Structural Insights into the Molecular Recognition Mechanism of the Cancer and Pathogenic Epitope, LacdiNAc by Immune-Related Lectins.

Interactions of glycan-specific epitopes to human lectin receptors represent novel immune checkpoints for investigating cancer and infection diseases. By employing a multidisciplinary approach that combines isothermal titration calorimetry, NMR spectroscopy, molecular dynamics simulations, and X-ray crystallography, we investigated the molecular determinants that govern the recognition of the tumour and pathogenic glycobiomarker LacdiNAc (GalNAcß1-4GlcNAc, LDN), including their comparison with the ubiquitous LacNAc epitope (Galß1-4GlcNAc, LN), by two human immune-related lectins, galectin-3 (hGal-3) and the macrophage galactose C-type lectin (hMGL). A different mechanism of binding and interactions was observed for the hGal-3/LDN and hMGL/LDN complexes, which explains the remarkable difference in the binding specificity of LDN and LN by these two lectins. The new structural clues reported herein are fundamental for the chemical design of mimetics targeting hGal-3/hMGL recognition process.

Role of MurT C-Terminal Domain in the Amidation of Staphylococcus aureus Peptidoglycan.

Glutamate amidation, a secondary modification of the peptidoglycan, was first identified in Staphylococcus aureus It is catalyzed by the protein products of the murT and gatD genes, which are conserved and colocalized in the genomes of most sequenced Gram-positive bacterial species. The MurT-GatD complex is required for cell viability, full resistance to ß-lactam antibiotics, and resistance to human lysozyme and is recognized as an attractive target for new antimicrobials. Great effort has been invested in the study of this step, culminating recently in three independent reports addressing the structural elucidation of the MurT-GatD complex. In this work, we demonstrate through the use of nonstructural approaches the critical and multiple roles of the C-terminal domain of MurT, annotated as DUF1727, in the MurT-GatD enzymatic complex. This domain provides the physical link between the two enzymatic activities and is essential for the amidation reaction. Copurification of recombinant MurT and GatD proteins and bacterial two-hybrid assays support the observation that the MurT-GatD interaction occurs through this domain. Most importantly, we provide in vivo evidence of the effect of substitutions at specific residues in DUF1727 on cell wall peptidoglycan amidation and on the phenotypes of oxacillin resistance and bacterial growth.

The Plasticity of the Carbohydrate Recognition Domain Dictates the Exquisite Mechanism of Binding of Human Macrophage Galactose-Type Lectin.

The human macrophage galactose-type lectin (MGL), expressed on macrophages and dendritic cells (DCs), modulates distinct immune cell responses by recognizing N-acetylgalactosamine (GalNAc) containing structures present on pathogens, self-glycoproteins, and tumor cells. Herein, NMR spectroscopy and molecular dynamics (MD) simulations were used to investigate the structural preferences of MGL against different GalNAc-containing structures derived from the blood group A antigen, the Forssman antigen, and the GM2 glycolipid. NMR spectroscopic analysis of the MGL carbohydrate recognition domain (MGL-CRD, C181-H316) in the absence and presence of methyl α-GalNAc (α-MeGalNAc), a simple monosaccharide, shows that the MGL-CRD is highly dynamic and its structure is strongly altered upon ligand binding. This plasticity of the MGL-CRD structure explains the ability of MGL to accommodate different GalNAc-containing molecules. However, key differences are observed in the recognition process depending on whether the GalNAc is part of the blood group A antigen, the Forssman antigen, or GM2-derived structures. These results are in accordance with molecular dynamics simulations that suggest the existence of a distinct MGL binding mechanism depending on the context of GalNAc moiety presentation. These results afford new perspectives for the rational design of GalNAc modifications that fine tune MGL immune responses in distinct biological contexts, especially in malignancy.

Protein-Glycan Quinary Interactions in Crowding Environment Unveiled by NMR Spectroscopy.

