Signal Transduction via the Two-component Pathway

The two-component pathway was originaUy discovered in bacteria. It was only recently recognized that this kind of signal transduction is also used in eucaryotes. Bacteria possess signal systems which they use to react to changes in N availability, osmolarity and to chemotactic substances. The signaling pathway responsible for this regulation is 

The two-component pathway is characterized by two functional elements. A histidine-specific protein kinase functions as a sensor that registers an external signal and passes this on to a downstream response regulator. The latter is activated by phosphorylation during the process of signal transduction, triggering other reactions in the cell (Fig. 12.3). 

The composition of the two-component pathway is very variable. The nature of the external signal and the reactions triggered in the cell may be very diverse. The sensor kinase may be a part of the receptor that registers the signal, or it may be on a polypeptide chain separate from the receptor. Furthermore, there are different mechanisms of coupling of the main functions of the two-component pathway. The sensor and reaction regulator may be on a single polypeptide or they may be on separate proteins. In addition, the proteins involved may be membrane proteins. 

The two-component system is a signaling pathway of great importance in bacteria. Similar proteins and signaling pathways can also be identified in plant cells and in yeast, based on sequence homology with the bacterial proteins (review Swanson et al., 1994). It is expected that signaling pathways using the principle of the two-component system will also be foimd in mammals. 

Fig 12.4 shows a model for signal transduction in the two-component system of yeast. The SLNl protein of yeast is a composite sensor regulator system in which the 

---

Regulation by Ca and diacylglycerol identify protein kinase C as a component of signal transduction pathways, in the course of which, phospholipase C is activated and the messenger substances lns(3,4,5)P3/Ca and diacylglycerol are produced. Activation of protein kinase C may take place via two central pathways ... 

---