassembly of the oligosaccharide substrate as a lipid-linked precursor, limiting access to protein and glycan substrates respectively. Here, we circumvent these bottlenecks by developing a facile glycoengineering platform that operates in the bacterial cytoplasm. The Glycoli platform leverages a recently discovered site-specific polypeptide glycosyltransferase together with variable glycosyltransferase modules to synthesize defined glycans, of bacterial or mammalian origin, directly onto recombinant proteins in the E. coli cytoplasm. We exploit the cytoplasmic localization of this glycoengineering platform to generate a variety of multivalent glycostructures, including self-assembling nanomaterials bearing hundreds of copies of the glycan epitope. This work establishes cytoplasmic glycoengineering as a powerful platform for producing glycoprotein structures with diverse future biomedical applications.

Established bacterial glycoengineering platforms limit access to protein and glycan substrates. Here the authors design a cytoplasmic protein glycosylation system, Glycoli, to generate a variety of multivalent glycostructures.

Current strategies in extending half-lives of therapeutic proteins

Macromolecular protein and peptide therapeutics have been proven to be effective in treating critical human diseases precisely. Thanks to biotechnological advancement, a huge number of proteins and peptide therapeutics were made their way to pharmaceutical market in past few decades. However, one of the biggest challenges to be addressed for protein therapeutics during clinical application is their fast degradation in serum and quick elimination owing to enzymatic degradation, renal clearance, liver metabolism and immunogenicity, attributing to the short half-lives. Size and hydrophobicity of protein molecules make them prone to kidney filtration and liver metabolism. On the other hand, proteasomes responsible for protein destruction possess the capability of specifically recognizing almost all kinds of foreign proteins while avoiding any unwanted destruction of cellular components. At present almost all protein-based drug formulations available in market are administered intravenously (IV) or subcutaneously (SC) with high dosing at frequent interval, eventually creating dose-fluctuation-related complications and reducing patient compliance vastly. Therefore, artificially increasing the therapeutic half-life of a protein by attaching to it a molecule that increases the overall size (eg, PEG) or helps with receptor mediated recycling (eg, albumin), or manipulating amino acid chain in a way that makes it more prone towards aggregate formation, are some of the revolutionary approaches to avoid the fast degradation in vivo. Half-life extension technologies that are capable of dramatically enhancing half-lives of proteins in circulation (2-100 folds) and thus improving their overall pharmacokinetic (PK) parameters have been successfully applied on a wide range of protein therapeutics from hormones and enzymes, growth factor, clotting factor to interferon. The focus of the review is to assess the technological advancements made so far in enhancing circulatory half-lives and improving therapeutic potency of proteins.

Metabolic engineering of glycoprotein biosynthesis in bacteria

The demonstration more than a decade ago that glycoproteins could be produced in Escherichia coli cells equipped with the N-linked protein glycosylation machinery from Campylobacter jejuni opened the door to using simple bacteria for the expression and engineering of complex glycoproteins. Since that time, metabolic engineering has played an increasingly important role in developing and optimizing microbial cell glyco-factories for the production of diverse glycoproteins and other glycoconjugates. It is becoming clear that future progress in creating efficient glycoprotein expression platforms in bacteria will depend on the adoption of advanced strain engineering strategies such as rational design and assembly of orthogonal glycosylation pathways, genome-wide identification of metabolic engineering targets, and evolutionary engineering of pathway performance. Here, we highlight recent advances in the deployment of metabolic engineering tools and strategies to develop microbial cell glyco-factories for the production of high-value glycoprotein targets with applications in research and medicine.

Design of glycosylation sites by rapid synthesis and analysis of glycosyltransferases

Glycosylation is an abundant post-translational modification that is important in disease and biotechnology. Current methods to understand and engineer glycosylation cannot sufficiently explore the vast experimental landscapes required to accurately predict and design glycosylation sites modified by glycosyltransferases. Here we describe a systematic platform for glycosylation sequence characterization and optimization by rapid expression and screening (GlycoSCORES), which combines cell-free protein synthesis and mass spectrometry of self-assembled monolayers. We produced six N- and O-linked polypeptide-modifying glycosyltransferases from bacteria and humans in vitro and rigorously determined their substrate specificities using 3,480 unique peptides and 13,903 unique reaction conditions. We then used GlycoSCORES to optimize and design small glycosylation sequence motifs that directed efficient N-linked glycosylation in vitro and in the Escherichia coli cytoplasm for three heterologous proteins, including the human immunoglobulin Fc domain. We find that GlycoSCORES is a broadly applicable method to facilitate fundamental understanding of glycosyltransferases and engineer synthetic glycoproteins.

Genetic engineering of host organisms for pharmaceutical synthesis

Pharmaceutical production hosts may be derived from almost any organism, from Chinese Hamster Ovary (CHO) cell lines to isolated actinomycetes. Each host can be improved, historically only through adaptive evolution. Recently, the maturation of organism engineering has expanded the available models, methods, and tools for altering host phenotypes. New tools like CRISPR-associated endonucleases promise to enable precise cellular reprogramming and to access previously intractable hosts. In this review, we discuss the most recent advances in engineering several types of pharmaceutical production hosts. These include model organisms, potential platform hosts with advantageous metabolism or physiology, specialized producers capable of unique biosynthesis, and CHO, the most widely used recombinant protein production host. To realize improved engineered hosts, an increasing number of approaches involving DNA sequencing and synthesis, host rewriting technologies, computational methods, and organism engineering strategies must be used. Integrative workflows that enable application of the right combination of methods to the right production host could enable economical production solutions for emerging human health treatments.