We performed bottom-up engineering of a synthetic pathway in E. coli for the production of eukaryotic trimannosyl chitobiose glycans and the transfer of these glycans to specific asparagine residues in target proteins. Glycan biosynthesis was enabled by four eukaryotic glycosyltransferases, including the yeast uridine diphosphate-N-acetylglucosamine transferases Alg13 and Alg14 and the mannosyltransferases Alg1 and Alg2. By including the bacterial oligosaccharyltransferase PglB from C. jejuni, glycans were successfully transferred to eukaryotic proteins.

N-Linked glycosylation is a frequent protein modification that occurs in all three domains of life. This process involves the transfer of a preassembled oligosaccharide from a lipid donor to asparagine side chains of polypeptides and is catalyzed by the membrane-bound oligosaccharyltransferase (OST). We characterized an alternative bacterial pathway wherein a cytoplasmic N-glycosyltransferase uses nucleotide-activated monosaccharides as donors to modify asparagine residues of peptides and proteins. N-Glycosyltransferase is an inverting glycosyltransferase and recognizes the NX(S/T) consensus sequence. It therefore exhibits similar acceptor site specificity as eukaryotic OST, despite the unrelated predicted structural architecture and the apparently different catalytic mechanism. The identification of an enzyme that integrates some of the features of OST in a cytoplasmic pathway defines a novel class of N-linked protein glycosylation found in pathogenic bacteria.N-Linked glycosylation is characterized by an N-glycosidic linkage between the side chain amide of an asparagine residue of proteins and an oligosaccharide. This type of glycosylation occurs in both prokaryotes and eukaryotes and requires the assembly of an oligosaccharide on a polyisoprenoid lipid by sequential addition of monosaccharides, catalyzed by cytosolic glycosyltransferase. The resulting lipid-linked oligosaccharide is translocated to the luminal side of the endoplasmic reticulum membrane or the plasma membrane of prokaryotes, where it may be further elongated. The glycan is then transferred to the ␦-amino group of asparagine residues within the consensus sequence NX(S/T) of polypeptides. This reaction is catalyzed by oligosaccharyltransferase (OST), 4 a membrane-bound enzyme that can be composed of several different subunits (1). As oligosaccharide transfer takes place in the endoplasmic reticulum or in the periplasm, N-glycosylation affects proteins trafficking along the secretory pathway.N-Glycosylation exhibits important physiological functions. In the early secretory pathway of eukaryotes, N-glycans present on newly synthesized proteins orchestrate the folding of glycoproteins and act as a signal for directing misfolded polypeptides to degradation (2). After being processed in the Golgi organelle, N-linked glycans are relevant, among other processes, for the modulation of the immune system and for the control of immune cell homeostasis and inflammation (3, 4). The importance of this protein modification is supported by its incidence: more than half of all eukaryotic proteins are glycosylated (5).About 8 years ago, a study uncovered that the extracellular HMW1A adhesin of the Gram-negative bacterium Haemophilus influenzae is modified with hexose monosaccharides on asparagine residues (6). The pioneering work of St Geme and co-workers (7, 8) subsequently showed that the modified asparagine residues are found within the same consensus sequence recognized by OST (i.e. NX(S/T)) and that the enzyme responsible for this modification is the HMW1C p...

(2016) Utility of a human FcRn transgenic mouse model in drug discovery for early assessment and prediction of human pharmacokinetics of monoclonal antibodies, mAbs, 8:6, 1064-1078, DOI: 10.1080/19420862.2016 To link to this article: https://doi.org/10. 1080/19420862.2016 ABSTRACTTherapeutic antibodies continue to develop as an emerging drug class, with a need for preclinical tools to better predict in vivo characteristics. Transgenic mice expressing human neonatal Fc receptor (hFcRn) have potential as a preclinical pharmacokinetic (PK) model to project human PK of monoclonal antibodies (mAbs). Using a panel of 27 mAbs with a broad PK range, we sought to characterize and establish utility of this preclinical animal model and provide guidance for its application in drug development of mAbs. This set of mAbs was administered to both hemizygous and homozygous hFcRn transgenic mice (Tg32) at a single intravenous dose, and PK parameters were derived. Higher hFcRn protein tissue expression was confirmed by liquid chromatography-high resolution tandem mass spectrometry in Tg32 homozygous versus hemizygous mice. Clearance (CL) was calculated using non-compartmental analysis and correlations were assessed to historical data in wild-type mouse, non-human primate (NHP), and human. Results show that mAb CL in hFcRn Tg32 homozygous mouse correlate with human (r 2 D 0.83, r D 0.91, p < 0.01) better than NHP (r 2 D 0.67, r D 0.82, p < 0.01) for this dataset. Applying simple allometric scaling using an empirically derived best-fit exponent of 0.93 enabled the prediction of human CL from the Tg32 homozygous mouse within 2-fold error for 100% of mAbs tested. Implementing the Tg32 homozygous mouse model in discovery and preclinical drug development to predict human CL may result in an overall decreased usage of monkeys for PK studies, enhancement of the early selection of lead molecules, and ultimately a decrease in the time for a drug candidate to reach the clinic.

