Botulinum neurotoxins (BoNTs) are exceptionally toxic proteins that cause paralysis but are also extensively used as treatment for various medical conditions. Most BoNTs bind two receptors on neuronal cells, namely, a ganglioside and a protein receptor. Differences in the sequence between the protein receptors from different species can impact the binding affinity and toxicity of the BoNTs. Here we have investigated how BoNT/B, /DC, and /G, all three toxins that utilize synaptotagmin I and II (Syt-I and Syt-II, respectively) as their protein receptors, bind to Syt-I and -II of mouse/rat, bovine, and human origin by isothermal titration calorimetry analysis. BoNT/G had the highest affinity for human Syt-I, and BoNT/DC had the highest affinity for bovine Syt-II. As expected, BoNT/B, /DC, and /G showed very low levels of binding to human Syt-II. Furthermore, we carried out saturation transfer difference (STD) and STD-TOCSY NMR experiments that revealed the region of the Syt peptide in direct contact with BoNT/G, which demonstrate that BoNT/G recognizes the Syt peptide in a model similar to that in the established BoNT/B-Syt-II complex. Our analyses also revealed that regions outside the Syt peptide's toxin-binding region are important for the helicity of the peptide and, therefore, the binding affinity.

中文翻译：

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突触结合素与肉毒杆菌神经毒素的结合。

肉毒杆菌神经毒素（BoNT）是极具毒性的蛋白质，会引起麻痹，但也广泛用于各种医学疾病的治疗。大多数BoNT结合神经元细胞上的两个受体，即神经节苷脂和蛋白质受体。来自不同物种的蛋白质受体之间的序列差异会影响BoNT的结合亲和力和毒性。在这里，我们研究了BoNT / B，/ DC和/ G这三种利用突触标签蛋白I和II（分别为Syt-I和Syt-II）作为其蛋白受体的毒素如何与Syt-I和-II结合等温滴定量热法分析小鼠/大鼠，牛和人来源。BoNT / G对人Syt-I的亲和力最高，而BoNT / DC对牛Syt-II的亲和力最高。不出所料，BoNT / B，/ DC和/ G与人Syt-II的结合水平非常低。此外，我们进行了饱和转移差异（STD）和STD-TOCSY NMR实验，揭示了Syt肽直接与BoNT / G接触的区域，这表明BoNT / G在与SNP相似的模型中识别Syt肽。建立的BoNT / B-Syt-II复合物。我们的分析还表明，Syt肽的毒素结合区以外的区域对于肽的螺旋度以及因此的结合亲和力都很重要。