Abstract
The sensor protein KdpD of Escherichia coli is composed of a large N-terminal hydrophilic region (aa 1–400), four transmembrane regions (aa 401–498) and a large hydrophilic region (aa 499–894) at the C-terminus. KdpD requires the signal recognition particle (SRP) for its targeting to the membrane. Deletions within KdpD show that the first 50 residues are required for SRP-driven membrane insertion. A fusion protein of the green fluorescent protein (GFP) with KdpD is found localized at the membrane only when SRP is present. The membrane targeting of GFP was not observed when the first 50 KdpD residues were deleted. A truncated mutant of KdpD containing only the first 25 amino acids fused to GFP lost its ability to specifically interact with SRP, whereas a specific interaction between SRP and the first 48 amino acids of KdpD fused to GFP was confirmed by pull-down experiments. Conclusively, a small amphiphilic region of 27 residues within the amino-terminal domain of KdpD (aa 22–48) is recognized by SRP and targets the protein to the membrane. This shows that membrane proteins with a large N-terminal region in the cytoplasm can be membrane-targeted early on to allow co-translational membrane insertion of their distant transmembrane regions.
Highlights
The KdpD protein of Escherichia coli is a membrane component of the KdpD/E sensory system involved in main-taining the intracellular osmolarity
The sensor protein KdpD of Escherichia coli is composed of a large N-terminal hydrophilic region, four transmembrane regions and a large hydrophilic region at the C-terminus
A fusion protein of the green fluorescent protein (GFP) with KdpD is found localized at the membrane only when signal recognition particle (SRP) is present
Summary
The KdpD protein of Escherichia coli is a membrane component of the KdpD/E sensory system involved in main-taining the intracellular osmolarity. In cases of low K+ concentrations in the medium, the expression of the highaffinity K+ pump KdpFABC is induced (Epstein and Davis, 1970). In this signalling cascade, the C-terminal domain of KdpD is phosphorylated most likely at His673 (Voelkner et al, 1993). The insertion of the transmembrane regions occurs independently of SecA, SecE and YidC, but involves the electrochemical membrane potential (Facey and Kuhn, 2003). The mutant showed the same insertion characteristics for the translocation of the P1 region as the wild-type protein and was Sec- and YidC-independent. It was concluded that short periplasmic loops can translocate without assistance of other known proteins (Facey and Kuhn, 2003)
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