Yang R Y, Hill P N, Hsu D K, Liu F T
Division of Allergy, La Jolla Institute for Allergy and Immunology, San Diego, California 92121, USA.
Biochemistry. 1998 Mar 24;37(12):4086-92. doi: 10.1021/bi971409c.
Galectin-3 is a member of a large family of beta-galactoside-binding animal lectins and is composed of a carboxyl-terminal lectin domain connected to an amino-terminal nonlectin part. Previous experimental results suggest that, when bound to multivalent glycoconjugates, galectin-3 self-associates through intermolecular interactions involving the amino-terminal domain. In this study, we obtained evidence suggesting that the protein self-associates in the absence of its saccharide ligands, in a manner that is dependent on the carboxyl-terminal domain. This mode of self-association is inhibitable by the lectin's saccharide ligands. Specifically, recombinant human galectin-3 was found to bind to galectin-3C (the carboxyl-terminal domain fragment) conjugated to Sepharose 4B and the binding was inhibitable by lactose. In addition, biotinylated galectin-3 bound to galectin-3 immobilized on plastic surfaces and the binding could also be inhibited by various saccharide ligands of the lectin. A mutant with a tryptophan to leucine replacement in the carboxyl-terminal domain, which exhibited diminished carbohydrate-binding activity, did not bind to galectin-3C-Sepharose 4B. Furthermore, galectin-3C formed covalent homodimers when it was treated with a chemical cross-linker and the dimer formation was completely inhibited by lactose. Therefore, galectin-3 can self-associate through intermolecular interactions involving both the amino- and the carboxyl-terminal domains and the relative contribution of each depends on whether the lectin is bound to its saccharide ligands.
半乳糖凝集素-3是β-半乳糖苷结合动物凝集素大家族的成员之一，由连接到氨基末端非凝集素部分的羧基末端凝集素结构域组成。先前的实验结果表明，当与多价糖缀合物结合时，半乳糖凝集素-3通过涉及氨基末端结构域的分子间相互作用进行自我缔合。在本研究中，我们获得的证据表明，该蛋白在没有其糖类配体的情况下也能自我缔合，且这种方式依赖于羧基末端结构域。这种自我缔合模式可被凝集素的糖类配体抑制。具体而言，发现重组人半乳糖凝集素-3与偶联到琼脂糖4B上的半乳糖凝集素-3C（羧基末端结构域片段）结合，且这种结合可被乳糖抑制。此外，生物素化的半乳糖凝集素-3与固定在塑料表面的半乳糖凝集素-3结合，这种结合也可被该凝集素的各种糖类配体抑制。在羧基末端结构域中色氨酸被亮氨酸取代的突变体，其碳水化合物结合活性降低，不与半乳糖凝集素-3C-琼脂糖4B结合。此外，用化学交联剂处理时，半乳糖凝集素-3C形成共价同型二聚体，且二聚体的形成被乳糖完全抑制。因此，半乳糖凝集素-3可通过涉及氨基末端和羧基末端结构域的分子间相互作用进行自我缔合，且每个结构域的相对贡献取决于凝集素是否与其糖类配体结合。
