The Che proteins are cytoplasmic proteins that are part of a signal transduction pathway between the attractant or repellent and the flagellar motor switch (1). CheA is histidine kinases which belongs to the class of signaling proteins. It has conserved amino acid sequences at the carboxy-terminal domain which is phosphorylated at a histidine residue, and transfers the phosphoryl group to a response regulator protein. CheY and CheB are response regulator proteins. They have amino-terminal domains similar to the other response regulators and undergo phosphorylation and dephosphorylation.

Chemoreceptor proteins for the chemoeffector are built into the bacterial cell membrane. The chemoreceptor proteins are also called receptor-transducer proteins, or simply transducer proteins. The cytoplasmic domain interacts with cytoplasmic components such as the CheW and CheA proteins, whereas the periplasmic domain interacts with chemoeffectors or periplasmic proteins to which athe chemoeffectors are bound. The cytoplasmic domain of the chemoreceptor proteins also contains four or five glutamate residues that become methylated and demethylated as part of the adapation response. Therefore, the chemoreceptor proteins are also called methyl-accepting chemotaxis proteins (MCPs). Treponema pallidum probably has four different MCPs. These are the Tsr proteins (taxis to serine and away from repellents), Tar (taxis to aspartate and maltose and away from repellents), Trg (taxis to ribose, glucose, and galactose), and Tap (taxis to dipeptides). The cytoplasmic domains of the receptor transducer proteins (tar, Tsr, Trg, and Tap) control the autophosphorylation activity of CheA in an activity that requires CheW. CheA phosphorylates CheY which has an autophosphatase activity. In the nonstimulated state there is a steady-state level of CheY-P that is determined by the rate of phosphorylation of CheY and the rate of dephosphorylation of CheY-P. CheY-P is a response regulator protein. It binds to the flagellar motor switch, FliM, FliG, FliNand initiates a reversal of ht motor so that it rotates in a CW (clockwise) direction, causing the cell to tumble (3). In the nonstimulated cell, there is a certain intermediate level of CheY-P that supports normal run-tumble behavior.

When an attractant or repellent stimulate a chemoreceptor protein, a signal is sent to the
cytoplasmic region of the receptor-transducer protein. The signal is presumably a conformational change in the periplasmic region of the chemoreceptor protein that is propagated to the cytoplasmic region of the chemoreceptor protein, which interacts with CheW/CheA. An attractant decreases the rate of autophosphorylation of CheA, and a repellent increases the rate of autophosphorylation. Thus attractants promote CCW (counterclockwise) rotation and smooth swimming, whereas repellents promote CW (clockwise) rotation and tumbling.

A model for signal transduction during chemotaxis in Treponema pallidum.

 
Reference
1.Iino, T., Y. Komeda, K. Kutsukake, R. M. Macnab, P. Matsumura, J. S. Parkinson, M. I. Simon, and S. Yamaguchi. 1988. New unified nomenclature for the flagellar genes of Escherichia coli and Salmonella typhimurium. microbiological reviews. 52(4):533-5.
2.Stock, J. B., A. J. Ninfa, and A. M. Stock. 1989. Protein phosphorylation and regulation of adaptive responses in bacteria. microbiological reviews. 53(4):450-90.
3.Welch, M., K. Oosawa, S. Aizawa, and M. Eisenbach. 1993. Phosphorylation-dependent binding of a signal molecule to the flagellar switch of bacteria. proceedings of the national academy of sciences of the united states of america. 90(19):8787-91.

This analysis was prepared by Gary Xie, and Staff. Please direct questions concerning this analysis to Gary Xie.