Carbohydrate-lectin interactions intervene in and mediate most biological processes, including ac rucial modulation of immune responsest op athogens. Despite growing interesti n investigating the association between host receptorl ectins and exogenous glycanl igands, the molecular mechanisms underlying bacterial recognition by human lectins are still not fully understood.H erein, an ovel molecular interaction between the human macrophage galactose-type lectin (MGL) and the lipooligosaccharide (LOS) of Escherichia coli strain R1 is described. Saturation transfer difference NMR spectroscopy analysis, supportedb yc omputational studies, demonstrated that MGL bound to the purified deacylated LOS R1 mainly throughr ecognition of its outer core and established crucial interactions with the terminal Gala(1,2)Gal epitope. These results assess the ability of MGL to recognise glycan moieties exposed on Gram-negative bacterial surfaces.Bacterialc ell surfaces are decorated with highly diverseg lycoconjugates,i nt he form of capsular polysaccharides, peptidoglycans, lipopolysaccharides( LPSs) and other glycolipids, [1] which perform severalf unctions, ranging from structural to protective roles. [2] Bacterial glycans take part in many keyb iological events, including pathogen recognition,receptor activation, cell adhesion and signal transduction. Additionally,t hese structures often serve as molecular patterns that are recognised by specific glycan-binding receptors of the host immune system,t hus triggeringapathogen-specific immune response.It is well known that LPSs, the major constituents of the outer membrane of Gram-negative bacteria, [3] are one of the main virulence factorsofseveral feared bacterialstrains,including enteropathogenic Escherichia coli,w hich is implicated in severe foodborne and urinary tract infections. [4] From as tructural point of view,L PS is composed of three structural motifs that can be distinguishedb ecause they are encoded by different gene clusters. Lipid A, which represents the glycolipid portion, is an acylated bis-phosphorylated glucosamined isaccharide that anchors the LPS to the outer membrane. Lipid Ai sc ovalentlyl inked to ac ore oligosaccharide (OS) that can be further divided into two different portions: the more conserved inner region, which is characterised by the presence of peculiar sugar residues,s uch as 3-deoxy-d-manno-oct-2-ulopyranosonic acid (KDO),a nd the more variableo uter core. Finally,t he O-antigen, which is ap olysaccharide composed of severalO S repeating units, extends to the extracellularm edium and acts as ah ydrophilic coating surface. [3,5] However,G ram-negative bacteria can also produce rough-typeL PS, namely,l ipooligosaccharide (LOS);atruncated version of LPS that lacks the Oantigen.To date, the receptor complex formed by the toll-like receptor 4a nd the small secreted MD2 protein is among the main speciesi nvolved in bacterial LPS recognition by host immune cells. [6] More recently,i th as been shown that LPSs are also intracellularly detected by caspases...

LipoPolySaccharides are a hallmark of Gram-negative bacteria and their presence at the cell surface is key for bacterial integrity. As surface exposed components, they are recognized by immunity C-type lectin receptors present on Antigen Presenting Cells. Human Macrophage Galactose Lectin bindsE. colisurface that presents a specific glycan motif. Nevertheless, this high affinity interaction occurs regardless of the integrity of its canonical calcium-dependent glycan binding site. Nuclear Magnetic Resonance of MGL carbohydrate recognition domain and complete extracellular domain revealed a new glycan binding site opposite to the canonical site. A model of trimeric Macrophage Galactose Lectin was determined based on a combination of Small Angle X-ray scattering and Alphafold. A disulphide bond positions the Carbohydrate Recognition Domain perpendicular to the coiled-coil domain. This unique configuration for a C-type lectin orients the six glycan sites of MGL in an ideal position to bind LipoPolySaccharides at the bacterial surface with high avidity.

No abstract