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Branch, Richard

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Branch, Richard
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    Adaptive Remodelling by FliN in the Bacterial Rotary Motor
    (Elsevier BV, 2014) Branch, Richard; Sayegh, Michael; Shen, Chong; Nathan, Vedavalli S.J.; Berg, Howard
    Sensory adaptation in the Escherichia coli chemosensory pathway has been the subject of interest for decades, with investigation focusing on the receptors that process extracellular inputs. Recent studies demonstrate that the flagellar motors responsible for cell locomotion also play a role, adding or subtracting FliM subunits to maximise sensitivity to pathway signals. It is difficult to reconcile this FliM remodelling with the observation that partner FliN subunits are relatively static fixtures in the motor. By fusing a fluorescent protein internally to FliN, we show that there is in fact significant FliN remodelling. The kinetics and stoichiometry of FliN in steady state and in adapting motors are investigated and found to match the behaviour of FliM in all respects except for timescale where FliN rates are about 4 times slower. We notice that motor adaptation is slower in the presence of the fluorescent protein, indicating a possible source for the difference. The behaviour of FliM and FliN is consistent with a kinetic and stoichiometric model that contradicts the traditional view of a packed, rigid motor architecture.
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    Adaptation at the output of the chemotaxis signalling pathway
    (2012) Yuan, Junhua; Branch, Richard; Hosu, Basarab; Berg, Howard
    In the bacterial chemotaxis network, receptor clusters process input1–3, and flagellar motors generate output4. Receptor and motor complexes are coupled by the diffusible protein CheY-P. Receptor output (the steady-state concentration of CheY-P) varies from cell to cell5. However, the motor is ultrasensitive, with a narrow [CheY-P] operating range6. How might the match between receptor output and motor input be optimized? Here we show that the motor can shift its operating range by changing its composition. The number of FliM subunits in the C-ring increases in response to a decrement in the concentration of CheY-P, increasing motor sensitivity. This shift in sensitivity explains the slow partial adaptation observed in mutants that lack the receptor methyltransferase and methylesterase7–8 and why motors exhibit signal-dependent FliM turnover9. Adaptive remodelling is likely to be a common feature in the operation of many molecular machines.