The Signal Transduction Cascade

Signal transduction is defined as the response of a cell to the application of an external stimulus. Stimuli can be chemical or physical, typical examples being light, oxygen, nutrients, hormones.

Signal transduction always involves the following phenomena:

  • signal reception

  • integration

  • amplification

  • a target that is affected

Fig. 1: the principle components of a simple signal transduction casca

The following modules are involved in signal transduction:

  • a receptor of the stimulus
    • (Description of a procaryotic signal reception module)
      Receptors need a receiver domain (e.g. a ligand-binding domain for chemotaxis, or a light-absorbing chromophore in phototaxis). Their activation is forwarded by a transducer protein that contains a signalling domain. The architecture of receptors and transducers is as heterogenous as the perceived signals. The receiver and signalling domain may be present in distinct proteins that form a complex or may be fused in a single polypeptide chain. While the signalling domain functions inside of the cell, the receiver domain is frequently on the outside. Consequently, the receptor/transducer is located in the membrane.
  • a signal transduction chain or a signal transduction network
    responsible for amplification, integration, and adaptation (see below)
    • The simplest transduction chain is that of the two-component systems of procaryotes.
    • A typical network are the phosphorylation cascades of the Map kinase pathway in eucaryotes.
  • a target that is affected and produces the cellular response
    Typical targets are:
    • the genome
      This results in regulation of gene expression.
    • the cytoskeleton in eukaryotes
    • the flagellar motor in prokaryotes
      This results in directed movements due to a "biased random walk".

The signal transduction chain (or network) is responsible for

  • integration
    • Integration indicates that several receptors activate/deactivate one and the same catalyst which thereby acts as a signal integrator.
    • In eucaryotic signal transduction networks, cross-talk between different systems adds another level of integration.
  • amplification
    • Amplification typically consists of activation of a catalyst, such as a protein kinase, which amplifies the input of a single unit (photon or molecule) into the phosphorylation of many target molecules.
  • adaptation
    • Adaptation is defined as return of the signalling system to the pre-stimulus level while the stimulus persists. This enables cells to perceive changes in stimulus size rather than absolute stimulus levels.
    • Example: Adaptation of the eye to bright sunlight or dim moonlight.

Keywords: signal transduction, chemotaxis, phototaxis, reception, amplification, integration, adaptation, receptor, transducer, two-component regulatory system, protein kinase, histidine kinase, response regulator

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