Artwork: Dan Nowakowski/Nicholas Taylor


Mechano-adaptation in a large protein complex

Soft, Living, Active and Adaptive Matter (SLAAM) Seminar
November 2021

Navish Wadhwa
Harvard University

                      

slides for this talk: WadhwaLab.com/talks

Machines perform specific tasks for us

Cells use molecular machines to perform specific tasks

ATP synthase

Ribosome

Replisome

Do cells have smart machines too?

Yes, they do.

We use the bacterium E. coli as a model organism


Inhabits guts of mammals


Some strains are harmful, most harmless


Multiple flagella per cell


Turner et al., J. Bacteriol., 2000

Bacteria are the smallest self-propelled swimmers

Many bacteria swim by rotating helical flagella

Slowed down 20 times




Turner et al., J. Bacteriol., 2000

A nanoscale motor powers
swimming in bacteria

Motor torque depends upon its rotation speed

Chen and Berg, Biophys. J., 2000
Yuan et al, PNAS, 2010

Automatic gearshift in cars allows the engine to adapt to changing terrains




Automatic gearshift in E. coli allows the motor to adapt to changing loads

Lele et al., PNAS, 2013

What is the physical and molecular mechanism underlying this automatic gearshift?

How can we change motor load?

Instantaneously

Reversibly

Controllably

Electrorotation allows
full control over motor load

Instantaneous

Reversible

Controllable

Electrorotation allows
full control over motor load

A change in load triggers stepwise changes in motor speed

Wadhwa et al., PNAS, 2019

The motor adapts to changes
in load by remodeling its stator


Wadhwa et al., PNAS, 2019
Wadhwa et al., PNAS, 2021

Remodeling kinetics depend on electrorotation speed

Wadhwa et al., PNAS, 2019

Higher electrorotation speed leads to lower torque

Hypothesis
Stator remodeling depends on torque

A quantitative model for stator assembly




We extracted the on rate ($k_+$) and the off rate ($k_-$) from the data


Wadhwa et al., PNAS, 2019

The off-rate decreases with torque


Free energy of the bound state decreases with torque

The off-rate decreases exponentially with torque

Molecular mechanism for torque-dependent unbinding rate

Low torque

High torque

Torque anisotropy allows us to test the model

Collapse of CCW and CW data validates the model

Wadhwa et al., PNAS, 2021

Conclusions and outlook


Cars and bacteria use different approaches


Cars adapt the transmission while bacteria adapt the engine itself



Flagellar motor is a bacterial mechanosensor






Molecular machines are not static, fixed structures, but flexible, dynamic, and responsive.

Acknowledgements


Howard Berg (Harvard)


Yuhai Tu (IBM)


Rob Phillips (Caltech)


Alberto Sassi (IBM)


NIH Pathway to Independence Award