![]() Subsequent studies showed that not only were the components of MAPK pathways conserved between yeast and humans, but also their arrangement into a cascade was conserved. ![]() In the early 1990s, when components of mammalian MAPK cascades were cloned, it was discovered that they were highly homologous to several yeast kinases (Ste11 MAP3K, Ste7 MAP2K, Kss1 MAPK and Fus3 MAPK) known to function in the signal transduction pathway by which yeast cells respond to peptide mating pheromone. The module consists of a MAP3K, which phosphorylates and thereby activates a MAP2K, which in turn phosphorylates and activates a MAPK. MAPK cascades are found embedded in signaling networks that transmit many different signals, including those initiated by growth and development factors, inflammatory stimuli, and cellular stresses. Do modules like kinase cascades always carry out similar functions regardless of which signaling pathway they are placed in? If not, what are the mechanisms that modulate kinase function to allow different input–output mappings? In a recent issue of Current Biology, Takahashi and Pryciak show that a scaffold protein can counteract the switch-like tendencies of the MAPK cascade to allow graded signaling, essentially turning an on–off switch into a dimmer. More recently, eukaryotic mitogen-activated protein kinase (MAPK) cascades have been shown to be inherently switch-like. Indeed, when the first protein kinase cascade was discovered, it was hypothesized to function as an amplifier. Like electronic circuits, intracellular signal transduction networks also appear to contain modular elements, such as protein kinase cascades.
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