Structural insights into the assembly mechanism of the molecular bushing in the bacterial flagellar motor

The LP-ring complex of the Salmonella flagellar motor acts as a bushing that supports high-speed rotation of the rod, which serves as a drive shaft. The L-ring, P-ring, and distal rod consist of FlgH, FlgI, and FlgG, respectively. LP-ring formation begins with P-ring assembly around the distal rod, where it remains firmly attached until the L-ring assembles above it. L-ring formation induces P-ring detachment, allowing the rod to freely rotate within the LP-ring, but this mechanism remains unclear. Here, we report the cryoEM structure of the P-ring assembled on a polyrod, an unusually elongated rod structure. The P-ring exhibits a slightly elliptical shape with a 2.7° tilt relative to the rod axis for its stable attachment. However, L-ring assembly on the P-ring causes steric clashes with the rod to induce P-ring detachment, allowing it to adopt a circular shape and complete the LP-ring as a functional bushing.