The small GTP-binding
p21 Ras protein plays a central role in the regulation of diverse cellular processes in invertebrates and vertebrates. Ras controls the specification of vulval or tail structures in the nematode Caenorhabditis elegans, the specification of neuronal and non-neuronal cell fates in Drosophila, and the choice between proliferation and differentiation in PC12 cells, to name just a few examples. Furthermore, Ras proteins play a critical role in oncogenesis. In some cases
p21 Ras appears to even control opposing pathways regulating cell growth and cell arrest and even cell death within the same cell. A variety of extracellular stimuli activate Ras by inducing exchange of Ras-bound GDP for GTP, a process that is facilitated by exchange factors such as son-of-sevenless. The binding of GTP to Ras triggers a conformational change whereby its effector domain, a loop of eight invariably conserved amino acids, is exposed. How can this simple change control a plethora of developmental and physiological precesses in different cells in vertebrates and invertebrates? Biochemical, genetic and structural studies have begun to shed light onto how Ras can pull so many strings attached to diverse cellular responses. Three broad concepts have emerged. Different effects of Ras activation can originate from first, the activation of parallel effector pathways, second, quantitatively different levels and duration of Ras activity and third, different cellular contexts that determine how the Ras signal is interpreted. After describing the two best known effector molecules of Ras, the protein Raf and the lipid kinase PI(3)K, we discuss how these three different models might account for the