FIGURE 6-3 A generalized signal transduction pathway (information transfer via a series of on-off switches. The active ligand is shown on the left, approaching the transmembrane receptor protein. Inside the cell is a multistep signal transduction pathway composed of switch-like intermediates, A, B, and C, that can exist in active and inactive states. In the absence of signal, the intermediates are inactive. During binding, the receptor becomes active and activates one intermediate that activates the next, and so on in series, until eventually a protein kinase is activated. This pathway transfers information, not energy or materials. The kinase enzyme is specific for transferring phosphate from adesine triphosphate to a serine, threonine, or tyrosine residue of select protein targets within the cell. Phosphorylated target proteins change activity, becoming either active or inactive, and therefore the basic cellular process of which they are a part, changes activity. In this way, the signal has effects (i.e., it triggers cellular responses). Target proteins might be components of the processes of transcription, translation, the cell cycle, cell movement, differentiation, or other signaling pathways. Transcription is a particularly frequent target. Fourteen of the 17 involve transmembrane receptors; the other two involve intracellular receptors and the ligands pass through the plasma membrane readily.
cations. Many of the initially identical genes later diversified their sequences, leading either to diversified functions of encoded products or to diversified cis-regulatory regions setting different conditions of expression. Despite this diversification, extensive redundancy still remains. For example, there are more than 24 TGFβ ligand genes and 11 Wnt ligand genes in mouse, but only 3-5 TGFβ and 1-3 Wnt genes in Drosophila. In mouse development, the various genes for a single step of a single kind of signaling pathway are expressed at different times and places in the embryo. When one gene is knocked out, the defect in development is limited in scope to a few times and places where no related gene is expressed to provide overlapping function.