de Rooij J, Zwartkruis F J, Verheijen M H, Cool R H, Nijman S M, Wittinghofer A, Bos J L
Laboratory for Physiological Chemistry and Centre for Biomedical Genetics, Utrecht University, The Netherlands.
Nature. 1998 Dec 3;396(6710):474-7. doi: 10.1038/24884.
Rap1 is a small, Ras-like GTPase that was first identified as a protein that could suppress the oncogenic transformation of cells by Ras. Rap1 is activated by several extracellular stimuli and may be involved in cellular processes such as cell proliferation, cell differentiation, T-cell anergy and platelet activation. At least three different second messengers, namely diacylglycerol, calcium and cyclic AMP, are able to activate Rap1 by promoting its release of the guanine nucleotide GDP and its binding to GTP. Here we report that activation of Rap1 by forskolin and cAMP occurs independently of protein kinase A (also known as cAMP-activated protein kinase). We have cloned the gene encoding a guanine-nucleotide-exchange factor (GEF) which we have named Epac (exchange protein directly activated by cAMP). This protein contains a cAMP-binding site and a domain that is homologous to domains of known GEFs for Ras and Rap1. Epac binds cAMP in vitro and exhibits in vivo and in vitro GEF activity towards Rap1. cAMP strongly induces the GEF activity of Epac towards Rap1 both in vivo and in vitro. We conclude that Epac is a GEF for Rap1 that is regulated directly by cAMP and that Epac is a new target protein for cAMP.
Rap1是一种小的、与Ras类似的GTP酶，最初被鉴定为一种能够抑制Ras介导的细胞致癌转化的蛋白质。Rap1可被多种细胞外刺激激活，并可能参与细胞增殖、细胞分化、T细胞无能及血小板活化等细胞过程。至少三种不同的第二信使，即二酰基甘油、钙和环磷酸腺苷（cAMP），能够通过促进Rap1释放鸟嘌呤核苷酸GDP并使其与GTP结合来激活Rap1。在此我们报道，福斯可林和cAMP对Rap1的激活独立于蛋白激酶A（也称为cAMP激活的蛋白激酶）。我们克隆了编码一种鸟嘌呤核苷酸交换因子（GEF）的基因，我们将其命名为Epac（cAMP直接激活的交换蛋白）。该蛋白含有一个cAMP结合位点以及一个与已知的Ras和Rap1的GEF结构域同源的结构域。Epac在体外结合cAMP，并在体内和体外均表现出对Rap1的GEF活性。cAMP在体内和体外均强烈诱导Epac对Rap1的GEF活性。我们得出结论，Epac是一种直接受cAMP调节的Rap1的GEF，并且Epac是cAMP的一种新的靶蛋白。
