Evolutionary assembly of the flagellar cage: repurposing a Type IV pili subcomplex

Flagella, the major motility organelles in bacteria, display great diversity and complexity in their motor structures. Aside from the classical model with a simple motor, the functions of the components of complex motors are beginning to be explored. Recently, flagellar motors in Campylobacterota were found to have homologs of Type IV pili (T4P). Here, we review how these homologs form a peripheral cage structure in the motor, enclosing stator units and influencing motor assembly and function. We highlight structural features, including conformational variability and a proposed two-step assembly process, and synthesize evidence for their possible origin via exaptation of T4P components in the common ancestor of Campylobacterota. This work focuses on the evolution, structure, and functional integration of a co-opted subcomplex, shedding light on the re-engineering of nanomachines.