Structure of Two Tandem-Repeat Galectin Proteins Binding a Model Glycolipid Membrane

Tandem-repeat galectins are a family of proteins containing two carbohydrate recognition domains (CRDs) with affinity to various glycoproteins and glycolipids involved in cell signaling. Galectin-4 is expressed in intestinal epithelial cells, and galectin-8 is essential in regulating cell adhesion and immune response. Misregulation of both tandem-repeat galectins is linked to variable cancer cell behavior. Structure models for the membrane-bound forms of galectin-4 and galectin-8 were constructed from X-ray reflectivity measurements coupled with molecular dynamics for galectin-4. The proteins were bound to lipid monolayers containing their respective ligands, gangliosides GM1 or GM3, to determine the membrane-bound structure. Galectin-4 contains two CRDs with weak affinity for GM1, and it bound with both CRDs arranged near the membrane while dynamically sampling alternative conformations. Galectin-8, in contrast, contains only one CRD with tight binding to GM3, and one CRD was pointed towards the membrane while the other oriented away from the membrane. Shortening the peptide linker between the CRDs altered protein binding to the membrane, suggesting the linker likely facilitates stabilizing contacts between the CRDs. Overall, this work helps to illustrate the conformational dynamics of tandem-repeat galectins, emphasizing the roles of ligand affinity, linker peptide dynamics, and contacts between CRDs.