The BglF sensor recruits the BglG transcription regulator to the membrane and releases it on stimulation.
The Escherichia coli BglF protein is a sugar-sensor that controls the activity of the transcriptional antiterminator BglG by reversibly phosphorylating it, depending on beta-glucoside availability. BglF is a membrane-bound protein, whereas BglG is a soluble protein, and they are both present in the cell in minute amounts. How do BglF and BglG find each other to initiate signal transduction efficiently? Using bacterial two-hybrid systems and the Far-Western technique, we demonstrated unequivocally that BglG binds to BglF and to its active site-containing domain in vivo and in vitro. Measurements by surface plasmon resonance corroborated that the affinity between these proteins is high enough to enable their stable binding. To visualize the subcellular localization of BglG, we used fluorescence microscopy. In cells lacking BglF, the BglG-GFP fusion protein was evenly distributed throughout the cytoplasm. In contrast, in cells producing BglF, BglG-GFP was localized to the membrane. On addition of beta-glucoside, BglG-GFP was released from the membrane, becoming evenly distributed throughout the cell. Using mutant proteins and genetic backgrounds that impede phosphorylation of the Bgl proteins, we demonstrated that BglG-BglF binding and recruitment of BglG to the membrane sensor requires phosphorylation but does not depend on the individual phosphorylation sites of the Bgl proteins. We suggest a mechanism for rapid response to environmental changes by preassembly of signaling complexes, which contain transcription regulators recruited by their cognate sensors-kinases, under nonstimulating conditions, and release of the regulators to the cytoplasm on stimulation. This mechanism might be applicable to signaling cascades in prokaryotes and eukaryotes.
Lopian
Livnat
L
Department of Molecular Biology, The Hebrew University-Hadassah Medical School, P.O. Box 12272, Jerusalem 91120, Israel.
Nussbaum-Shochat
Anat
A
O'Day-Kerstein
Kathryn
K
Wright
Andrew
A
Amster-Choder
Orna
O
D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism*; D005960:Glucosides / Q000378:metabolism; D007700:Kinetics; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism*; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction; D020349:Surface Plasmon Resonance
D001426:Bacterial Proteins; C079638:BglF protein, E coli; D029968:Escherichia coli Proteins; D005960:Glucosides; D008565:Membrane Proteins; D016601:RNA-Binding Proteins; D011993:Recombinant Fusion Proteins; C064872:antiterminator proteins, Bacteria; D011494:Protein Kinases