Rapid biosynthesis of glycoprotein therapeutics and vaccines from freeze-dried bacterial cell lysates

Cell-Free Translation Quantification via a Fluorescent Minihelix

Cell-free gene expression systems are used in numerous applications, including medicine making, diagnostics, and educational kits. Accurate quantification of nonfluorescent proteins in these systems remains a challenge. To address this challenge, we report the adaptation and use of an optimized tetra-cysteine minihelix both as a fusion protein and as a standalone reporter with the FlAsH dye. The fluorescent reporter helix is short enough to be encoded on a primer pair to tag any protein of interest via PCR. Both the tagged protein and the standalone reporter can be detected quantitatively in real time or at the end of cell-free expression reactions with standard 96/384-well plate readers, an RT-qPCR system, or gel electrophoresis without the need for staining. The fluorescent signal is stable and correlates linearly with the protein concentration, enabling product quantification. We modified the reporter to study cell-free expression dynamics and engineered ribosome activity. We anticipate that the fluorescent minihelix reporter will facilitate efforts in engineering in vitro transcription and translation systems.

Establishing a Cell-Free Glycoprotein Synthesis System for Enzymatic N-GlcNAcylation

N-linked glycosylation plays a key role in the efficacy of many therapeutic proteins. One limitation to the bacterial glycoengineering of human N-linked glycans is the difficulty of installing a single N-acetylglucosamine (GlcNAc), the reducing end sugar of many human-type glycans, onto asparagine in a single step (N-GlcNAcylation). Here, we develop an in vitro method for N-GlcNAcylating proteins using the oligosaccharyltransferase PglB from Campylobacter jejuni. We use cell-free protein synthesis (CFPS) to test promiscuous PglB variants previously reported in the literature for the ability to produce N-GlcNAc and successfully determine that PglB with an N311V mutation (PglBN311V) exhibits increased GlcNAc transferase activity relative to the wild-type enzyme. We then improve the transfer efficiency by producing CFPS extracts enriched with PglBN311V and further optimize the reaction conditions, achieving a 98.6 ± 0.5% glycosylation efficiency. We anticipate this method will expand the glycoengineering toolbox for therapeutic research and biomanufacturing.

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.

Cell-Free Systems for the Production of Glycoproteins

Cell-free protein synthesis (CFPS), whereby cell lysates are used to produce proteins from a genetic template, has matured as an attractive alternative to standard biomanufacturing modalities due to its high volumetric productivity contained within a distributable platform. Initially, cell-free lysates produced from Escherichia coli, which are both simple to produce and cost-effective for the production of a wide variety of proteins, were unable to produce glycosylated proteins as E. coli lacks native glycosylation machinery. With many important therapeutic proteins possessing asparagine-linked glycans that are critical for structure and function, this gap in CFPS production capabilities was addressed with the development of cell-free expression of glycoproteins (glycoCFE), which uses the supplementation of extracted lipid-linked oligosaccharides and purified oligosaccharyltransferases to enable glycoprotein production in the CFPS reaction environment. In this chapter, we highlight the basic methods for the preparation of reagents for glycoCFE and the protocol for expression and glycosylation of a model protein using a more productive, yet simplified, glycoCFE setup. Beyond this initial protocol, we also highlight how this protocol can be extended to a wide range of alternative glycan structures, oligosaccharyltransferases, and acceptor proteins as well as to a one-pot cell-free glycoprotein synthesis reaction.