Unraveling the GM Specificity of Galectin‑1 Binding to Lipid Membranes.

Galectin-1 (Gal-1) is a galactose-binding protein involved in various cellular functions. Gal-1's activity has been suggested to be connected to two molecular concepts, which are, however, lacking experimental proof: a) enhanced binding affinity of Gal-1 toward membranes containing monosialotetrahexosylganglioside (GM) over disialoganglioside GDa and b) cross-linking of GM's by homodimers of Gal-1. We provide evidence about the specificity and the nature of the interaction of Gal-1 with model membranes containing GM or GDa, employing a broad panel of fluorescence-based and label-free experimental techniques, complemented by atomistic biomolecular simulations. Our study demonstrates that Gal-1 indeed binds specifically to GM and not to GDa when embedded in membranes over a wide range of concentrations (i.e., 30 nM to 20 μM). The apparent binding constant is about tens of micromoles. On the other hand, no evidence of Gal-1/GM cross-linking was observed. Our findings suggest that cross-linking does not result from sole interactions between GM and Gal-1, indicating that in a physiological context, additional triggers are needed, which shift the GM/Gal-1 equilibria toward the membrane-bound homodimeric Gal-1.