Like the communication that takes place through G protein-coupled receptors, signaling through receptor kinases causes cells to respond in many ways. During embryonic development, receptor kinase signaling is responsible for the formation and elongation of limb buds that eventually become our arms and legs. When we cut a finger, platelet-derived growth factor (PDGF) is released from platelets in the blood and binds to its receptor kinase on the surface of cells at the site of the wound, where it triggers cell division necessary to repair the wound.

The cellular responses that result from receptor kinase activation tend to involve changes in gene expression, which in turn allow cells to grow, divide, differentiate, or change shape. In contrast, the activation of G protein-coupled receptors typically leads to shorter-term changes in the cell, like activating enzymes or opening ion channels.

Signaling through receptor kinases takes place in most eukaryotic organisms, and the structure and function of these receptors have been conserved as organisms have evolved over hundreds of millions of years. A well-studied receptor kinase called Kit provides an example. In vertebrates, signaling through the Kit receptor kinase is important for the production of pigment in skin, feathers, scales, and hair. The conserved function of this receptor can be seen in individuals with mutations in the kit gene, as shown in Fig. 9.13. (By convention, the name of a protein, like Kit, is capitalized and in roman type. The name of the gene that encodes the protein, like kit, is lower case and italicized.) As you can see from Fig. 9.13, mammals, reptiles, birds, and fish with a mutation in the kit gene have a similar appearance. This observation indicates that the function of this gene has remained fairly constant since the appearance of the last common ancestor of these groups, more than 500 million years ago.

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