Yang Y, Inouye M
Department of Biochemistry, University of Medicine and Dentistry of New Jersey, Rutgers-Robert Wood Johnson Medical School, Piscataway 08854.
Proc Natl Acad Sci U S A. 1991 Dec 15;88(24):11057-61. doi: 10.1073/pnas.88.24.11057.
Taz1 is a hybrid signal-transducing membrane receptor between Tar, an aspartate chemoreceptor, and EnvZ, an osmosensor of Escherichia coli that is able to induce ompC expression by phosphorylating OmpR (a transcriptional activator) in response to aspartate. When the Taz1 His-277, the proposed autophosphorylation site in the cytoplasmic EnvZ domain, was replaced with a valine residue, the mutant Taz1 was unable to induce ompC expression. Similarly, when approximately two-thirds of the EnvZ domain was deleted, Taz1 was nonfunctional. However, when these two defective Taz1 proteins were coexpressed in a cell, ompC was constitutively expressed. Coinciding with this result, two mutant Taz1 molecules were able to intermolecularly complement each other to restore the OmpR kinase activity but not phosphatase activity in vitro. The identical result was also obtained with EnvZ. The present results suggest that the autophosphorylation of Taz1 and EnvZ is an intermolecular phosphorylation reaction, requiring formation of a dimer (or oligomer), and that ligand-dependent ompC expression requires not only kinase but also phosphatase activity.
Taz1是一种介于Tar（一种天冬氨酸化学感受器）和EnvZ（大肠杆菌的一种渗透压感受器）之间的混合信号转导膜受体，它能够通过响应天冬氨酸使OmpR（一种转录激活因子）磷酸化来诱导ompC表达。当Taz1的His-277（推测为细胞质EnvZ结构域中的自磷酸化位点）被缬氨酸残基取代时，突变型Taz1无法诱导ompC表达。同样，当EnvZ结构域大约三分之二被缺失时，Taz1无功能。然而，当这两种有缺陷的Taz1蛋白在细胞中共表达时，ompC组成型表达。与此结果一致，两个突变型Taz1分子能够在分子间相互互补，以恢复体外OmpR激酶活性，但不能恢复磷酸酶活性。EnvZ也得到了相同的结果。目前的结果表明，Taz1和EnvZ的自磷酸化是一种分子间磷酸化反应，需要形成二聚体（或寡聚体），并且依赖配体的ompC表达不仅需要激酶活性，还需要磷酸酶活性。