Background: Actinobacillus pleuropneumoniae N-glycosyltransferase is a cytoplasmic glycosyltransferase catalyzing N-glycosylation of polypeptides. Results: In depth analysis of a reconstituted A. pleuropneumoniae glycosylation system in Escherichia coli showed a surprisingly relaxed peptide substrate specificity of N-glycosyltransferase. Conclusion: N-Glycosyltransferase constitutes a general glycosylation system with a preference for autotransporters. Significance: Our study could provide the basis for a novel route for the engineering of N-glycoproteins in bacteria.

A number of proteobacteria carry the genetic information to perform N-linked glycosylation, but only the protein glycosylation (pgl) pathway of Campylobacter jejuni has been studied to date. Here, we report that the pgl gene cluster of Campylobacter lari encodes for a functional glycosylation machinery that can be reconstituted in Escherichia coli. We determined that the N-glycan produced in this system consisted of a linear hexasaccharide. We found that the oligosaccharyltransferase (OST) of C. lari conserved a predominant specificity for the primary sequence D/E-X(-1)-N-X(+1)-S/T (where X(-1) and X(+1) can be any amino acid but proline). At the same time, we observed that this enzyme exhibited a relaxed specificity toward the acceptor site and modified asparagine residues of a protein at sequences DANSG and NNNST. Moreover, C. lari pgl glycosylated a native E. coli protein. Bacterial N-glycosylation appears as a useful tool to establish a molecular description of how single-subunit OSTs perform selection of glycosyl acceptor sites.

The initial glycan transfer in asparagine-linked protein glycosylation is catalysed by the integral membrane enzyme oligosaccharyltransferase (OST). Here we study the mechanism of the bacterial PglB protein, a single-subunit OST, using chemically synthesized acceptor peptide analogues. We find that PglB can glycosylate not only asparagine but also glutamine, homoserine and the hydroxamate Asp(NHOH), although at much lower rates. In contrast, N-methylated asparagine or 2,4-diaminobutanoic acid (Dab) are not glycosylated. We find that of the various peptide analogues, only asparagine-or Dab-containing peptides bind tightly to PglB. Glycopeptide products are unable to bind, providing the driving force of product release. We find no suitably positioned residues near the active site of PglB that can activate the acceptor asparagine by deprotonation, making a general base mechanism unlikely and leaving carboxamide twisting as the most likely mechanistic proposal for asparagine activation.

Glycosylation is the predominant protein modification to diversify the functionality of proteins. In particular, N-linked protein glycosylation can increase the biophysical and pharmacokinetic properties of therapeutic proteins. However, the major challenges in studying the consequences of protein glycosylation on a molecular level are caused by glycan heterogeneities of currently used eukaryotic expression systems, but the discovery of the N-linked protein glycosylation system in the ε-proteobacterium Campylobacter jejuni and its functional transfer to Escherichia coli opened up the possibility to produce glycoproteins in bacteria. Toward this goal, we elucidated whether antibody fragments, a potential class of therapeutic proteins, are amenable to bacterial N-linked glycosylation, thereby improving their biophysical properties. We describe a new strategy for glycoengineering and production of quantitative amounts of glycosylated scFv 3D5 at high purity. The analysis revealed the presence of a homogeneous N-glycan that significantly increased the stability and the solubility of the 3D5 antibody fragment. The process of bacterial N-linked glycosylation offers the possibility to specifically address and alter the biophysical properties of proteins.

No abstract