Galectin multimerization and lattice formation are regulated by linker region structure

Galectins regulate cellular functions by binding to glycan ligands on cell surface glycoprotein receptors. Prototype galectins, such as galectin-1, are one carbohydrate recognition domain (CRD) monomers that noncovalently dimerize, whereas tandem-repeat galectins, such as galectin-9, have two non-identical CRDs connected by a linker domain. Dimerization of prototype galectins, or both CRDs in tandem-repeat galectins, is typically required for the crosslinking of glycoprotein receptors and subsequent cellular signaling. Several studies have found that tandem-repeat galectins are more potent than prototype galectins in triggering many cell responses, including cell death. These differences could be due to CRD specificity, the presence or absence of a linker domain between CRDs, or both. To interrogate the basis for the increased potency of tandem-repeat galectins compared with prototype galectins in triggering cell death, we created three tandem-repeat galectin constructs with different linker regions joining identical galectin-1 CRDs, so that any differences we observed would be due to the contribution of the linker region rather than due to CRD specificity. We found that random-coil or rigid a-helical linkers that permit separation of the two galectin-1 CRDs facilitated the formation of higher-order galectin multimers and that these galectins were more potent in binding to glycan ligands and cell surface glycoprotein receptors, as well as triggering T cell death, compared with native galectin-1 or a construct with a short rigid linker. Thus, the increased potency of tandem-repeat galectins compared with prototype galectins is likely due to the ability of the linker domain to permit intermolecular CRD interactions, resulting in the formation of higher-order multimers with increased valency, rather than differences in CRD specificity.