Control of MAP kinase signaling specificity or how not to go HOG wild: Figure 1.

Mitogen-activated protein kinase (MAPK cascades; also called ERK cascade, for extracellular signal-regulated protein kinase) are at the center of many signal transduction pathways in eukaryotic cells. These cascades are composed of a trio of sequentially acting protein kinases that relay an extracellular physiological signal to the targets that orchestrate the appropriate cellular response. A given mammalian cell type typically contains multiple MAPK cascades (for review, see Robinson and Cobb 1997), which raises a number of intriguing questions. Is each cascade dedicated to a specific physiological response? What ensures the specificity of action of each cascade? Do distinct MAPK cascades share some components? What mechanisms restrict or regulate cross talk between distinct pathways? These issues can be difficult to decipher in mammalian cells, but recent studies in yeast have shed light on these questions. Strikingly, these studies have revealed that the MAP kinases can function as specificity factors.