Signal transduction
Also known as: cell signaling, signaling pathway
Signal transduction is the process by which a cell converts an external chemical or physical signal into an internal response. A ligand binds a receptor, the receptor activates intracellular messengers, and a cascade amplifies the message into a change in cell behavior.
Every pathway follows the same skeleton: reception, transduction, response. A signaling molecule — hormone, neurotransmitter, growth factor, cytokine — binds a specific receptor. The receptor changes conformation and activates downstream proteins, which relay and amplify the signal through a cascade. The endpoint is a cellular response: an enzyme is switched on, a channel opens, a gene is transcribed, or the cell divides, migrates, or dies.
Receptors fall into a few major classes. G protein-coupled receptors span the membrane seven times and activate heterotrimeric G proteins; Gs stimulates adenylate cyclase to raise cAMP, Gi inhibits it, and Gq activates phospholipase C to generate IP3 and DAG, releasing calcium and activating protein kinase C. Receptor tyrosine kinases such as the EGF and insulin receptors dimerize on ligand binding — the insulin receptor is already a disulfide-linked dimer, so it changes conformation instead — then autophosphorylate and launch the Ras–MAPK and PI3K–Akt pathways. Ligand-gated ion channels convert binding directly into ion flux on a millisecond timescale. Intracellular receptors bind lipid-soluble ligands like steroid and thyroid hormones that cross the membrane, then act as transcription factors in the nucleus.
Two features make these systems powerful. Amplification: one bound receptor can activate many G proteins, each producing many second messenger molecules, so a handful of hormone molecules can generate a large intracellular response. Integration and specificity: the same second messenger produces different outcomes in different cell types depending on which effector proteins are present, and pathways cross-talk. Termination matters equally — GTP hydrolysis, phosphodiesterases, phosphatases, and receptor desensitization all shut the signal down.
Pathology often reflects hijacked signaling. Cholera toxin locks Gs in its active state, driving runaway cAMP and secretory diarrhea; pertussis toxin disables Gi with a related effect. Constitutively active Ras or receptor tyrosine kinases drive many cancers.
USMLE Step 1 covers signal transduction in cell and molecular biology and returns to it in bacteriology when explaining toxin mechanisms, while the MCAT tests it under protein structure and non-enzymatic protein function. Know the receptor classes and the second messengers each generates.
Key takeaways
- Signal transduction converts an extracellular signal into an intracellular response through reception, transduction, and response.
- Major receptor classes are GPCRs, receptor tyrosine kinases, ligand-gated ion channels, and intracellular nuclear receptors.
- Second messengers such as cAMP, IP3, DAG, and calcium relay and amplify the signal inside the cell.
- Cascades amplify enormously, so a few ligand molecules can produce a large cellular response.
- Termination by GTP hydrolysis, phosphodiesterases, and phosphatases is essential; toxins like cholera toxin cause disease by blocking it.