Soluble immunoglobulin M (IgM) forms a pentamer containing a joining (J) chain polypeptide. While IgM pentamer has various immune functions, it also behaves as a carrier of circulating apoptosis inhibitor of macrophage (AIM; also called CD5L) protein that facilitates repair during different diseases. AIM binds to the IgM pentamer solely in the presence of the J chain. Here, using a single-particle negative-stain electron microscopy, we found that the IgM pentamer exhibits an asymmetric pentagon containing one large gap, which is markedly different from the textbook symmetric pentagon model. A single AIM molecule specifically fits into the gap, cross-bridging two IgM-Fc that form the edges of the gap through a disulfide bond at one side and a charge-based interaction at the other side. The discovery of the bona fide shape of the IgM pentamer advances our structural understanding of the pentameric IgM and its binding mode with AIM.

中文翻译：

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IgM五聚体是不对称的五边形，具有与AIM蛋白结合的开槽。

可溶性免疫球蛋白M（IgM）形成含有连接（J）链多肽的五聚体。尽管IgM五聚体具有多种免疫功能，但它也可作为循环性巨噬细胞凋亡抑制剂（AIM；也称为CD5L）蛋白的载体，在不同疾病中促进修复。AIM仅在J链存在的情况下才与IgM五聚体结合。在这里，使用单粒子负染色电子显微镜，我们发现IgM五聚体表现出一个不对称的五边形，其中包含一个大的缺口，这与教科书的对称五边形模型有明显的不同。一个单一的AIM分子特异性地插入间隙中，通过一侧的二硫键和另一侧的基于电荷的相互作用，将形成间隙边缘的两个IgM-Fc交叉桥接。