Evolution of complexity in a molecular machine

What are the evolutionary mechanisms behind increasing complexity of molecular machines? To tackle this question, we focus on the diversification of structure and mechanical output in the bacterial flagellar motor family. To understand how complexity arises, we first asked what the selective benefits of diversification are.
It is known that diverse bacterial flagellar motors produce different torques, leading us to hypothesize that torque is a clear selective benefit of altering motor structure. To understand this better, we combined genetic analyses with 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 enteric motor. We identify 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.