Bacterial Chemotaxis – The root of P.L.A.N.T.
Two-component systems are found in eukaryotes and prokaryotes and allow sensing of environmental conditions in a stimulus-response mechanism. Since two-component mechanism can be found in plants and bacteria and is well conserved throughout various species, it is feasible to engineer chimeric two component systems. Our signaling cascade in plants is based on a two-component system which is involved in chemotaxis of Escherichia coli (E. coli). Chemotaxis is a complex mechanism that allows cells to sense and move towards attractants or move away from repellents. The following passage focuses on the chemotaxis of E. coli, since it has been proven as the organism we successfully established the signaling cascade in.
The first step of the complex signaling during the chemotaxis is the binding of the ligand, for example a carbon source such as ribose or glucose, to a specific receptor located in the periplasm, the periplasmic binding protein (PBP). PBPs are homologous proteins, that consist of two globular domains and a connecting hinge region. Upon binding of a ligand, a drastic conformational change is induced. The so-called “Venus-flytrap”-mechanism enables embedding of the ligand between the globular domains and allows the binding of the periplasmic binding protein by membrane-bound receptors that are specific for each PBP . During our work, we mainly focused on the ribose binding protein (RBP). In the case of RBP, as well as the galactose/glucose binding protein (GBP), the membrane bound receptor involved in the signaling cascade is the Trg receptor. This receptor type officiates as a homodimer and consists of a periplasmic ligand-binding domain, a helical transmembrane segment and a helical cytoplasmic region . The cytoplasmic part of the receptor serves as a framework for a super-molecular receptor-kinase signaling complex. In this complex the histidine kinase CheA is bound to the receptor. The binding is facilitated by a conformational change upon PBP binding to Trg . The activity of the kinase on the other hand is downregulated upon ligand binding by Trg, which in turn inhibits the activity of the two response regulators CheB and CheY . CheB is a methylase that acts antagonistic to the methylation enzyme CheR. CheR methylates Trg and makes it less responsive to the binding of the ligand, while CheB reverts this reaction, therefore increasing sensitivity to a ligand. Phosphorylated CheY docks to the flagellar motor protein and induces a tumbling state . The inhibition of the activity of both CheB and CheY by the binding of the ligand results in a stable forward swimming state along an increasing attractant gradient.