Single-Site Cys-Substituting Mutation of Human Lectin Galectin-2: Modulating Solubility in Recombinant Production, Reducing Long-Term Aggregation, and Enabling Site-Specific MonoPEGylation

Structural mechanisms for the S-nitrosylation-derived protection of mouse galectin-2 from oxidation-induced inactivation revealed by NMR

Galectin-2 (Gal-2) is a lectin thought to play protective roles in the gastrointestinal tract. Oxidation of mouse Gal-2 (mGal-2) by hydrogen peroxide (H₂ O₂ ) results in the loss of sugar-binding activity, whereas S-nitrosylation of mGal-2, which does not change its sugar-binding profile, has been shown to protect the protein from H₂ O₂ -induced inactivation. One of the two cysteine residues, C57, has been identified as being responsible for controlling H₂ O₂ -induced inactivation; however, the underlying molecular mechanism has not been elucidated. We performed structural analyses of mGal-2 using nuclear magnetic resonance (NMR) and found that residues near C57 experienced significant chemical shift changes following S-nitrosylation, and that S-nitrosylation slowed the H₂ O₂ -induced aggregation of mGal-2. We also revealed that S-nitrosylation improves the thermal stability of mGal-2 and that the solvent accessibility and/or local dynamics of residues near C57 and the local dynamics of the core-forming residues in mGal-2 are reduced by S-nitrosylation. Structural models of Gal-2 indicated that C57 is located in a hydrophobic pocket that can be plugged by S-nitrosylation, which was supported by the NMR experiments. Based on these results, we propose two structural mechanisms by which S-nitrosylation protects mGal-2 from H₂ O₂ -induced aggregation without changing its sugar-binding profile: (a) stabilization of the hydrophobic pocket around C57 that prevents oxidation-induced destabilization of the pocket, and (b) prevention of oxidation of C57 during the transiently unfolded state of the protein, in which the residue is exposed to H₂ O₂ .

DATABASE

Nuclear magnetic resonance assignments for non-S-nitrosylated mGal-2 and S-nitrosylated mGal-2 have been deposited in the BioMagResBank (http://www.bmrb.wisc.edu/) under ID code 27237 for non-S-nitrosylated mGal-2 and ID code 27238 for S-nitrosylated mGal-2.

Cysteine mutagenesis improves the production without abrogating antigenicity of a recombinant protein vaccine candidate for human chagas disease

A therapeutic vaccine for human Chagas disease is under development by the Sabin Vaccine Institute Product Development Partnership. The aim of the vaccine is to significantly reduce the parasite burden of Trypanosoma cruzi in humans, either as a standalone product or in combination with conventional chemotherapy. Vaccination of mice with Tc24 formulated with monophosphoryl-lipid A (MPLA) adjuvant results in a Th1 skewed immune response with elevated IgG2a and IFNγ levels and a statistically significant decrease in parasitemia following T. cruzi challenge. Tc24 was therefore selected for scale-up and further evaluation. During scale up and downstream process development, significant protein aggregation was observed due to intermolecular disulfide bond formation. To prevent protein aggregation, cysteine codons were replaced with serine codons which resulted in the production of a non-aggregated and soluble recombinant protein, Tc24-C4. No changes to the secondary structure of the modified molecule were detected by circular dichroism. Immunization of mice with wild-type Tc24 or Tc24-C4, formulated with E6020 (TLR4 agonist analog to MPLA) emulsified in a squalene-oil-in-water emulsion, resulted in IgG2a and antigen specific IFNγ production levels from splenocytes that were not significantly different, indicating that eliminating putative intermolecular disulfide bonds had no significant impact on the immunogenicity of the molecule. In addition, vaccination with either formulated wild type Tc24 or Tc24-C4 antigen also significantly increased survival and reduced cardiac parasite burden in mice. Investigations are now underway to examine the efficacy of Tc24-C4 formulated with other adjuvants to reduce parasite burden and increase survival in pre-clinical studies.

