Architecture of the Type IVA Pilus Machine

Type IV pili are part of a widespread superfamily of bacterial and archaeal cell surface structures important for biofilm formation, motility, host adhesion, predation, DNA uptake, protein secretion and virulence. Type IVa pili are anchored in the cell envelope and undergo cycles of extension, adhesion to surfaces and retraction thereby pulling cells forward. The extension/retraction cycles are powered by the type IVa pilus machine (T4PM), the strongest molecular motor identified. Here, we use electron cryotomography of intact Myxococcus xanthus cells to solve the complete three-dimensional architecture of wild-type T4PM in situ in the piliated and non-piliated states at 3-4 nm resolution. T4PM comprise a multi-layered structure that spans the entire cell envelope including an outer membrane pore, four interconnected ring structures in the periplasm and cytoplasm, a cytoplasmic disc and dome, and a periplasmic stem. Comparison of the two structures reveal that piliation is accompanied by large conformational changes. By systematically imaging mutants lacking defined T4PM proteins or with individual proteins fused to tags, the locations of all ten T4PM core components and minor pilins are mapped to these structural features revealing the overall architecture and component map of T4PM. Using available atomic structures, the component maps were informed by building hypothetical models of both piliated and non-piliated forms, which fit the EM maps well and satisfy all known connectivities and structural constraints. The architecture of the T4PM provides new mechanistic insights into pilus extension and retraction and explains the enigmatic switch from extension to retraction