Protein-glycan interactions as modulators for quinary structures in crowding environments were explored. The interaction between human galectin 3 (Gal-3) and distinct macromolecular crowders, such as bovine and human serum albumin (BSA and HSA), Ficoll 70 and PEG3350, was scrutinized. The molecular recognition event of the specific ligand, lactose, by Gal-3 in crowding conditions was evaluated. Gal-3 interactions were monitored by NMR analysing chemical shift perturbation (CSP) and line broadening of 1 H15 N-HSQC signals. The intensity of the Gal-3 1 H15 N-HSQC signals decreased in the presence of all crowders, due to the increase in the solution viscosity and to the formation of large protein complexes. When glycosylated containing samples of BSA and HSA were used, signal broadening was more severe than that observed in the presence of the more viscous solutions of PEG3350 and Ficoll 70. However, for the samples containing glycoproteins, the signal intensity of 1 H15 N-HSQC recovered upon addition of lactose. We show that serum proteins interact with Gal-3, through their α2,3-linked sialylgalactose moieties exposed at their surfaces, competing with lactose for the same binding site. The quinary interaction between Gal-3 and serum glycoproteins, could help to co-localize Gal-3 at the cell surface, and may play a role in adhesion and signalling functions of this protein.

Atomic and Specificity Details of Mucin 1 O-Glycosylation Process by Multiple Polypeptide GalNAc-Transferase Isoforms Unveiled by NMR and Molecular Modeling.

The large family of polypeptide GalNAc-transferases (GalNAc-Ts) controls with precision how GalNAc O-glycans are added in the tandem repeat regions of mucins (e.g., MUC1). However, the structural features behind the creation of well-defined and clustered patterns of O-glycans in mucins are poorly understood. In this context, herein, we disclose the full process of MUC1 O-glycosylation by GalNAc-T2/T3/T4 isoforms by NMR spectroscopy assisted by molecular modeling protocols. By using MUC1, with four tandem repeat domains as a substrate, we confirmed the glycosylation preferences of different GalNAc-Ts isoforms and highlighted the importance of the lectin domain in the glycosylation site selection after the addition of the first GalNAc residue. In a glycosylated substrate, with yet multiple acceptor sites, the lectin domain contributes to orientate acceptor sites to the catalytic domain. Our experiments suggest that during this process, neighboring tandem repeats are critical for further glycosylation of acceptor sites by GalNAc-T2/T4 in a lectin-assisted manner. Our studies also show local conformational changes in the peptide backbone during incorporation of GalNAc residues, which might explain GalNAc-T2/T3/T4 fine specificities toward the MUC1 substrate. Interestingly, we postulate that a specific salt-bridge and the inverse γ-turn conformation of the PDTRP sequence in MUC1 are the main structural motifs behind the GalNAc-T4 specificity toward this region. In addition, in-cell analysis shows that the GalNAc-T4 isoform is the only isoform glycosylating the Thr of the immunogenic epitope PDTRP in vivo, which highlights the relevance of GalNAc-T4 in the glycosylation of this epitope. Finally, the NMR methodology established herein can be extended to other glycosyltransferases, such as C1GalT1 and ST6GalNAc-I, to determine the specificity toward complex mucin acceptor substrates.

The first structure from the SOUL/HBP family of heme-binding proteins, murine P22HBP.

Murine p22HBP, a 22-kDa monomer originally identified as a cytosolic heme-binding protein ubiquitously expressed in various tissues, has 27% sequence identity to murine SOUL, a heme-binding hexamer specifically expressed in the retina. In contrast to murine SOUL, which binds one heme per subunit via coordination of the Fe(III)-heme to a histidine, murine p22HBP binds one heme molecule per subunit with no specific axial ligand coordination of the Fe(III)-heme. Using intrinsic protein fluorescence quenching, the values for the dissociation constants of p22HBP for hemin and protoporphyrin-IX were determined to be in the low nanomolar range. The three-dimensional structure of murine p22HBP, the first for a protein from the SOUL/HBP family, was determined by NMR methods to consist of a 9-stranded distorted beta-barrel flanked by two long alpha-helices. Although homologous domains have been found in three bacterial proteins, two of which are transcription factors, the fold determined for p22HBP corresponds to a novel alpha plus beta fold in a eukaryotic protein. Chemical shift mapping localized the tetrapyrrole binding site to a hydrophobic cleft formed by residues from helix alphaA and an extended loop. In an attempt to assess the structural basis for tetrapyrrole binding in the SOUL/HBP family, models for the p22HBP-protoporphyrin-IX complex and the SOUL protein were generated by manual docking and automated methods.