Human galectin-2 interacts with carbohydrates and peptides non-classically: new insight from X-ray crystallography and hemagglutination

Galectin-2 (Gal-2) plays a role in cancer, myocardial infarction, immune response, and gastrointestinal tract diseases. The only reported crystal structure of Gal-2 shows that it is a dimer in which the monomer subunits have almost identical structures, each binding with one molecule of lactose. In this study, we crystallized Gal-2 under new conditions that produced three crystal structures. In each Gal-2 dimer structure, lactose was shown to be bound to only one of the carbohydrate recognition domain subunits. In solution studies, the thermal shift assay demonstrated that inequivalent monomer subunits in the Gal-2 dimer become equivalent upon ligand binding. In addition, galectin-mediated erythrocyte agglutination assays using lactose and larger complex polysaccharides as inhibitors showed the structural differences between Gal-1 and Gal-2. Overall, our results reveal some novel aspects to the structural differentiation in Gal-2 and expand the potential for different types of molecular interactions that may be specific to this lectin.

Oxidation Protection in Metal-Binding Peptide Motif and Its Application to Antibody for Site-Selective Conjugation

Here, we demonstrate that a metal ion binding motif could serve as an efficient and robust tool for site-specific conjugation strategy. Cysteine-containing metal binding motifs were constructed as single repeat or tandem repeat peptides and their metal binding characteristics were investigated. The tandem repeats of the Cysteine-Glycine-Histidine (CGH) metal ion binding motif exhibited concerted binding to Co(II) ions, suggesting that conformational transition of peptide was triggered by the sequential metal ion binding. Evaluation of the free thiol content after reduction by reducing reagent showed that metal-ion binding elicited strong retardation of cysteine oxidation in the order of Zn(II)>Ni(II)>Co(II). The CGH metal ion binding motif was then introduced to the C-terminus of antibody heavy chain and the metal ion-dependent characteristics of oxidation kinetics were investigated. As in the case of peptides, CGH-motif-introduced antibody exhibited strong dependence on metal ion binding to protect against oxidation. Zn(II)-saturated antibody with tandem repeat of CGH motif retains the cysteine reactivity as long as 22 hour even with saturating O₂ condition. Metal-ion dependent fluorophore labeling clearly indicated that metal binding motifs could be employed as an efficient tool for site-specific conjugation. Whereas Trastuzumab without a metal ion binding site exhibited site-nonspecific dye conjugation, Zn(II) ion binding to antibody with a tandem repeat of CGH motif showed that fluorophores were site-specifically conjugated to the heavy chain of antibody. We believe that this strong metal ion dependence on oxidation protection and the resulting site-selective conjugation could be exploited further to develop a highly site-specific conjugation strategy for proteins that contain multiple intrinsic cysteine residues, including monoclonal antibodies.

Identification of the cysteine residue responsible for oxidative inactivation of mouse galectin-2

Galectins are a group of animal lectins characterized by their specificity for β-galactosides. Mouse galectin-2 (mGal-2) is predominantly expressed in the gastrointestinal tract and has been identified as one of the main gastric mucosal proteins that are uniquely sensitive to S-nitrosylation. We have previously reported that oxidation of mGal-2 by hydrogen peroxide (H₂O₂) resulted in the loss of sugar-binding ability, whereas pre-treatment of mGal-2 with S-nitrosocysteine prevented H₂O₂-induced inactivation. In this study, we used point-mutated recombinant mGal-2 proteins to study which of the two highly conserved Cys residues in mGal-2 must be S-nitrosylated for protection against oxidative inactivation. Mutation of Cys⁵⁷ to a Met residue (C57M) did not result in lectin inactivation following H₂O₂ treatment, whereas Cys⁷⁵ mutation to Ser (C75S) led to significantly reduced lectin activity, as is the case for wild-type mGal-2. However, pre-treatment of the C75S mutant with S-nitrosocysteine protected the protein from H₂O₂-induced inactivation. Therefore, Cys⁵⁷ is suggested to be responsible for oxidative inactivation of the mGal-2 protein, and protection of the sulfhydryl group of the Cys⁵⁷ in mGal-2 by S-nitrosylation is likely important for maintaining mGal-2 protein function in an oxidative environment such as the gastrointestinal tract.

