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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 15-10. An example of the G-protein activity cycle. Ras is a member of the G-protein family. When Ras binds GDP, it does not signal. Through direct interactions with Ras, another protein called Sos causes conformational changes in Ras so that it preferentially binds GTP. The RasGTP complex is able to interact in turn with a cytoplasmic serine/threonine kinase, activating its kinase activity, and thus transmits the signal to the next step in the signal transduction pathway. When RasGTP is released from Sos, it hydrolyzes GTP to GDP and reassumes the inactive RasGDP state. (Adapted from J. D. Watson, M. Gilman, J. Witkowski, and M. Zoller, Recombinant DNA, 2d ed. Copyright © 1992 by James D. Watson, Michael Gilman, Jan Witkowski, and Mark Zoller.)

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