Figure 34. A simplified, typical signal transduction pathway.

Many growth factors (secreted proteins) interact with a receptor at the cell surface and both ligand and receptor can act in the form of a monomer or in a complex. The interaction causes the receptor to become active, and this will lead to a signalling cascade (depicted by the dashed red lines), passed from one molecule to the next. The signal may culminate in the activation of a transcription factor with a consequent change in gene expression, or influence mitochondrial membrane integrity and cell survival, or have an alternative destination to elicit a cellular response. The activation signal may be transmitted in several ways, the most common method is through the action of kinases, enzymes which phosphorylate their substrates (depicted by P), thereby activating the substrate for the next step. While a simple pathway is depicted, the reality is that a vast network of complex interactions occur, integrating numerous signals allowing for an extensive array of subtly different responses.