Figure 1. Two-component signal transduction system in bacteria. This is one of the most common signalling mechanisms in bacteria. Binding of an external ligand, for example a toxic compound, induces a conformational change in a sensor kinase receptor which is propagated through the membrane, through a piston-like effect. This triggers in the cytoplasmic domain an autophosphorylation reaction on a specific histidine residue, consuming ATP in the process. The activated sensor kinase then transfers the phosphate to a specific aspartic acid of the associated responder (response regulator). The now active response regulator in most cases is able specifically to activate transcription of certain genes in collaboration with RNA polymerase. In a few cases, stimulation of a particular sensor receptor triggers the corresponding response regulator which is specially adapted, not to active RNA polymerase but to bind directly to the motor of the flagellum, thereby controlling the direction of swimming.

Figure 2. Modular structure of an idealized two component signal transduction system. Incoming signals are picked up by the input domain of the receptor on the cell surface, inducing autophosphorylation in the output domain. A phospho-relay then propagates a conformational change in the input domain of the responder (response regulator) which finally activates the output domain of the responder. Sensor input domains are highly variable and signal specific. Response regulators, although specifically associating only with their own cognate receptor, are very similar in sequence. The simplest possible relay is portrayed here but some pathways can contain multiple modules of cytoplasmic input–output domains, some fused into a single polypeptide, some separate. TM1 and TM2 and external loop sense and communicate the signal.

Figure 3. Simplified view of a signalling relay in animal cells. The receptor binds a ligand, a conformational signal is propagated across the membrane bilayer to the cytoplasmic domain and then, according to the type of receptor, several mechanisms of propagating the effect of the signal through the cytoplasm are possible. On the right is illustrated the autophosphorylation of several (e.g. 1, 2, 3) Ser, Thr or Tyr residues stimulated by the binding of a ligand. The activated cytoplasmic domain of the receptor can then in turn activate a specific cytoplasmic protein to trigger a second messenger cascade ( Ca²⁺ or cAMP), modify an ion channel directly, or initiate a phospho-relay. Other pathways may be triggered by interaction through a G protein. In other variations, illustrated on the left, for some receptors autophosphorylation is sufficient to activate a downstream partner without phospho-transfer, whilst in yet other cases, the activated receptor acquires and can ‘pass on’ conformational changes without the intervention of phosphorylation. Downstream events in all cases often terminate in an activated transcription factor arriving in the nucleus to control the expression of specific genes.