Many cellular processes are highly spatially ordered, with spatial separation regulated by cellular factors called landmark proteins. Examples of compartmentalized processes are those involved in bacterial motility. In bacteria, swimming and swarming require the formation of flagella - long, rotating, helical filaments driven by a membrane-embedded motor. Landmark proteins likely regulate the numerous flagellation patterns found in a variety of bacterial species. In polarly flagellated bacteria, primarily encountered in marine habitats, the SRP-like GTPase FlhF and the MinD-like ATPase FlhG are known to control flagellar positioning and number. HubP, another polar factor identified in Vibrio cholerae, was shown to be involved in the localization of proteins which are a part of other cellular processes such as chemotaxis, enabling the cell to navigate efficiently towards more favorable conditions.
The polarly flagellated gammaproteobacterium Shewanella putrefaciens CN-32 possesses two flagellar systems encoded in two gene clusters, enabling the cell to form a single polar and multiple lateral flagella. However, genes for only a single chemotaxis system are located on the chromosome. The primary polar system is required for the main propulsion of the cell. Since only the motor switch protein of the polar system, FliM1, harbors the binding domain of the chemotaxis response regulator CheY, the chemotaxis
system also acts exclusively on this flagellar motor. Secondary, lateral flagella enable the cell to turn more efficiently by biasing the directional changes of the swimming cell towards smaller turn angles. This leads to a higher directional persistence in the swimming path of the cell. Since the positions of both the polar and lateral flagella play key roles in this special movement pattern, the mode of action of the regulators FlhF and FlhG on the dual flagellation was examined. While FlhF determinates the position of the nascent flagellum by recruiting flagellar components to the cell pole, direct interaction, likely at the cell pole, of FlhF and FlhG estricts polar accumulation of FlhF by stimulating its GTPase activity. The placement of the lateral flagellar system seems to be FlhF-independent. In addition to interaction with FlhF, FlhG was shown to be involved in the assembly of the cytoplasmic portion of the flagellar motor. For this purpose, FlhG binds FliM1 at the binding motif also recognized by CheY. As the motor switch protein of the lateral system, FliM2, lacks this binding domain, lateral flagella assemble independently of FlhG. Since FlhG was also shown to act on flagellar transcription, polar localization of FlhG might form a part of a feedback loop regulating flagellar transcription and assembly.
In V. cholerae, the polar landmark HubP was shown to interact with both FlhF and FlhG. The ortholog of V. cholerae HubP was identified in S. putrefaciens and affected its flagella-mediated motility. In addition to its interaction with FlhFG and the chemotaxis system, SpHubP and VcHubP appears to be involved in chromosome segregation and polar recruitment of other yet unidentified factors.
These results indicate that the polar flagellar system requires the presence of several factors to assemble a functional flagellum and to function in concert with the chemotaxis system. These factors do not affect assembly and function of the lateral flagellum, which seems to assemble stochastically and independently.