Difference between revisions of "Team:Technion Israel/Chemotaxis"

Fig. 1: Chemotaxis concept, adapted from iGEM gottingen 2012.

The E.coli chemotaxis system is considered a model system that demonstrates some of the core principles of chemotaxis. Through use of its flagella, E.coli has the ability to move rapidly towards attractants and away from repellents, by means of a random movement, with “runs” and “tumbles” by rotating its flagellum counter-clockwise and clockwise, respectively.

Fig. 2: Random and run movement of E.coli. (2).

The response of E.coli to environmental signals is achieved by intercellular reactions. Chemo-sensing is carried out by a broad repertoire of chemoreceptors. Most have a N-terminal region that spans the membrane twice, which results in an intertwined periplasmic domain that can sense an extracellular signal. The C-terminal cytoplasmic region comprises a HAMP region (that connects the extracellular sensory domain with intracellular signaling domains), a dimerization domain and a kinase domain that interacts with its RR (3)

Ligands bind to the periplasmic domain of the chemoreceptor, at the interface between the two monomers of the dimer, with residues from both monomers being involved in the binding process. Ligand binding alters the interactions between the periplasmic domains, and between the transmembrane dimer (3).

The cytoplasmic region of the chemoreceptors interacts with two additional proteins - CheA and CheW, to form stable ternary complexes that generate stimulus signals. The activation of the receptor by an external stimulus causes auto-phosphorylation in the histidine kinase, CheA. CheA, in turn, transfers phosphoryl groups to residues in the response regulators CheB and CheY, which are cytoplasmic proteins. CheW is an adaptor protein, physically bridges CheA to the chemoreceptor to allow regulated phosphor-transfer to CheY and CheB. CheB (methylesterase) regulates the chemoreceptor methylation state. It works oppositely with CheR, a methyltransferase. When an attractant is bound to the receptors binding domain – the chemoreceptor is methylated by CheR. Then, CheY transmits the signals to the flagellar motors. The phosphate in CheY is continuously removed by CheZ. The short life time of phosphorylated CheY means that the bacteria are very responsive to changes in attractant concentration. This mechanism of signal transduction is called two component system, and is common form of signal transduction in bacteria (1).

Fig. 3: Signaling components and circuit logic (2).