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Catch Bond Drives Stator Mechanosensitivity in The Catch bond drives stator mechanosensitivity in the bacterial flagellar motor Ashley L Norda, Emilie Gachona, Ruben Perez-Carrascob, Jasmine A. Nirodyc, Alessandro Barduccia, Richard M. Berryd, and Francesco Pedacia,1 aCentre de Biochimie Structurale (CBS), INSERM, CNRS, Universite´ de Montpellier, 34090 Montpellier, France; bDepartment of Mathematics, University College London, London WC1E 6BT, United Kingdom; cBiophysics Graduate Group, University of California, Berkeley, CA 94720; and dDepartment of Physics, Clarendon Laboratory, University of Oxford, Oxford OX1 2JD, United Kingdom Edited by Steven M. Block, Stanford University, Stanford, CA, and approved October 27, 2017 (received for review September 11, 2017) The bacterial flagellar motor (BFM) is the rotary motor that rotates function, and sensitivity, they share one key feature: The confor- each bacterial flagellum, powering the swimming and swarming mational state, and thus the function, of the protein is directly of many motile bacteria. The torque is provided by stator units, dependent upon mechanical stress, mediated by the surrounding ion motive force-powered ion channels known to assemble and cell membrane. Two recent works have shown that stator recruit- disassemble dynamically in the BFM. This turnover is mechanosen- ment in the BFM depends on the viscous load placed upon the sitive, with the number of engaged units dependent on the vis- motor (17, 18). The property of mechanosensing (likely relevant cous load experienced by the motor through the flagellum. How- for the cell to overcome local inhomogeneities and obstacles) ever, the molecular mechanism driving BFM mechanosensitivity is has important consequences for the interpretation of previous unknown. Here, we directly measure the kinetics of arrival and data and ultimately for successfully modeling the torque gener- departure of the stator units in individual motors via analysis ation of the BFM. It implies, in fact, that previously measured of high-resolution recordings of motor speed, while dynamically torque–speed relationships (11, 21–23) are likely composed of varying the load on the motor via external magnetic torque. The motors with a dynamically changing number of stator units. The- kinetic rates obtained, robust with respect to the details of the oretical models must now take this novel fact into consider- applied adsorption model, indicate that the lifetime of an assem- ation (12). bled stator unit increases when a higher force is applied to its Here, to better elucidate the molecular mechanism responsi- anchoring point in the cell wall. This provides strong evidence ble for the mechanosensitivity of the BFM, we investigate sin- BIOPHYSICS AND that a catch bond (a bond strengthened instead of weakened by gle motors and quantify stator stoichiometry. Using an exter- COMPUTATIONAL BIOLOGY force) drives mechanosensitivity of the flagellar motor complex. nal magnetic field and magnetic microbeads of different sizes These results add the BFM to a short, but growing, list of sys- bound to the hook of individual E. coli motors, we rapidly manip- tems demonstrating catch bonds, suggesting that this “molecu- ulate the load experienced by the motor by reversibly stalling lar strategy” is a widespread mechanism to sense and respond to its rotation. The external load manipulation directly probes the mechanical stress. We propose that force-enhanced stator adhe- mechanosensitivity of the BFM: We stimulate stator binding sion allows the cell to adapt to a heterogeneous environmental during the period of stall and observe and quantify stator unbind- viscosity and may ultimately play a role in surface-sensing during ing after release. We perform these experiments for various swarming and biofilm formation. viscous loads, each of which imposes a different initial stator occupancy. This allows us to statistically characterize the kinetics bacterial flagellar motor j molecular motor j mechanosensitivity j of stator stoichiometry both in steady-state conditions and fol- catch bond j Escherichia coli lowing a rapid change in external load. Our analysis suggests that a catch-bond mechanism (a bond counterintuitively strength- ened, instead of weakened, by force) (24–26) is at the heart of he bacterial flagellar motor (BFM) is a large molecular com- Tplex found in many species of motile bacteria which actively rotates each flagellum of the cell, enabling swimming, chemo- Significance taxis, and swarming (1, 2). The rotor of the BFM is embedded within and spans the cellular membranes, coupling rotation to The bacterial flagellar motor (BFM) is the rotary motor power- the extracellular hook and flagellar filament. Multiple transmem- ing swimming of many motile bacteria. Many of the compo- brane complexes, called stator units, are anchored around the nents of this molecular machine are dynamic, a property which perimeter of the rotor and bound to the rigid