Signal transduction is the process by which a cell converts an extracellular signal — most often the binding of a ligand to a receptor — into a defined intracellular response. The process typically comprises a receptor, one or more transducer proteins (G proteins, kinases, adapter proteins), second-messenger molecules and downstream effectors such as transcription factors or ion channels.
Signal-transduction pathways are organised as cascades that amplify the input signal and integrate it with other cellular cues. They are regulated at every level by phosphorylation, ubiquitylation, feedback loops and receptor internalisation, all of which shape the magnitude and duration of the eventual response.
Every mechanism-of-action page on this site is, structurally, a description of a signal-transduction pathway.