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| Modern Genetic Analysis Chapter 15. Regulation of Cell Number: Normal and Cancer Cells Controlling the Cell Proliferation and Death Machinery Figure 15-9. The consequences of ligand binding on RTK activity and the initiation of the signal transduction cascade. On dimerization of the RTK, autophosphorylation occurs. The ability of activated RTK to initiate signal transduction is through two different routes: (a) by the activated RTK serving as an anchor site for some signal transduction proteins and (b) by the activated RTK phosphorylating other proteins. (a) Autophosphorylation creates binding sites for adaptor, or "docking," proteins that bind to sites that include specific phosphorylated tyrosine residues. Through the docking of different adaptor proteins in close proximity to one another, protein protein conformational changes lead to induction of the signal transduction activities of these proteins. (b) The other consequence of dimerization and autophosphorylation is that the RTK directly phosphorylates substrate proteins and, through this phosphorylation, induces their signal transduction activities. (Adapted from H. Lodish, D. Baltimore, A. Berk, S. L. Zipursky, P. Matsudaira, and J. Darnell, Molecular Cell Biology, 3d ed. Copyright © 1995 by Scientific American Books.)

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