Architecture of the type IVa pilus machine

How the bacterial pilus works Many bacteria, including important pathogens, move by projecting grappling-hook–like extensions called type IV pili from their cell bodies. After these pili attach to other cells or objects in their environment, the bacteria retract the pili to pull themselves forward. Chang et al. used electron cryotomography of intact cells to image the protein machines that extend and retract the pili, revealing where each protein component resides. Putting the known structures of the individual proteins in place like pieces of a three-dimensional puzzle revealed insights into how the machine works, including evidence that ATP hydrolysis by cytoplasmic motors rotates a membrane-embedded adaptor that slips pilin subunits back and forth from the membrane onto the pilus. Science, this issue p. 10.1126/science.aad2001 Structural models show how bacteria switch from pilus extension to retraction. INTRODUCTION Type IVa pili are bacterial cell surface structures that perform critical functions in motility, surface adhesion, virulence, and biofilm formation. Type IVa pili are anchored in the cell envelope and pull cells forward through cycles of extension, adhesion to surfaces, and retraction, all powered by the type IVa pilus machine (T4PM). Although the structures and connectivities of the 10 core T4PM proteins and minor pilins have already been determined, the overall architecture of the T4PM and its extension and retraction mechanisms have not. RATIONALE To elucidate the architecture of the intact T4PM, we directly imaged T4PMs within intact Myxococcus xanthus cells by cryo–electron tomography. Mutants that either lacked T4PM components or contained individual T4PM proteins fused to a tag were then imaged. Difference maps revealed the locations of all components of the T4PM machine. Hypothetical models were then built by fitting the known atomic structures of the components together in their relative positions. RESULTS Both piliated and nonpiliated T4PMs are multilayered structures that span the entire cell envelope. T4PMs include an outer membrane pore, three interconnected periplasmic ring structures and another in the cytoplasm, a cytoplasmic disc and dome, and a periplasmic stem. The PilQ secretin forms the outer membrane pore; TsaP forms a periplasmic ring around PilQ; periplasmic domains of PilQ together with PilP constitute the mid-periplasmic ring; and the globular domains of PilO and PilN constitute the lower periplasmic ring and connect via coiled coils across the inner membrane to PilM, which forms the cytoplasmic ring. The cytoplasmic domains of the inner membrane protein PilC form the cytoplasmic dome on the T4PM axis inside the PilM ring. The short stem in the nonpiliated state is composed of minor pilins and PilA, the major subunit of the pilus. In the piliated state, the pilus extends from the cell exterior through the PilQ pore and the periplasmic rings to PilC in the inner membrane. In the piliated structure, the hexameric adenosine triphosphatases (ATPases) PilB and PilT bind in a mutually exclusive manner to the base of the T4PM, where they appear as the cytoplasmic disc during extensions and retractions, respectively. Next, we asked whether the known atomic structures of the proteins could be fit within the map where our imaging results indicated, while still satisfying all known constraints of size, connectivities, and interfaces. This successful effort resulted in “pseudo-atomic” working models of both states of the T4PM. The models suggest that through ATP hydrolysis, PilB rotates PilC, incrementally moving it into positions that facilitate incorporation of new PilA subunits one by one from the inner membrane onto the base of the growing helical pilus. Pilus retraction is driven by replacement of PilB with PilT, which rotates PilC into positions that promote PilA departure from the base of the pilus back into the membrane. CONCLUSION We determined the architecture of the T4PM in the piliated and nonpiliated states and mapped all known components onto this architecture, producing a complete structural map of the T4PM. The results illustrate how the structure and function of macromolecular complexes that defy purification and traditional structural approaches can nonetheless be interrogated through cryo–electron tomography of intact cells and model building. Bacterial type IVa pilus machine (T4PM). Two T4PMs are depicted on the left, spanning the envelope of a Gram-negative bacterial cell. T4PMs extend and retract pili to pull cells forward. The structural data presented here support the hypothesis that ATP hydrolysis in the cytoplasm causes an adapter protein in the inner membrane to rotate, facilitating the transfer of pilin subunits from the inner membrane onto the growing pilus. The process is reversed during retraction. Type IVa pili are filamentous cell surface structures observed in many bacteria. They pull cells forward by extending, adhering to surfaces, and then retracting. We used cryo–electron tomography of intact Myxococcus xanthus cells to visualize type IVa pili and the protein machine that assembles and retracts them (the type IVa pilus machine, or T4PM) in situ, in both the piliated and nonpiliated states, at a resolution of 3 to 4 nanometers. We found that T4PM comprises an outer membrane pore, four interconnected ring structures in the periplasm and cytoplasm, a cytoplasmic disc and dome, and a periplasmic stem. By systematically imaging mutants lacking defined T4PM proteins or with individual proteins fused to tags, we mapped the locations of all 10 T4PM core components and the minor pilins, thereby providing insights into pilus assembly, structure, and function.