FIGURE 15-7 A signal transduction pathway involving a G protein, a second messenger, a protein kinase, and several target proteins. The figure depicts a generalized signal transduction pathway; in parentheses are listed the molecules involved in the specific signaling pathway discussed in Section 15-5. Binding of the hormone to its cell-surface receptor 1 triggers activation of a G protein 2 by the receptor functioning as a GEF and causing loss of GDP and binding of GTP. The active G protein binds to and activates an enzyme 3 that synthesizes a second messenger 4, which in turn binds to and activates a protein kinase 5. The kinase, in turn, phosphorylates, and thus changes the activity of, one or more target proteins. These proteins can either be cytosolic proteins 6a that induce changes in cellular function, metabolism, or movement or transcription factors 6b that induce changes in gene expression.