EFFECT OF CYSTEINE REPLACEMENTS ON THE PROPERTIES OF THE TURGOR SENSOR KDPD OF ESCHERICHIA-COLI

Escherichia coli responds rapidly to K+-limitation or high osmolarity by induction of the kdpFABC operon coding for the high affinity K+-tra nslocating Kdp-ATPase. This process is controlled by the membrane-boun d histidine kinase KdpD and the response regulator KdpE. Here, it is d emonstrated that replacements of the native Cys residues at positions 409, 852, and 874 influence distinct activities of KdpD, whereas repla cements of Cys residues at positions 32, 256, and 402 have no effect. Replacements of Cys(409) in KdpD reveal that transmembrane domain I is important for perception and/or propagation of the stimulus. When Cys (409) is replaced with Ala, kdpFABC expression becomes constitutive re gardless of the external stimuli. In contrast, when Cys(409) is replac ed with Val or Tyr, induction of kdpFABC expression in response to dif ferent stimuli is drastically reduced. KdpD with Ser at position 309 s upports levels of kdpFABC expression comparable to those seen in wildt ype. Since neither the kinase nor phosphatase activity of these protei ns is affected, it is proposed that different amino acid side-chains a t position 409 alter the switch between the inactive and active forms of the kinase. When Cys(852) or Cys(874) is replaced with Ala or Ser, kinase activity is reduced to 10% of the wild-type level. However, kin etic studies reveal that the apparent ATP binding affinity is not affe cted. Surprisingly, introduction of Cys(852) and Cys(874) into a KdpD protein devoid of Cys residues leads to full recovery of the kinase ac tivity. Labeling studies support the idea that a disulfide bridge form s between these two residues. (C) 1998 Elsevier Science B.V. All right s reserved.