Researchers at the University of Osaka revealed how bacteria build the molecular bushing of the flagellar motor. The study showed that the P-ring binds to the rod before the L-ring forms. "The bacterial flagellum achieves this process with remarkable precision through self-organization," said senior author Keiichi Namba.
The bacterial flagellum is a motility organelle that enables bacteria to swim and is composed of more than 30 different proteins. Researchers have long sought to understand how this complex nanomachine self-assembles from many parts in the correct order and at the correct positions. In a study published in Communications Biology, a research team at the University of Osaka revealed the mechanism by which the flagellar motor assembles its bushing around the drive shaft.
The LP-ring acts as a molecular bushing surrounding the drive shaft called the rod and supports high-speed rotation of the rod with low friction. During motor assembly, the rod first polymerizes, after which the P-ring forms around a specific part of the rod. The L-ring subsequently assembles on top of the P-ring, and the completed LP-ring is released from the rod surface to become a bushing.
"Because the rod rotates within the mature LP-ring without physical contact, it has long been unclear how the LP-ring is accurately positioned during assembly and how it is later released from the rod," says Tomoko Yamaguchi. "To understand how the LP-ring assembles, we needed structural information on the assembly intermediate."
To capture the assembly intermediate, the researchers analyzed a polyrod–P-ring complex (PaPR), in which multiple P-rings assemble around genetically modified elongated rods. Using electron cryomicroscopy (cryoEM) single-particle analysis, they found that the P-ring initially adopts a slightly elliptical and tilted conformation to increase the contact area with the rod surface to bind to a specific position of the rod.
Then, upon formation of the L-ring, steric hindrance arises between the rod and the L-ring, causing the entire LP-ring to detach from the rod surface and become a functional bushing.
"Artificially reproducing the autonomous assembly of a complex biomolecular machine remains a major technological challenge," says the senior author Keiichi Namba. "The bacterial flagellum achieves this process with remarkable precision through self-organization."
The newly identified mechanism for constructing and releasing a molecular bearing provides important design principles for self-assembling nanomachines and could contribute to future nanotechnology applications, including highly efficient drug delivery systems (DDSs).