Evolution of bacterial flagellar motors studied by electron cryo-tomography


What evolutionary principles underlie the increasing diversity of life on earth? To tackle this question, we used electron cryo-tomography to study the diversification of the structures and mechanical outputs of molecular machines in bacteria, with a focus on bacterial flagellar motors. Flagellar motors are ~50-nm diameter molecular rotary motors that span the bacterial cell envelope to drive rotation of a multi-micron filament to coil and function as a helical propeller.

We recently showed that flagellar motors have diversified into a range of variants while maintaining their core function of cellular motility. To understand how this diversity arose, we first asked what the selective benefits of diversification may have been. Diverse bacterial flagellar motors produce different torques, leading us to hypothesize that torque was a selective benefit of modifying motor structure. To explore this speculation, we combined genetic analyses with high-throughput electron cryo-tomography subtomogram averaging to determine in situ structures of flagellar motors that produce different torques.

For the first time, our results unambiguously locate the torque-generating stator complexes and show that diverse high-torque motors use variants of an ancestrally related family of structures to scaffold incorporation of additional stator complexes at wider radii from the axial driveshaft than in the model Escherichia coli and Salmonella enterica motors. We identified the protein components of these additional scaffold structures and elucidate their sequential assembly, demonstrating that they are required for stator-complex incorporation. These proteins are widespread, suggesting that different bacteria have tailored torques to specific environments by scaffolding alternative stator placement and number.

Our results quantitatively account for different torques, complete the assignment of the locations of the major flagellar components, and provide crucial constraints for understanding mechanisms of torque generation and the evolution of multiprotein complexes. I will conclude by discussing possible pathways to evolve this diversity in the flagellar motors.