Katada T, Tamura M, Ui M
Arch Biochem Biophys. 1983 Jul 1;224(1):290-8. doi: 10.1016/0003-9861(83)90212-6.
Islet-activating protein (IAP), pertussis toxin, is an oligomeric protein composed of as A protomer and a B oligomer. IAP and its A protomer were equipotent, on a molar basis, in enhancing GTP-dependent adenylate cyclase activity and in causing ADP-ribosylation of the 41,000 Mr protein when directly added to the cell-free membrane preparation from rat C6 glioma cells. Similar actions of IAP observed upon its addition to intact C6 cells were not mimicked by its A protomer, indicating that the A protomer had to be associated with the B oligomer to become accessible to its site of action on the inner surface of the membrane of intact cells. The A protomer, but not IAP, exhibited NAD-glycohydrolase activity in the reaction mixture lacking cellular components but containing dithiothreitol. Their actions on membranes were not accelerated by dithiothreitol, but markedly suppressed by oxidized glutathione. Thus, C6 cell membranes may possess certain "processing" enzyme(s) responsible for releasing the A protomer from the IAP molecule and for reductive cleavage of an intrachain disulfide bond in the released protomer, thereby producing an active peptide which functions to cause ADP-ribosylation of one of the subunits of guanine nucleotide regulatory protein in the receptor-adenylate cyclase system.
胰岛激活蛋白（IAP），即百日咳毒素，是一种由A原体和B寡聚体组成的寡聚蛋白。当直接添加到大鼠C6胶质瘤细胞的无细胞膜制剂中时，IAP及其A原体在摩尔基础上，在增强GTP依赖性腺苷酸环化酶活性和导致41,000 Mr蛋白的ADP核糖基化方面具有同等效力。将IAP添加到完整的C6细胞上时观察到的类似作用，其A原体并不能模拟，这表明A原体必须与B寡聚体结合，才能接触到完整细胞内膜表面的作用位点。在缺乏细胞成分但含有二硫苏糖醇的反应混合物中，A原体而非IAP表现出NAD-糖水解酶活性。二硫苏糖醇不会加速它们对膜的作用，但会被氧化型谷胱甘肽显著抑制。因此，C6细胞膜可能具有某些“加工”酶，负责从IAP分子中释放A原体，并对释放的原体中的链内二硫键进行还原裂解，从而产生一种活性肽，其作用是导致受体-腺苷酸环化酶系统中鸟嘌呤核苷酸调节蛋白的一个亚基发生ADP核糖基化。
