Wilkins A, Chubb J R, Insall R H
MRC Laboratory for Molecular Cell Biology, University College London, Gower Street, WC1E 6BT, London, UK.
Curr Biol. 2000 Nov 16;10(22):1427-37. doi: 10.1016/s0960-9822(00)00797-1.
Dictyostelium possesses a surprisingly large number of Ras proteins and little is known about their activators, the guanine nucleotide exchange factors (GEFs). It is also unclear, in Dictyostelium or in higher eukaryotes, whether Ras pathways are linear, with each Ras controlled by its own GEF, or networked, with multiple GEFs acting on multiple Ras proteins.
We have identified the Dictyostelium gene that encodes RasGEFB, a protein with homology to known RasGEFs such as the Son-of-sevenless (Sos) protein. Dictyostelium cells in which the gene for RasGEFB was disrupted moved unusually rapidly, but lost the ability to perform macropinocytosis and therefore to grow in liquid medium. Crowns, the sites of macropinocytosis, were replaced by polarised lamellipodia. Mutant cells were also profoundly defective in early development, although they eventually formed tiny but normally proportioned fruiting bodies. This defect correlated with loss of discoidin Igamma mRNA, a starvation-induced gene, although other genes required for development were expressed normally or even precociously. RasGEFB was able to rescue a Saccharomyces CDC25 mutant, indicating that it is a genuine GEF for Ras proteins.
RasGEFB appears to be the principal activator of the RasS protein, which regulates macropinocytosis and cell speed, but it also appears to regulate one or more other Ras proteins.
盘基网柄菌拥有数量惊人的Ras蛋白，而对其激活因子鸟嘌呤核苷酸交换因子（GEF）却知之甚少。在盘基网柄菌或高等真核生物中，Ras信号通路是呈线性的，即每个Ras由其自身的GEF控制，还是呈网络状的，即多个GEF作用于多个Ras蛋白，目前也尚不清楚。
我们鉴定出了盘基网柄菌中编码RasGEFB的基因，该蛋白与已知的RasGEF如七号less之子（Sos）蛋白具有同源性。RasGEFB基因被破坏的盘基网柄菌细胞移动异常迅速，但失去了进行巨胞饮作用的能力，因此无法在液体培养基中生长。巨胞饮作用发生的位点——冠状结构，被极化的片状伪足所取代。突变细胞在早期发育中也存在严重缺陷，尽管它们最终形成了微小但比例正常的子实体。这种缺陷与盘状球蛋白Iγ mRNA的缺失有关，盘状球蛋白Iγ mRNA是一种饥饿诱导基因，尽管发育所需的其他基因表达正常甚至提前表达。RasGEFB能够挽救酿酒酵母CDC25突变体，表明它是Ras蛋白的真正GEF。
RasGEFB似乎是RasS蛋白的主要激活因子，RasS蛋白调节巨胞饮作用和细胞速度，但它似乎也调节一种或多种其他Ras蛋白。
