(193z) Mechanical Force-Based Regulation of Protein Assemblies

                                             Mechanical force-based
regulation of protein assemblies

Ravi Chawla, Katie M. Ford, Pushkar P.
Lele*

Artie McFerrin Department of Chemical
Engineering

Texas A&M University, College
Station, TX-77843

*Corresponding author: plele@tamu.edu

The bacterial flagellar motor undergoes structural
changes in order to adapt to long-lived perturbations in the cell’s thermal,
chemical, electrical and mechanical environments. Remodeling of motor-assemblies
improves the chances of survival. For example, the remodeling of the
motor-switch extends the range of signal detection and enhances the ability to
respond to chemical signals during chemotaxis.

The stator-complex that rotates the motor is
responsible for surface sensing and remodels in response to perturbations in
viscous loads [1]. The stator builds itself by recruiting additional stator
units under high loads; however the mechanism for such load-dependent
self-assembly is currently unknown. Here, we tested a hypothesis that the
amount of force generated by each stator-unit modulates its association with
the rotor. To do this, we measured stator-binding in mutants strains in which
the motors reportedly develop lower torque compared to wild-type motors. Our
measurements indicated that the stator retains at best one stator-unit when
delivering a low force to the rotor. On the other hand, when delivering a
higher force the stator retains as many as eleven stator-units. An analytical
model was developed that incorporated an exponential dependence of the off-rate
for individual units on the force delivered to the rotor. The model provided
accurate fits to measurements of stator-rotor binding over a range of loads. It
is possible that the tensile forces that develop when a stator-unit delivers a
high torque uncover cryptic binding sites that stabilize the stator-rotor
association.  Our experimental and modeling results represent the first steps
towards establishing a plausible mechanism for mechanical force-based
regulation of the flagellar stator.

[1] Lele et al., Proc Natl Acad Sci USA, 110 (29), 11839-11844, 2013.

[2] Chawla et al., Sci Rep, under review.