Two site genetic incorporation of varying length polyethylene glycol into the backbone of one peptide

Polyethylene glycol (PEG) of different lengths was genetically incorporated into the backbone of a polypeptide using stop-anticodon and frameshift anticodon-containing tRNAs, which were acylated with PEG-containing amino acids.

Lectin engineering, a molecular evolutionary approach to expanding the lectin utilities

In the post genomic era, glycomics—the systematic study of all glycan structures of a given cell or organism—has emerged as an indispensable technology in various fields of biology and medicine. Lectins are regarded as “decipherers of glycans”, being useful reagents for their structural analysis, and have been widely used in glycomic studies. However, the inconsistent activity and availability associated with the plant-derived lectins that comprise most of the commercially available lectins, and the limit in the range of glycan structures covered, have necessitated the development of innovative tools via engineering of lectins on existing scaffolds. This review will summarize the current state of the art of lectin engineering and highlight recent technological advances in this field. The key issues associated with the strategy of lectin engineering including selection of template lectin, construction of a mutagenesis library, and high-throughput screening methods are discussed.

S-nitrosylation of mouse galectin-2 prevents oxidative inactivation by hydrogen peroxide

Galectins are a group of animal lectins characterized by their specificity for β-galactosides. Galectin-2 (Gal-2) is predominantly expressed in the gastrointestinal tract. A proteomic analysis identified Gal-2 as a protein that was S-nitrosylated when mouse gastric mucosal lysates were reacted with S-nitrosoglutathione, a physiologically relevant S-nitrosylating agent. In the present study, recombinant mouse (m)Gal-2 was S-nitrosylated using nitrosocysteine (CysNO), which had no effect on the sugar-binding specificity and dimerization capacity of the protein. On the other hand, mGal-2 oxidation by H2O2 resulted in the loss of sugar-binding ability, while S-nitrosylation prevented H2O2-inducted inactivation, presumably by protecting the Cys residue(s) in the protein. These results suggest that S-nitrosylation by nitric oxides protect Gal-2 from oxidative stress in the gastrointestinal tract.

Single-site mutational engineering and following monoPEGylation of the human lectin galectin-2: effects on ligand binding, functional aspects, and clearance from serum

The emerging insights into the physiological significance of endogenous lectins prompted us to characterize the effect of monosubstitution with poly(ethylene glycol) (PEG; 5 kDa) on a human lectin. As role model, we used a member of the galectin family, that is, galectin-2, the Cys57Met (single-site) mutant and its monoPEGylated derivative. The activities of these three proteins were comparatively studied by biochemical, cell biological, and histochemical methods, using surface-immobilized glycoproteins, different types of cells presenting gangliosides or (glyco)proteins as counterreceptors in vitro and tissue sections. PEGylation led to decreases in affinity/signal intensity with context dependence. The introduction of the mutation, too, can influence reactivity. Assays on haemagglutination and inhibition of cell proliferation underscored that mutational engineering and substitution can (but must not necessarily) affect this protein's activity. Serum clearance in rats was markedly retarded by PEGylation. Overall, the bulky substitution, spatially comparable to N-glycans, can markedly reduce binding of the galectin to physiological binding sites.

Genetic PEGylation

Polyethylene glycol (PEG) was genetically incorporated into a polypeptide. Stop-anticodon-containing tRNAs were acylated with PEG-containing amino acids and were then translated into polypeptides corresponding to DNA sequences containing the stop codons. The molecular weights of the PEG used were 170, 500, 700, 1000, and 2000 Da, and the translation was confirmed by mass spectrometry. The PEG incorporation ratio decreased as the molecular weight of PEG increased, and PEG with a molecular weight of 1000 Da was only slightly incorporated. Although improvement is required to increase the efficiency of the process, this study demonstrates the possibility of genetic PEGylation.