The integrin receptor α_Mβ₂ mediates phagocytosis of complement-opsonized objects, adhesion to the extracellular matrix, and transendothelial migration of leukocytes. However, the mechanistic aspects of α_Mβ₂ signaling upon ligand binding are unclear. Here, we present the first atomic structure of the human α_Mβ₂ headpiece fragment in complex with the nanobody (Nb) hCD11bNb1 at a resolution of 3.2 Å. We show that the receptor headpiece adopts the closed conformation expected to exhibit low ligand affinity. The crystal structure indicates that in the R77H α_M variant, associated with systemic lupus erythematosus, the modified allosteric relationship between ligand binding and integrin outside–inside signaling is due to subtle conformational effects transmitted over a distance of 40 Å. Furthermore, we found the Nb binds to the αI domain of the α_M subunit in an Mg²⁺-independent manner with low nanomolar affinity. Biochemical and biophysical experiments with purified proteins demonstrated that the Nb acts as a competitive inhibitor through steric hindrance exerted on the thioester domain of complement component iC3b attempting to bind the α_M subunit. Surprisingly, we show that the Nb stimulates the interaction of cell-bound α_Mβ₂ with iC3b, suggesting that it may represent a novel high-affinity proteinaceous α_Mβ₂-specific agonist. Taken together, our data suggest that the iC3b–α_Mβ₂ complex may be more dynamic than predicted from the crystal structure of the core complex. We propose a model based on the conformational spectrum of the receptor to reconcile these observations regarding the functional consequences of hCD11bNb1 binding to α_Mβ₂.

整合素受体 α _M β ₂介导补体调理物的吞噬作用、与细胞外基质的粘附以及白细胞的跨内皮迁移。_{然而，配体结合后 α M} β ₂信号传导的机制方面尚不清楚。在这里，我们展示了与纳米体 (Nb) hCD11bNb1 复合的人类 α _M β _{2头饰片段的第一个原子结构，分辨率为 3.2 Å。}我们表明受体头件采用预期表现出低配体亲和力的封闭构象。晶体结构表明，在 R77H α _M变体，与系统性红斑狼疮相关，配体结合和整合素内外信号之间的修饰变构关系是由于在 40 Å 距离上传输的微妙构象效应。此外，我们发现Nb以不依赖Mg ^{2+的方式与}_αM亚基的αI结构域结合，具有低纳摩尔亲和力。纯化蛋白质的生化和生物物理实验表明，Nb 通过施加在补体成分 iC3b 的硫酯结构域上的空间位阻作为竞争性抑制剂，试图结合 α _M亚基。令人惊讶的是，我们表明 Nb 刺激细胞结合的 α _M β _{2的相互作用}与 iC3b 结合，表明它可能代表一种新型的高亲和力蛋白质 α _M β ₂特异性激动剂。总之，我们的数据表明 iC3b-α _M β ₂复合物可能比核心复合物的晶体结构所预测的更具动态性。我们提出了一个基于受体构象谱的模型，以协调这些关于 hCD11bNb1 与 α _M β ₂结合的功能后果的观察结果。