Crystal structure of the cysteine-rich domain of scavenger receptor MARCO reveals the presence of a basic and an acidic cluster that both contribute to ligand recognition
Crystal structure of the cysteine-rich domain of scavenger receptor MARCO reveals the presence of a basic and an acidic cluster that both contribute to ligand recognition
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DOI：
10.1074/jbc.m701750200
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发表时间：
2007-06-01
影响因子：
4.8
通讯作者：
Tryggvason, Karl
中科院分区：
文献类型：
--
作者：
Ojala, Juha R. M.;Pikkarainen, Timo;Tryggvason, Karl
MARCO是巨噬细胞和树突状细胞的一种三聚体A类清道夫受体，它能识别多阴离子颗粒和病原体。该受体胞外部分的远端清道夫受体富含半胱氨酸（SRCR）结构域与配体结合有关。为了给理解MARCO的配体结合机制提供结构基础，我们测定了小鼠MARCO SRCR结构域的晶体结构。重组的SRCR结构域纯化后呈现单体和二聚体形式，它们的结构分别在1.78 Å和1.77 Å分辨率下被测定。单体具有紧密的球状折叠结构，包含一个扭曲的五链反平行β -折叠和一个覆盖单个β -螺旋的长环，而二聚体是通过两个单体的β -链交换形成的，因此包含一个大的八链β -折叠。表面静电势的计算显示，带有几个精氨酸的β -折叠区域形成一个碱性簇。出乎意料的是，在长环区域发现了一个酸性簇。在单体中，酸性簇参与金属离子结合。对表达各种SRCR结构域突变体的细胞的研究表明，碱性簇中的所有精氨酸都参与配体结合，这表明存在一种协同结合机制。配体结合还依赖于酸性簇和Ca²⁺离子，Ca²⁺离子的缺失似乎至少通过调节静电势或相对结构域取向来影响配体结合。我们提出，SRCR结构域二聚化可以通过为MARCO受体寡聚化提供一种方式来促进对大配体的识别。
MARCO is a trimeric class A scavenger receptor of macrophages and dendritic cells that recognizes polyanionic particles and pathogens. The distal, scavenger receptor cysteine-rich (SRCR) domain of the extracellular part of this receptor has been implicated in ligand binding. To provide a structural basis for understanding the ligand-binding mechanisms of MARCO, we have determined the crystal structure of the mouse MARCO SRCR domain. The recombinant SRCR domain purified as monomeric and dimeric forms, and their structures were determined at 1.78 and 1.77A resolution, respectively. The monomer has a compact globular fold with a twisted five-stranded antiparallel beta-sheet and a long loop covering a single beta-helix, whereas the dimer is formed via beta-strand swapping of two monomers, thus containing a large eight- stranded beta-sheet. Calculation of the surface electrostatic potential revealed that the beta-sheet region with several arginines forms a basic cluster. Unexpectedly, an acidic cluster was found in the long loop region. In the monomer, the acidic cluster is involved in metal ion binding. Studies with cells expressing various SRCR domain mutants showed that all of the arginines of the basic cluster are involved in ligand binding, suggesting a cooperative binding mechanism. Ligand binding is also dependent on the acidic cluster and Ca2+ ions whose depletion appears to affect ligand binding at least by modulating the electrostatic potential or relative domain orientation. We propose that the SRCR domain dimerization can contribute to the recognition of large ligands by providing a means for the MARCO receptor oligomerization.