Structures of the Streptococcus sanguinis SrpA Binding Region with Human Sialoglycans Suggest Features of the Physiological Ligand
Structures of the Streptococcus sanguinis SrpA Binding Region with Human Sialoglycans Suggest Features of the Physiological Ligand
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DOI：
10.1021/acs.biochem.6b00704
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发表时间：
2016-10-25
期刊：
影响因子：
2.9
通讯作者：
Iverson, T. M.
中科院分区：
文献类型：
--
作者：
Loukachevitch, Lioudmila V.;Bensing, Barbara A.;Iverson, T. M.
血链球菌是细菌性感染性心内膜炎的主要原因，这是一种威胁生命的心脏瓣膜感染。血链球菌以很高的亲和力与人的血小板结合，这种粘附性可能会增强毒力。以前的研究表明，一个被称为SRPA的富含丝氨酸的粘附素重复序列通过与受体GPIBα上的唾液酸聚糖相互作用来介导血链球菌与人血小板的结合。然而，与SRPA和已定义的唾液酸聚糖的体外结合试验未能确定特定的高亲和力配体。为了更好地了解SRPA与人血小板之间的相互作用，我们测定了SRPA唾液酸聚糖结合区(SRPA(BR))与五个低亲和力配体的共晶结构：三个唾液酸化三糖(唾液酸化T抗原、3‘-唾液酸化乳糖和3’-唾液酸化-N-乙酰乳糖胺)、一个唾液化四糖(唾液酸化路易斯(X))和这些唾液酸化半乳糖二糖组分。然后，我们将结构分析与突变相结合，进一步确定我们观察到的SRPA(BR)与多糖之间的相互作用是否对与血小板结合很重要，并更好地定位生理受体的结合位置。我们发现SRPA(BR)的唾液酸聚糖结合部位明显大于本研究中共结晶的唾液酸聚糖，这表明SRPA与血小板的结合要么是多价的，要么是通过更大的二唾液酸聚糖发生的。
Streptococcus sanguinis is a leading cause of bacterial infective endocarditis, a life-threatening infection of heart valves. S. sanguinis binds to human platelets with high avidity, and this adherence is likely to enhance virulence. Previous studies suggest that a serine-rich repeat adhesin termed SrpA mediates the binding of S. sanguinis to human platelets via its interaction with sialoglycans on the receptor GPIb alpha. However, in vitro binding assays with SrpA and defined sialoglycans failed to identify specific high-affinity ligands. To improve our understanding of the interaction between SrpA and human platelets, we determined cocrystal structures of the SrpA sialoglycan binding region (SrpA(BR)) with five low-affinity ligands: three sialylated trisaccharides (sialyl-T antigen, 3'-sialyllactose, and 3'-sialyl-N-acetyllactosamine), a sialylated tetrasaccharide (sialyl-Lewis(X)), and a sialyl galactose disaccharide component common to these sialoglyans. We then combined structural analysis with mutagenesis to further determine whether our observed interactions between SrpA(BR) and glycans are important for binding to platelets and to better map the binding site for the physiological receptor. We found that the sialoglycan binding site of SrpA(BR) is significantly larger than the sialoglycans cocrystallized in this study, which suggests that binding of SrpA to platelets either is multivalent or occurs via a larger, disialylated glycan.