Signal transduction: Taking the Rap

Signalling through different growth factors can lead to different outcomes. For example, whereas nerve growth factor (NGF) leads to differentiation of the PC12 cell line, signalling through epidermal growth factor (EGF) does not. How is such specificity achieved? Differentiation occurs because NGF leads to sustained activation of the extracellular-signal-regulated kinases (ERKs), whereas EGF leads to only transient activation of the ERKs. Moreover, the pathway leading to activation of ERKs was thought to hinge on the small GTPase Ras, but one group has now found that the key may be another small GTPase called Rap1.