Signaling specificity: the RTK/RAS/MAP kinase pathway in metazoans.

The molecular basis by which commonly used signaling pathways are able to elicit tissue-specific responses in multicellular organisms is an important yet poorly understood problem. In this review, we use the receptor tyrosine kinase (RTK)/RAS/MAP kinase signaling cascade as a model to discuss various hypotheses that have been proposed to explain signaling specificity. Specificity can arise at the level of the receptor, through the modulation of signaling kinetics, through the interaction of different signaling pathways, and at the level of downstream signaling components. Mechanisms of specificity used by the RTK/RAS/MAP kinase signaling pathway might apply to other signaling pathways as well, and might help explain how multicellular organisms are able to generate tissues of diverse forms and functions from a small set of common signaling pathways.