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The BglF Sensor Recruits the BglG Transcription Regulator to the Membrane and Releases It on Stimulation
Livnat Lopian, Anat Nussbaum-Shochat, Kathryn O'Day-Kerstein, Andrew Wright and Orna Amster-Choder
Proceedings of the National Academy of Sciences of the United States of America
Vol. 100, No. 12 (Jun. 10, 2003), pp. 7099-7104
Published by: National Academy of Sciences
Stable URL: http://www.jstor.org/stable/3139438
Page Count: 6
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The Escherichia coli BglF protein is a sugar-sensor that controls the activity of the transcriptional antiterminator BglG by reversibly phosphorylating it, depending on β-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 β-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.
Proceedings of the National Academy of Sciences of the United States of America © 2003 National Academy of Sciences