peptidoglycan (PG) allows the cell to optimize its behavior in accordance with the layer (3–5). By harnessing the ion motive force, the stator units surrounding environment. A prime example is the stator unit, are responsible for torque generation, acting upon the common a membrane-bound ion channel that is responsible for apply- ring formed by FliG proteins on the cytosolic side of the rotor ing torque to the rotor. The stator units are mechanosensitive, (Fig. 1A) (6, 7). with the number of engaged units dependent on the viscous Several studies have revealed the continuous exchange of var- ious BFM molecular constituents (8–10), demonstrating that a load on the motor. We measure the kinetics of the stators as a static model for the BFM structure is not adequate. A prime function of the viscous load and find that the mechanosensi- example, and in contrast to macroscopic rotary motors, the stator tivity of the BFM is governed by a catch bond: a counterintu- of the BFM is dynamic; while each bound stator unit acts upon itive type of bond that becomes stronger under force. the rotor independently (11, 12), their stoichiometry in the motor varies. Once anchored around the rotor, stator units dynamically Author contributions: A.L.N. and F.P. designed research; A.L.N. and E.G. performed research; R.P.-C. contributed new reagents/analytic tools; A.L.N., R.P.-C., J.A.N., A.B., turn over, eventually diffusing away in the inner membrane (10). R.M.B., and F.P. analyzed data; and A.L.N., R.M.B., and F.P. wrote the paper. Each additional recruited stator unit increases the total torque and thus the measured rotational speed of the motor (11, 13), The authors declare no conflict of interest. and up to 11 units have been observed to engage in an individual This article is a PNAS Direct Submission. motor in Escherichia coli (10, 14–16). This open access article is distributed under Creative Commons Attribution- Recently, novel observations revealed that the stator units NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND). are also mechanosensors (17–19). A variety of mechanosensitive 1To whom correspondence should be addressed. Email: [email protected]. membrane protein complexes exist in all three kingdoms of life This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. (20). Although these complexes vary widely in their structure, 1073/pnas.1716002114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1716002114 PNAS Early Edition j 1 of 6 Downloaded by guest on September 25, 2021 laser held at stall for 300 s. The magnets are then raised to the original A B bright field Hook height; the load is returned to that supplied by the bead in its vis- cous environment; and the rotation of the motor is recorded for L ring OM at least another 5 min. Each movement of the magnets occurs within 3 s. The load felt by the BFM is therefore quickly and P ring PG B dynamically manipulated twice during the measurement of an Mot B Mot A individual motor; an exemplification of this procedure is shown Rod in the torque–speed plane in Fig. 1D. The torque of individual MS ring IM motors is measured first at steady state before stall and then again immediately after stall. An example torque trace for the FliG flagellar motor viscous load γ500 is shown in Fig. 1 C, Upper (see also Fig. S1 FliM ref. bead C ring E.coli for a collection of individual traces at different viscous loads). FliN An increase in torque during stall is evident, followed by a step- 5 min C stall D wise relaxation to a torque value close to the original steady- state value. release load line Viscous Load Dependency of Stator Assembly Dynamics. Under the assumption that torque traces represent noisy constant signals demarcated by discrete discontinuities due to stator association or dissociation, we use a recently developed step-detection algo- stall steady-state Torque rithm (30, 31) to fit the individual torque traces. By using this fit, a developed algorithm (Materials and Methods and SI Materials and Methods) is used to calculate stator stoichiometry, extracting stator number as a function of time, N (t), for each individual Speed trace. Contrary to previous works, this algorithm determines sta- tor stoichiometry based upon the discrete discontinuities of the Fig. 1. The experimental assay. (A) Schematic of the BFM showing rotor torque traces, not upon the absolute value of the torque; given and stator protein complexes. Stator units bind to the PG at the periph- the broad distributions of single-stator torque values (Fig. 2A), ery of the rotor, providing torque via an interaction with FliG. IM, inner this approach greatly reduces the error in stator stoichiometry membrane; OM, outer membrane. (B) Experimental setup. Bacterial cells estimation. Simulations (Fig. S2) suggest that, given the aver- are immobilized onto a coverslip, and a rotating superparamagnetic bead age noise in our torque measurements, the algorithms used here attached to the hook of a BFM is imaged and tracked.
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