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Ligand Binding to the Receptor Domain Regulates the Ratio of Kinase to Phosphatase Activities of the Signaling Domain of the Hybrid Escherichia coli Transmembrane Receptor, Taz1

https://doi.org/10.1006/jmbi.1993.1404Get rights and content

Abstract

Taz1 is a hybrid receptor, in which the periplasmic receptor domain of Tar, an aspartate chemoreceptor, is fused with the cytoplasmic signaling domain of EnvZ, an osmosensor. Taz1 is able to induce ompC-lacZ expression in response to aspartate added to the medium. We introduced amino acid substitution mutations in the highly conserved region of the signaling domain of Tar near the Tar-EnvZ junction. The same mutations in Tsr, a serine chemoreceptor, are known to lock the flagella rotation in either a clockwise (CW) or in a counter-clockwise (CCW) mode. It was found that a CW-biased mutation in Taz1 resulted in ompC-lacZ expression in the "off mode", or low ompC-lacZ expression in both the absence and presence of aspartate, while CCW-biased mutations caused ompC-lacZ expression in the "on mode", or constitutive expression regardless of aspartate. The OmpR kinase and phospho-OmpR phosphatase activities of the wild-type and mutant Taz proteins were also examined in response to aspartate. The phosphatase activity of the wild-type Taz1 was found to decrease in the presence of aspartate, while the OmpR kinase activity remained constant. This indicated that aspartate binding to the Taz1 receptor domain modulates the ratio of kinase to phosphatase activity of the signaling domain. An increased kinase to phosphatase ratio in the presence of aspartate resulted in higher levels of phospho-OmpR in the cell and therefore induced ompC-lacZ expression. In contrast to the wild-type Taz1 protein, the enzymatic activities of CW as well as CCW mutants did not change in response to aspartate, indicating that mutant Taz proteins are incapable of transducing the signal across the membrane as a result of a locked conformation of the signaling domain in either the on or off mode.

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    In some two-component signaling systems, dephosphorylation of the phosphorylated RR via the HK (the so-called “phosphatase activity”) limits the level of the activated RR and serves to reset the system. Although it was proposed that EnvZ phosphatase activity was reduced in the presence of high osmolality (Jin and Inouye, 1993), Fig. 3 clearly shows that high osmolality leads to a direct effect on EnvZ activation via autophosphorylation (Wang et al., 2012). These results are consistent with ATPase measurements of EnvZ-stimulated OmpR ∼ P dephosphorylation at in-vivo ratios of EnvZ to OmpR (Kenney, 2010).

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    The level of Taz1 was estimated to be ∼20-fold higher than normal EnvZ levels. Both Taz1 autophosphorylation and phosphotransfer to OmpR were not affected by aspartate, but extremely small aspartate-induced decreases in phosphatase activity were observed [49]. A concern about the physiological relevance of the aspartate response mediated by Taz1 is that, in contrast to Tar, Taz1 requires very high concentrations of aspartate to stimulate ompC-lacZ expression.

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