1.12'Pivot hypothesis': a signalling mechanism in a bacterial chemotaxis receptor

摘要: Bacteria in general can sense the increase in the concentration of attractants or decrease in repellent concentration in their environment and move toward attractants or away from repellents by suppressing tumbling motions caused by a clockwise rotation of their flagella (Adler 1975; Koshland 1988). Most bacterial chemotaxis receptors known so far in Escherichia coli and Salmonella typhimurium are composed of three domains: a ligand-binding domain; a transmembrane domain; and a cytoplasmic domain (Fig. 1 (a)). The signalling in the cytoplasm is accomplished by a fast process of phosphate transfer between several pairs of cytoplasmic proteins counterbalanced by phosphatases (Borkovich et al. 1989; Bourret et al. 1989), finally activating the rotor of each flagellum. This signalling is further modulated by a slower methylation and demethylation process of the cytoplasmic domains as an adaptation process (Russo and Koshland 1983). Although the biochemistry and regulation of the signalling by these proteins have been extensively studied, the mechanism of transmembrane signalling is not known. We present a simple, hypothetical mechanism for transmembrane signalling based on the crystal structures of a ligand-binding domain of a bacterial chemotaxis receptor with and without a bound ligand.