Differential Ligand Binding Specificities of the Pulmonary Collectins Are Determined by the Conformational Freedom of a Surface Loop.
Differential Ligand Binding Specificities of the Pulmonary Collectins Are Determined by the Conformational Freedom of a Surface Loop.
复制标题
肺集合素的差异配体结合特异性由表面环的构象自由度决定。
DOI：
10.1021/acs.biochem.6b01313
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发表时间：
2017
期刊：
影响因子：
2.9
通讯作者：
McCormack,FrancisX
中科院分区：
文献类型：
--
作者：
Rynkiewicz,MichaelJ;Wu,Huixing;Cafarella,TanyaR;Nikolaidis,NikolaosM;Head,JamesF;Seaton,BarbaraA;McCormack,FrancisX
肺表面活性蛋白（SP）在肺表面活性物质功能和天然免疫中起着关键作用。SP-A和SP-D是C型凝集素的凝集素家族的成员，尽管具有广泛的结构同源性，但它们表现出不同的配体特异性、对表面活性剂结构的影响和宿主防御功能。SP-A结合二棕榈酰磷脂酰胆碱（DPPC），主要的表面活性剂脂质成分，但不磷脂酰肌醇（PI），而SP-D显示相反的偏好。此外，SP-A和SP-D识别广泛不同的病原体相关分子模式。先前的研究表明，SP-A独特的配体诱导的表面环构象变化有助于脂质结合亲和力。为了检验这一假设并确定决定其配体结合特异性的SP-A和SP-D的结构特征，使用结构指导方法将SP-D的关键特征引入SP-A。发现将SP-D样环稳定钙结合位点引入碳水化合物识别结构域的四重突变体（E171 D/P175 E/R197 N/K203 D）将SP-A配体结合偏好互变为SP-D表型，将DPPC交换为PI特异性，并导致脂质A结合的丧失和获得更亲合的甘露聚糖结合特性。组成的单一或三重突变的突变体显示其脂质和糖的结合特性之间的SP-A和SP-D的中间的改变。与肌醇或甲基-甘露糖的突变体复合物的结构显示相对于野生型SP-A的配体诱导的构象变化的衰减。这些研究表明，在一个关键的表面环的灵活性支持SP-A的独特的脂质结合功能，从而有助于其在表面活性剂的结构和调节，宿主防御的多种功能。
Lung surfactant proteins (SPs) play critical roles in surfactant function and innate immunity. SP-A and SP-D, members of the collectin family of C-type lectins, exhibit distinct ligand specificities, effects on surfactant structure, and host defense functions despite extensive structural homology. SP-A binds to dipalmitoylphosphatidylcholine (DPPC), the major surfactant lipid component, but not phosphatidylinositol (PI), whereas SP-D shows the opposite preference. Additionally, SP-A and SP-D recognize widely divergent pathogen-associated molecular patterns. Previous studies suggested that a ligand-induced surface loop conformational change unique to SP-A contributes to lipid binding affinity. To test this hypothesis and define the structural features of SP-A and SP-D that determine their ligand binding specificities, a structure-guided approach was used to introduce key features of SP-D into SP-A. A quadruple mutant (E171D/P175E/R197N/K203D) that introduced an SP-D-like loop-stabilizing calcium binding site into the carbohydrate recognition domain was found to interconvert SP-A ligand binding preferences to an SP-D phenotype, exchanging DPPC for PI specificity, and resulting in the loss of lipid A binding and the acquisition of more avid mannan binding properties. Mutants with constituent single or triple mutations showed alterations in their lipid and sugar binding properties that were intermediate between those of SP-A and SP-D. Structures of mutant complexes with inositol or methyl-mannose revealed an attenuation of the ligand-induced conformational change relative to wild-type SP-A. These studies suggest that flexibility in a key surface loop supports the distinctive lipid binding functions of SP-A, thus contributing to its multiple functions in surfactant structure and regulation, and host defense.