Abstract：
The mouse intestinal helminth Heligmosomoides polygyrus modulates host immune responses by secreting a transforming growth factor (TGF)-β mimic (TGM), to expand the population of Foxp3+ Tregs. TGM comprises five complement control protein (CCP)-like domains, designated D1-D5. Though lacking homology to TGF-β, TGM binds directly to the TGF-β receptors TβRI and TβRII and stimulates the differentiation of naïve T-cells into Tregs. However, the molecular determinants of binding are unclear. Here, we used surface plasmon resonance, isothermal calorimetry, NMR spectroscopy, and mutagenesis to investigate how TGM binds the TGF-β receptors. We demonstrate that binding is modular, with D1-D2 binding to TβRI and D3 binding to TβRII. D1-D2 and D3 were further shown to compete with TGF-β(TβRII)2 and TGF-β for binding to TβRI and TβRII, respectively. The solution structure of TGM-D3 revealed that TGM adopts a CCP-like fold but is also modified to allow the C-terminal strand to diverge, leading to an expansion of the domain and opening potential interaction surfaces. TGM-D3 also incorporates a long structurally ordered hypervariable loop, adding further potential interaction sites. Through NMR shift perturbations and binding studies of TGM-D3 and TβRII variants, TGM-D3 was shown to occupy the same site of TβRII as bound by TGF-β using both a novel interaction surface and the hypervariable loop. These results, together with the identification of other secreted CCP-like proteins with immunomodulatory activity in H. polygyrus, suggest that TGM is part of a larger family of evolutionarily plastic parasite effector molecules that mediate novel interactions with their host.
摘要：
小鼠肠道蠕虫通过分泌转化生长因子（TGF）-β模拟物（TGM）来调节宿主的免疫反应，扩大Foxp3+Tregs的种群。TGM包含五个补体控制蛋白（CCP）样结构域，指定D1-D5。虽然缺乏与TGF-β的同源性，TGM直接结合TGF-β受体TβRI和TβRII并刺激初始T细胞分化为Treg。然而,结合的分子决定因素尚不清楚。这里,我们使用表面等离子体共振，等温量热法,核磁共振波谱,和诱变研究TGM如何结合TGF-β受体。我们证明绑定是模块化的，D1-D2与TβRI结合，D3与TβRII结合。D1-D2和D3进一步显示与TGF-β（TβRII）2和TGF-β竞争结合TβRI和TβRII，分别。TGM-D3的溶液结构表明，TGM采用CCP样折叠，但也经过修饰以允许C末端链发散，导致域的扩展和开放潜在的相互作用表面。TGM-D3还包含一个长的结构有序超变环，增加更多潜在的相互作用位点。通过TGM-D3和TβRII变体的NMR位移扰动和结合研究，使用新的相互作用表面和高变环,显示TGM-D3占据与TGF-β结合的TβRII的相同位点。这些结果,结合其他分泌的具有免疫调节活性的CCP样蛋白的鉴定，表明TGM是一个更大的进化可塑性寄生虫效应分子家族的一部分，该分子介导与宿主的新型相互作用。
