Kahn R A, Gilman A G
J Biol Chem. 1984 May 25;259(10):6235-40.
We have utilized purified reactants and cofactors to examine the form of the stimulatory guanine nucleotide-binding regulatory component (Gs) of adenylate cyclase that serves as a substrate for ADP-ribosylation by cholera toxin; we have also investigated some of the consequences of that covalent modification. Activation of Gs with nonhydrolyzable analogs of GTP, which causes dissociation of its subunits, completely inhibits the toxin-catalyzed covalent modification. However, this effect cannot be explained by subunit dissociation, since activation of Gs by fluoride is not inhibitory and ADP ribosylation of the alpha (45,000-Da) subunit of Gs proceeds equally well in the presence and absence of the beta (35,000-Da) subunit. ADP-ribosylation of the alpha subunit of Gs decreases its apparent affinity for the beta subunit; however, the affinity of alpha and ADP-ribosyl-alpha for GTP appear to be approximately the same. ADP-ribosylation of Gs thus promotes the dissociation of its alpha and beta subunits. This effect may account for or contribute to the activation of adenylate cyclase by cholera toxin.
我们利用纯化的反应物和辅因子来研究作为霍乱毒素ADP-核糖基化底物的腺苷酸环化酶刺激性鸟嘌呤核苷酸结合调节成分（Gs）的形式；我们还研究了这种共价修饰的一些后果。用不可水解的GTP类似物激活Gs会导致其亚基解离，从而完全抑制毒素催化的共价修饰。然而，这种效应不能用亚基解离来解释，因为氟化物对Gs的激活没有抑制作用，并且在存在和不存在β（35,000道尔顿）亚基的情况下，Gs的α（45,000道尔顿）亚基的ADP核糖基化进行得同样良好。Gs的α亚基的ADP核糖基化降低了其对β亚基的表观亲和力；然而，α和ADP-核糖基-α对GTP的亲和力似乎大致相同。因此，Gs的ADP核糖基化促进了其α和β亚基的解离。这种效应可能解释或促成了霍乱毒素对腺苷酸环化酶的激活。
