Osaka team resolves cryoEM structure of Salmonella P-ring on an elongated flagellar rod
The P-ring binds the rod as a slightly elliptical, tilted ring. L-ring assembly then forces it off, leaving a bushing for the rotating rod, the authors report.
Researchers at the University of Osaka, with Keiichi Namba as senior author, determined the cryoEM structure of the P-ring of the Salmonella flagellar motor assembled on a polyrod, an unusually elongated rod. The study was published in Communications Biology in 2026. The P-ring turned out to be slightly elliptical and tilted by 2.7° relative to the rod axis nature.com
Communications Biologynature.comOpen source ↗.
The bacterial flagellum enables swimming and consists of more than 30 different proteins. Its LP-ring acts as a bushing around the rod, which serves as a drive shaft, and supports high-speed rotation at low friction. The L-ring, P-ring and distal rod consist of FlgH, FlgI and FlgG, respectively.
Order of LP-ring assembly
The P-ring forms first, around the distal rod, and stays firmly attached to it until the L-ring assembles above it. FlgI monomers form the P-ring. FlgH monomers are translocated to the outer membrane and assemble on the P-ring after the rod grows and penetrates the peptidoglycan layer.
In the mature motor the rod rotates inside the LP-ring without physical contact. How the ring is positioned accurately during assembly and how it is later released from the rod had remained unclear, said first author Tomoko Yamaguchi.
Structure of the assembly intermediate
To capture the intermediate, the team analyzed a polyrod–P-ring complex (PaPR), in which multiple P-rings assemble around genetically modified elongated rods. Single particle cryoEM analysis showed that the elliptical, tilted shape increases the contact area with the rod surface, so the ring binds at a specific position on the rod nature.com
Communications Biologynature.comOpen source ↗.
In the PaPR the P-ring is tilted by about 3° relative to the rod axis and is slightly elliptical. In the basal body, the matured form, the P-ring is perpendicular to the rod axis and nearly a perfect circle.
When the L-ring forms on the tilted P-ring, steric clashes arise between the L-ring and the rod. According to the authors, these clashes detach the P-ring from the rod surface and let it adopt a circular shape, so that the LP-ring becomes a functional bushing around a freely rotating rod.
Namba noted that artificially reproducing the autonomous assembly of a complex biomolecular machine remains a major technological challenge, while the bacterial flagellum achieves it through self-organization.
