Domain-swap polymerization drives the self-assembly of the bacterial flagellar motor.
Baker MA., Hynson RM., Ganuelas LA., Mohammadi NS., Liew CW., Rey AA., Duff AP., Whitten AE., Jeffries CM., Delalez NJ., Morimoto YV., Stock D., Armitage JP., Turberfield AJ., Namba K., Berry RM., Lee LK.
Large protein complexes assemble spontaneously, yet their subunits do not prematurely form unwanted aggregates. This paradox is epitomized in the bacterial flagellar motor, a sophisticated rotary motor and sensory switch consisting of hundreds of subunits. Here we demonstrate that Escherichia coli FliG, one of the earliest-assembling flagellar motor proteins, forms ordered ring structures via domain-swap polymerization, which in other proteins has been associated with uncontrolled and deleterious protein aggregation. Solution structural data, in combination with in vivo biochemical cross-linking experiments and evolutionary covariance analysis, revealed that FliG exists predominantly as a monomer in solution but only as domain-swapped polymers in assembled flagellar motors. We propose a general structural and thermodynamic model for self-assembly, in which a structural template controls assembly and shapes polymer formation into rings.