Conserved machinery of the bacterial flagellar motor.

Novel periplasmic and cytoplasmic structural modules of the bases of bacterial flagella have been observed in situ and isolated using new biochemical protocols. Flagellar rotation may depend upon interactions of these modules with the intramembrane particle rings, a ubiquitous feature of flagellar bases necessary for torque generation. The outer membrane-associated basal disk of the Wolinella succinogenes polar flagellum has architecture well suited for interaction with the ring particles. However, antibody against the main W. succinogenes basal disk protein did not cross-react with flagella-enriched fractions from Salmonella typhimurium and Bacillus firmus; nor have such structures been observed in these species thus far. Antibodies against two S. typhimurium proteins, FliG and FliM, known to be involved in motor function and part of the cytoplasmic module in this species cross-reacted with flagella-enriched fractions from both W. succinogenes and B. firmus. In addition, flagellar cytoplasmic structure could be isolated from B. firmus. The basal disk may anchor the flagellar motor to the cell wall in some polar bacteria, but this does not seem to be a unique strategy. In contrast, the data indicate that the cytoplasmic module is conserved.

Nucleotide sequence of the Wolinella succinogenes flagellin, which contains in the antigenic domain two conserved regions also present in Campylobacter spp. and Helicobacter pylori.

Wolinella succinogenes possesses one polar flagellum, which shows a characteristic surface pattern of parallel lines along the axis of the filament in electron microscopic images. We determined the gene sequence of the Wolinella flagellin, which is, as in most other bacteria, the only structural component of the filament. Sequence comparison with other members of the Proteobacteria revealed two highly conserved regions in the central part of the flagellin molecule among Campylobacter spp. and Helicobacter pylori, an area that had previously been described as highly variable. Similar surface patterns are found in related polarly flagellated bacteria, but not in Escherichia coli and Bacillus subtilis, which also lack these conserved regions.

An archimedian spiral: the basal disk of the Wolinella flagellar motor.

The motor that powers the rotation of the bacterial flagellum reaches through both membranes into the cytoplasm of Gram-negative bacteria. The flagellum is connected by a flexible link (hook) to the motor axis, which passes through the center of a structure called the basal disk. The basal disk functions with the L-P ring complex as a bushing, enabling the rotation of the motor in the cell wall. The protein subunits of the basal disk of Wolinella succinogenes form an Archimedian spiral. The polymerization of subunits from a nucleation point at the motor in the form of a spiral allows constant growth of the basal disk. The disk is thought to provide a reinforcement at the flagellar insertion at the cell pole and to disperse forces that are generated by the momentum of the flagellar rotation.

Location of the basal disk and a ringlike cytoplasmic structure, two additional structures of the flagellar apparatus of Wolinella succinogenes.

The basal body of Wolinella succinogenes consists of a central rod, a set of two rings (L and P rings), a basal disk from 70 to 200 nm in diameter, and a terminal knob. In negatively stained preparations of flagellar hook-basal body complexes, some disks remain fixed perpendicularly to the grid and show that such a disk is located on the distal side of the P ring. The basal disks have been isolated with and without the P ring; in both cases there is a hole in the center of the disk. The diameter of the disk is smaller in the presence of the P ring. The L-P ring complex is therefore assumed to be a bushing for the rod. Thin sections of whole bacteria and spheroplasts reveal that the disk is attached to the inner surface of the outer membrane. At the insertions of the flagellar hook-basal body-basal disk complexes, depressions are visible in negatively stained preparations of whole bacteria and spheroplasts. A new ringlike structure is connected to an elongation of the basal body into the cytoplasm in both preparations. Its diameter (60 nm) is larger than that of the M ring. A heavily stained compartment can be seen in between the new ringlike structure and the basal disk, which may be formed by the energy transducing units.