Unique Conformer Selection of Human Growth-Regulatory Lectin Galectin-1 for Ganglioside GM₁ versus Bacterial Toxins^,

Abstract

Endogenous lectins induce effects on cell growth by binding to antennae of natural glycoconjugates. These complex carbohydrates often present more than one potential lectin-binding site in a single chain. Using the growth-regulatory interaction of the pentasaccharide of ganglioside GM(1) with homodimeric galectin-1 on neuroblastoma cell surfaces as a model, we present a suitable strategy for addressing this issue. The approach combines NMR spectroscopic and computational methods and does not require isotope-labeled glycans. It involves conformational analysis of the two building blocks of the GM(1) glycan, i.e., the disaccharide Galbeta1-3GalNAc and the trisaccharide Neu5Acalpha2-3Galbeta1-4Glc. Their bound-state conformations were determined by transferred nuclear Overhauser enhancement spectroscopy. Next, measurements on the lectin-pentasaccharide complex revealed differential conformer selection regarding the sialylgalactose linkage in the tri- versus pentasaccharide (Phi and Psi value of -70 degrees and 15 degrees vs 70 degrees and 15 degrees, respectively). To proceed in the structural analysis, the characteristic experimentally detected spatial vicinity of a galactose unit and Trp68 in the galectin's binding site offered a means, exploiting saturation transfer from protein to carbohydrate protons. Indeed, we detected two signals unambiguously assigned to the terminal Gal and the GalNAc residues. Computational docking and interaction energy analyses of the entire set of ligands supported and added to experimental results. The finding that the ganglioside's carbohydrate chain is subject to differential conformer selection at the sialylgalactose linkage by galectin-1 and GM(1)-binding cholera toxin (Phi and Psi values of -172 degrees and -26 degrees, respectively) is relevant for toxin-directed drug design. In principle, our methodology can be applied in studies aimed at blocking galectin functionality in malignancy and beyond glycosciences.

Assessing the inhibitory potency of galectin ligands identified from combinatorial (glyco)peptide libraries using surface plasmon resonance spectroscopy

“…The further structural analysis of these complexes may disclose peptide-galectin contacts beyond the central lactose moiety. Whether they involve sites of the lectin active in binding of histo-blood group A/B-tetrasaccharides or the ganglioside GM1-derived pentasaccharide [29,30] is an open question. …”

Galectin-1 Interacts with the α5β1 Fibronectin Receptor to Restrict Carcinoma Cell Growth via Induction of p21 and p27

“…This is an intriguing demonstration for selective interaction of Gal-1 with distinct target structures on epithelial tumor cells, despite the ubiquitous abundance of ␤-galactosides on the cell surface. Structural research will have to resolve how Gal-1 recognizes distinct target sites in complex glycans, as was accomplished recently for ganglioside G M1 , the main binding partner for growth regulation on neuroblastoma cells (7).…”

Siglec-7 Undergoes a Major Conformational Change When Complexed with the α(2,8)-Disialylganglioside GT1b

“…For a number of lectins there is a distinct conformerdependent selection of ligands (54,55). Such conformer selection may allow for additional hierarchies other than glycan composition in directing lectins to their physiological ligands, i.e.…”