Biology subjects

Berg, H. C.

Publications and source records attributed to Berg, H. C..

5 recordsLinked to original sources

bioRxiv2021

A multi-state dynamical process underpins mechano-adaptation in the bacterial flagellar motor

Adaptation is a defining feature of living systems. The bacterial flagellar motor adapts to changes in the external mechanical environment by adding or removing torque-generating stator units. However, the molecular mechanism for mechanosensitive motor remodeling remains unclear. Here, we induced stator disassembly using electrorotation, followed by the time-dependent assembly of the individual stator units into the motor after electrorotation was terminated. From these experiments, we extracted detailed statistics of the dwell times that comprise the stochastic dynamics of the binding and unbinding of stator units. Dwell times reveal multiple timescales, indicating the existence of multiple binding states of the stator units. Based on these results, we propose a minimal model in which the stator unit can occupy four different states - two bound states with very different rates of unbinding, a diffusive unbound state, and a transiently detached state. Our minimal model quantitatively explains multiple features of the experimental data and allows us to determine the transition rates among all four states. Our experiments and modeling suggest a mechanism of mechano-adaptive remodeling of the bacterial flagellar motor in which torque generated by bound stator units controls their effective unbinding rate by modulating the transition between the two bound states. Furthermore, the binding rate of stator units with the motor has a non-monotonic dependence on the number of bound units, likely because of two counteracting effects of motor rotation on the binding process.

biophysics↗

bioRxiv2021

Mechanosensitive remodeling of the bacterial flagellar motor is independent of direction of rotation

Motility is critical for the survival and dispersal of bacteria, and it plays an important role during infection. How bacteria regulate motility is thus a question of broad interest. Regulation of bacterial motility by chemical stimuli is well studied, but recent work has added a new dimension to the problem of motility control. The bidirectional flagellar motor of the bacterium Escherichia coli recruits or releases torque-generating units (stator units) in response to changes in load. Here, we show that this mechanosensitive remodeling of the flagellar motor is independent of direction of rotation. Remodeling rate constants in clockwise rotating motors and in counterclockwise rotating motors, measured previously, fall on the same curve if plotted against torque. Increased torque decreases the off rate of stator units from the motor, thereby increasing the number of active stator units at steady state. A simple mathematical model based on observed dynamics provides quantitative insight into the underlying molecular interactions. The torque-dependent remodeling mechanism represents a robust strategy to quickly regulate output (torque) in response to changes in demand (load). SignificanceMacromolecular machines carry out most of the biological functions in living organisms. Despite their significance, we do not yet understand the rules that govern the self-assembly of large multi-protein complexes. The bacterial flagellar motor tunes the assembly of its torque-generating stator complex with changes in external load. Here, we report that clockwise and counterclockwise rotating motors have identical remodeling response to changes in the external load, suggesting a purely mechanical mechanism for this regulation. Autonomous control of self-assembly may be a general strategy for tuning the functional output of protein complexes. The flagellar motor is a prime example of a macromolecular machine in which the functional regulation of assembly can be rigorously studied.

biophysics↗

bioRxiv2020

Structure and function of stator units of the bacterial flagellar motor

Many bacteria use the flagellum for locomotion and chemotaxis. Its bi-directional rotation is driven by the membrane-embedded motor, which uses energy from the transmembrane ion gradient to generate torque at the interface between stator units and rotor. The structural organization of the stator unit (MotAB), its conformational changes upon ion transport and how these changes power rotation of the flagellum, remain unknown. Here we present ~3 [A]-resolution cryo-electron microscopy reconstructions of the stator unit in different functional states. We show that the stator unit consists of a dimer of MotB surrounded by a pentamer of MotA. Combining structural data with mutagenesis and functional studies, we identify key residues involved in torque generation and present a mechanistic model for motor function and switching of rotational direction. One Sentence SummaryStructural basis of torque generation in the bidirectional bacterial flagellar motor

biophysics↗

bioRxiv2018

Cargo Transport Shapes the Spatial Organization of a Microbial Community

The human microbiome is an assemblage of diverse bacteria that interact with one another to form communities. Bacteria in a given community are arranged in a three-dimensional matrix with many degrees of freedom. Snapshots of the community display well-defined structures, but the steps required for their assembly are not understood. Here, we show that this construction is carried out with the help of gliding bacteria. Gliding is defined as the motion of cells over a solid or semi-solid surface without the necessity of growth or the aid of pili or flagella. Genomic analysis suggests that gliding bacteria are present in human microbial communities. We focus on Capnocytophaga gingivalis which is present in abundance in the human oral microbiome. Tracking of fluorescently-labeled single cells and of gas bubbles carried by fluid flow shows that swarms of C. gingivalis are layered, with cells in the upper layers moving more rapidly than those in the lower layers. Thus, cells also glide on top of one another. Cells of non-motile bacterial species attach to the surface of C. gingivalis and are propelled as cargo. The cargo cell moves along the length of a C. gingivalis cell, looping from one pole to the other. Multi-color fluorescent spectral imaging of cells of different live but non-motile bacterial species reveals their long-range transport in a polymicrobial community. A swarm of C. gingivalis transports some non-motile bacterial species more efficiently than others and helps shape the spatial organization of a polymicrobial community.\n\nSignificanceWe describe a situation in which bacteria typical of the human oral microbiome are organized spatially by gliding cells, species of Capnocytophaga, that move backwards and forwards over the substratum. The mobile adhesins that pull the cells over the substratum also attach to cells of non-motile bacterial species, which are carried up and down the motile cells as cargo. The synchronized transport of non-motile cargo bacteria helps shape a polymicrobial community.

microbiology↗

bioRxiv2018

A molecular rack and pinion actuates a cell-surface adhesin and enables bacterial gliding motility

The mechanism for bacterial gliding is not understood. The gliding bacterium Flavobacterium johnsoniae is known to have an adhesin, SprB, that moves along the cell surface on a spiral track. When cells are sheared by passage of a suspension through thin tubing, they stop gliding but can be tethered by addition of an anti-SprB antibody. Tethered cells spin about 3 Hz. We labeled the Type 9 secretion system (T9SS) with a yellow-fluorescent-protein (YFP) fusion of GldL. When labeled cells were tethered, a yellow fluorescent spot was found near the rotation axis, which shows that the motor that drives the rotation localizes with the T9SS. The spiral track was labeled by following the motion of Cy3 attached to SprB via an antibody. The distance between the rotation axis and the track was determined by a measurement involving both labels, YFP and Cy3, yielding 90 nm. If a rotary motor spins a pinion of radius 90 nm 3 Hz, a spot on its periphery will move 1.5 m/s, the speed at which cells glide. We suggest that the pinion drives a flexible tread that carries SprB along a track fixed to the cell surface. Cells glide when such an adhesin adheres to the solid substratum.

microbiology↗