Interactions between galectins and polysaccharides are crucial to many biological processes, and yet these are some of the least understood, usually being limited to studies with small saccharides and short oligosaccharides. The present study is focused on human galectin-3 (Gal-3) interactions with a 60 kDa rhamnogalacturonan RG-I-4 that we use as a model to garner information as to how galectins interact with large polysaccharides, as well as to develop this agent as a therapeutic against human disease. Gal-3 is unique among galectins, because as the only chimera-type, it has a long N-terminal tail (NT) that has long puzzled investigators due to its dynamic, disordered nature and presence of numerous prolines. Here, we use ¹⁵N–¹H heteronuclear single quantum coherence NMR spectroscopy to demonstrate that multiple sites on RG-I-4 provide epitopes for binding to three sites on ¹⁵N-labeled Gal-3, two within its carbohydrate recognition domain (CRD) and one at a novel site within the NT encompassing the first 40 residues that are highly conserved among all species of Gal-3. Moreover, strong binding of RG-I-4 to the Gal-3 NT occurs on a very slow time scale, suggesting that it may be mediated by cis–trans proline isomerization, a well-recognized modulator of many biological activities. The NT binding epitope within RG-I-4 appears to reside primarily in the side chains of the polysaccharide, some of which are galactans. Our results provide new insight into the role of the NT in Gal-3 function.

中文翻译：

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新的多糖结合到galectin-3的N末端尾巴可能被脯氨酸异构化调节

半乳糖凝集素和多糖之间的相互作用对于许多生物学过程至关重要，但这些相互作用是鲜为人知的，通常仅限于使用小糖和短寡糖的研究。本研究的重点是人类半乳糖凝集素3（Gal-3）与60 kDa鼠李糖半乳糖醛酸RG-I-4的相互作用，我们以此为模型来收集有关半乳糖凝集素如何与大多糖相互作用的信息，并以此来发展药物作为人类疾病的治疗剂。Gal-3在半乳凝素中是唯一的，因为它是唯一的嵌合体类型，它具有长的N末端尾巴（NT），由于其动态，无序的性质和大量脯氨酸的存在，长期困扰着研究者。在这里，我们使用¹⁵ N– ¹H异核单量子相干NMR光谱法证明RG-1-4上的多个位点提供了与^15个N标记的Gal-3上的三个位点结合的表位，其中两个位于其碳水化合物识别域（CRD）内，一个位于内部的一个新位点NT包含所有Gal-3物种中高度保守的前40个残基。此外，RG-I-4与Gal-3 NT的强结合发生在很慢的时间范围内，这表明它可能是由顺式-反式介导的。脯氨酸异构化，一种公认的许多生物活性调节剂。RG-1-4中的NT结合表位似乎主要存在于多糖的侧链中，其中一些是半乳聚糖。我们的结果为NT在Gal-3功能中的作用提供了新的见解。