GTP BINDING PROTEINS AND ADENYLATE CYCLASE

霍乱原(霍乱毒素)，负责 以霍乱为特征的毁灭性腹泻疾病，导致其 激活激素敏感的腺苷对细胞的影响 循环酶。激活源于GSA的ADP-核糖化，a 环化酶鸟嘌呤核苷酸结合调节蛋白 很复杂。这种毒素还可以使其他蛋白质发生二磷酸腺苷核糖基化。 作为简单的胍基化合物，如胍丁胺。ADP-核糖基化 通过所谓的ADP来增强Gsα和其他底物的 核糖化因子(ARF)，一个约19 kDa的家族 可溶性和膜上的鸟嘌呤核苷酸结合蛋白，具有 是从牛脑中提纯出来的。在GTP存在的情况下，但不是 GDP，ARF与毒素相互作用导致 NAD和ADP-核糖受体对NO的表观KMS 对Vmax有显著影响。ARF对毒素的激活作用 将0.003%的十二烷基硫酸钠提高约4倍 (十二烷基硫酸钠)，这对活性影响很小或在没有 无论有无ARF，0.01%的十二烷基硫酸钠均不能抑制毒素活性。在……里面 刺激浓度的十二烷基硫酸钠的存在，ARF进一步 增加了NAD和胍丁胺的亲和力，此外， 提高了Vmax。观察ARF和十二烷基硫酸钠的刺激作用 在含有毒素A亚单位的二硫苏糖醇和 不含二硫苏糖醇，带有还原和烷化的毒素A1 多肽。因此，ARF和SDS似乎不会通过 促进毒素催化单元的释放。更确切地说，ARF 直接与催化的A1肽相互作用，因此可能 增强霍乱毒素的致病性。

Choleragen (cholera toxin), the agent responsible for the devastating diarrheal disease characteristic of cholera, causes its effects on cells by activating the hormone-sensitive adenylyl cyclase. Activation results from the ADP-ribosylation of Gsa, a guanine nucleotide-binding regulatory protein of the cyclase complex. The toxin can also ADP-ribosylate other proteins as well as simple guanidino compounds such as agmatine. ADP-ribosylation of Gs alpha and other substrates is enhanced by so-called ADP- ribosylation factors (ARFs), a family of approximately 19 kDa soluble and membrane guanine nucleotide-binding proteins that have been purified from bovine brain. In the presence of GTP, but not GDP, ARF interacts with the toxin resulting in a decrease in apparent Kms for both NAD and ADP-ribose acceptor with no significant effect on Vmax. Activation of toxin by ARF was increased approximately 4-fold by 0.003% sodium dodecyl sulfate (SDS), which had little or no effect on activity in the absence of ARF; 0.01% SDS abolished toxin activity with or without ARF. In the presence of SDS at stimulatory concentrations, ARF further increased affinities for NAD and agmatine and, in addition, increased Vmax. Stimulatory effects of ARF and SDS were observed in the presence of dithiothreitol with toxin A subunit and in the absence of dithiothreitol, with the reduced and alkylated toxin A1 peptide. Thus, it appears that ARF and SDS do not activate by facilitating release of the toxin catalytic unit. Rather, ARF interacts directly with the catalytic A1 peptide and may thus potentiate the pathogenicity of cholera toxin.