A Chaperone for the Stator Units of a Bacterial Flagellum

Abstract

The stator units of the flagellum supply power to the flagellar motor via ion transport across the cytoplasmic membrane and generate torque on the rotor for rotation. Flagellar motors across bacterial species have evolved adaptations that impact and enhance stator function to meet the demands of each species, including producing stator units using different fuel types or various stator units for different motility modalities. Campylobacter jejuni produces one of the most complex and powerful flagellar motors by positioning 17 stator units at a greater radial distance than in most other bacteria to increase power and torque for high velocity of motility. We report another evolutionary adaptation impacting flagellar stators by identifying FlgX as a chaperone for C. jejuni stator units to ensure sufficient power and torque for flagellar rotation and motility. We discovered that FlgX maintains MotA and MotB stator protein integrity likely through a direct interaction with MotA that prevents their degradation. Suppressor analysis suggested that the physiology of C. jejuni drives the requirement for FlgX to protect stator units from proteolysis by the FtsH protease complex. C. jejuni ΔflgX was strongly attenuated for colonization of the natural avian host, but colonization capacity was greatly restored by a single mutation in MotA. These findings suggest that the likely sole function of FlgX is to preserve stator unit integrity for the motility required for host interactions. Our findings demonstrate another evolved adaptation in motile bacteria to ensure the equipment of the flagellar motor with sufficient power to generate torque for motility. IMPORTANCE The bacterial flagellum is a reversible rotating motor powered by ion transport through stator units, which also exert torque on the rotor component to turn the flagellum for motility. Species-specific adaptations to flagellar motors impact stator function to meet the demands of each species to sufficiently power flagellar rotation. We identified another evolutionary adaptation by discovering that FlgX of Campylobacter jejuni preserves the integrity of stator units by functioning as a chaperone to protect stator proteins from degradation by the FtsH protease complex due to the physiology of the bacterium. FlgX is required to maintain a level of stator units sufficient to power the naturally high-torque flagellar motor of C. jejuni for motility in intestinal mucosal layers to colonize hosts. Our work continues to identify an increasing number of adaptations to flagellar motors across bacterial species that provide the mechanics necessary for producing an effective rotating nanomachine for motility.

Binding of Phage-Encoded FlaGrab to Motile Campylobacter jejuni Flagella Inhibits Growth, Downregulates Energy Metabolism, and Requires Specific Flagellar Glycans

“…This increased proton flow would presumably disrupt proton motive force homeostasis in the cell, perhaps due to electron leakage across the electron transport chain, as has been proposed by Flint et al, which could explain the observed growth inhibition (Flint et al, 2014). To begin to examine this hypothesis, we tested whether FlaGrab could inhibit the growth of motA or motB mutants, which have a "paralyzed" flagellar phenotype as they each lack a key element of the flagellar motor (Ribardo et al, 2019). We predicted that these mutants would be bound, but not cleared by FlaGrab.…”

“…This increased proton flow might in turn disrupt the electron transport chain (as proposed by Flint et al, 2014), which could prompt cells to respond by altering transcription of energy metabolism pathways. We predicted that if this were true, flagellar motility mutants, such as motA and motB, which do not express functional flagellar motors, would resist FlaGrab-induced clearing (Beeby et al, 2016;Ribardo et al, 2019). To test this, we generated insertional mutants in each of motA and motB in C. jejuni 11168 and confirmed that these mutants were non-motile ( Figure S2).…”

Novel transient cytoplasmic rings stabilize assembling bacterial flagellar motors

“…The motors of E. coli and S. enterica have dynamic stators that continually associate and disassociate from the rotor in response to the external environment (Lele et al , 2013). The motors of some other species generate higher torque with wider, more highly‐occupied stator rings stabilized by periplasmic scaffolds and chaperones that directly interact with the stators to maintain their integrity (Beeby et al , 2016; Kaplan et al , 2019a; Ribardo et al , 2019).…”

Bacterial flagella hijack type IV pili proteins to control motility

“…Complexes containing MotB were isolated using antibodies and proteins identified by mass spectrometry. By applying a stringent criterion for interaction specificity (>2 6 -fold difference in protein amount between the anti-MotB antibody and IgG control), we identified six significant MotB interactions: PilO, PilN, PilM, a FlgX homolog that is a stator-unit chaperone in Campylobacter jejuni ( 21 ), a hypothetical protein, and MotB’s known interaction partner MotA ( Fig. 2 B and SI Appendix , Fig.…”