Gentile T C, Dierks S E, Watt R M
Biochim Biophys Acta. 1984 Nov 23;791(1):102-11. doi: 10.1016/0167-4838(84)90287-5.
Intrigued by reports that the mitogenic effect of protein A on B lymphocytes was due to a direct interaction of cell surface immunoglobulin with protein A, the binding of 19 S, 8 S, and Fab mu fragments of 125I-labeled IgM isolated from porcine serum was investigated. Approx. 60% of purified 19 S porcine IgM interacted specifically with Protein A-Sepharose. Mild reduction and alkylation of 19 S IgM to yield monomeric IgM did not appear to alter its ability to bind to protein A. Elution of either molecular species of IgM from protein A and subsequent repassage over Protein A-Sepharose resulted in nearly quantitative rebinding of the IgM to protein A. Fab mu fragments prepared by digestion of 19 S IgM with pepsin exhibited binding characteristics similar to that observed for intact and monomeric IgM. These results suggest that: (1) there are at least two populations of porcine serum IgM, one that binds to protein A and one that does not; (2) these populations are not interconverting; (3) the ability of IgM to bind to protein A is not dependent on the 19 S pentameric structure extant in sera, but rather is an intrinsic property of some and not all four chain IgM protomers; and (4) a binding site for protein A on porcine IgM is localized in the Fab mu (including the C mu 2 domain) regions of the molecule.
有报道称蛋白A对B淋巴细胞的促有丝分裂作用是由于细胞表面免疫球蛋白与蛋白A的直接相互作用，受此报道的启发，研究了从猪血清中分离出的125I标记IgM的19S、8S和Fabμ片段的结合情况。约60%的纯化猪19S IgM与蛋白A - 琼脂糖特异性相互作用。将19S IgM轻度还原和烷基化以产生单体IgM，似乎并未改变其与蛋白A结合的能力。从蛋白A上洗脱任何一种分子形式的IgM，随后再次通过蛋白A - 琼脂糖，结果显示IgM几乎能定量地重新结合到蛋白A上。用胃蛋白酶消化19S IgM制备的Fabμ片段表现出与完整和单体IgM相似的结合特性。这些结果表明：（1）猪血清IgM至少有两个群体，一个群体能与蛋白A结合，另一个群体则不能；（2）这些群体不会相互转化；（3）IgM与蛋白A结合的能力不依赖于血清中存在的19S五聚体结构，而是某些而非所有四链IgM原聚体的固有特性；（4）猪IgM上蛋白A的结合位点定位于分子的Fabμ（包括Cμ2结构域）区域。
