4.2 Introduction to Signal Transduction

Describe the components of a signal transduction pathway.

• Signal transduction pathways link signal receptions with cellular responses.
• Many signal transduction pathways include protein modifications and involve phosphorylation cascades.

Describe the role of components of a signal transduction pathway in producing a cellular response.

• Signaling begins with the recognition of a chemical messenger—a ligand—by a receptor protein in a target cell.
  • i. The ligand-binding domain of a receptor recognizes a specific chemical messenger, which can be a peptide (protein) or a small molecule.
  • ii. G protein-coupled receptors are an example of a receptor protein in eukaryotes.
  • iii. Receptors may be located on the surface of a target cell or in the cytoplasm or nucleus of the target cell.
• Signaling cascades relay signals from receptors to cell targets, often amplifying the incoming signals, resulting in the appropriate responses by the cell. Responses could include cell growth, secretion of molecules, or gene expression.
  • i. After the ligand binds, the intracellular domain of a receptor protein changes shape, initiating transduction of the signal.
  • ii. Enzymes and second messengers such as cyclic AMP (cAMP) relay and amplify the intracellular signal.
  • iii. Hormones are an example of a signaling messenger that can travel long distances in the bloodstream.
  • iv. The binding of ligands to ligand-gated channels can cause the channel to open or close.