Structure of Vibrio FliL, a New Stomatin-like Protein That Assists the Bacterial Flagellar Motor Function

Abstract

Many motile bacteria swim or swarm using a filamentous rotating organelle, the flagellum. FliL, a component protein of the flagellar motor, is known to enhance the motor performance under high-load conditions in some bacteria. Here we determined the structure of the periplasmic region of FliL (FliLPeri) of the polar flagellum of Vibrio alginolyticus. FliLPeri shows a remarkable structural similarity to the stomatin/prohibitin/flotillin/HflK/C (SPFH) domain of stomatin family proteins, some of which are involved in modulation of ion channel activities in various organisms. FliLPeri forms a ring assembly in the crystal with an inner diameter of around 8 nm, which is comparable to the size of the stator unit. Mutational analyses suggest that the FliL ring forms a complex with the stator unit and that the length of the periplasmic linkers of FliL and the stator B-subunit is essential for the complex formation. We propose a model of the FliL-stator complex to discuss how Vibrio FliL modulates stator function in the bacterial flagellar motor under conditions of high viscosity. IMPORTANCE Some flagellated bacteria regulate motor torque in response to the external load change. This behavior is critical for survival, but the mechanism has remained unknown. Here, we focused on a key protein, FliL of Vibrio alginolyticus, and solved the crystal structure of its periplasmic region (FliLPeri). FliLPeri reveals striking structural similarity to a conserved domain of stomatin, which is involved in ion channel regulation in some organisms, including mammals. FliLPeri forms a ring with an inner diameter that is comparable in size to the stator unit. The mutational analyses suggested that the presence of the ring-like assembly of FliL around the stator unit enhances the surface swarming of Vibrio cells. Our study data also imply that the structural element for the ion channel regulation is conserved from bacteria to mammals.

Protein expression in the obligate hydrocarbon‐degrading psychrophile Oleispira antarctica RB‐8 during alkane degradation and cold tolerance

“…Flagella assembly is controlled by FleQ, an NtrC‐like transcriptional regulator which controls synthesis of flagellar genes and its expression, in other flagellated bacteria, was shown to be cold‐induced, based on an 83% decrease in fleQ transcripts at 30°C compared with 4°C (Soutourina et al ., ). The flagellar protein, FliL, which is a single‐transmembrane protein with a large periplasmic region and associates with the flagellar basal body, has been found to be important for the function of the motor under high‐load conditions, such as highly viscous environments, by recruiting or stabilizing the stators or by increasing their efficiency, leading to greater torque generation (Partridge et al ., ; Takekawa et al ., ). Flagellin is the major structural protein of the flagella filaments, which in O .…”

Characterization of FliL Proteins in Bradyrhizobium diazoefficiens: Lateral FliL Supports Swimming Motility, and Subpolar FliL Modulates the Lateral Flagellar System

“…The motor of this spirochete also possesses a particular structure, called the collar, to which FliL is associated (18). Furthermore, biochemical and X-ray crystallography studies in V. alginolyticus suggested that rings of FliL might associate around stator units (19,20). From these and other studies, it may be concluded that FliL is a small membrane protein that is part of the flagellar basal body and that might have a role in stabilizing the stator (21).…”

“…In addition to the paucity of studies on the role of FliL in alphaproteobacteria, there are no comparative works in bacterial species possessing two flagellar systems except for recent studies in V. alginolyticus (19,20). Since B. diazoefficiens uses its two flagellar systems for swimming (2), this species brings an opportunity to compare the role of FliL in each system for swimming.…”

Living in a Foster Home: The Single Subpolar Flagellum Fla1 of Rhodobacter sphaeroides

“…Besides, in Vibrio the proteins MotY and MotX form the periplasmic T-ring that interacts with PomB (equivalent to MotB in Vibrio) and stabilize the stator complexes [25,26]. Activation of the proton channel requires extensive remodeling of the periplasmic region of MotB [27][28][29][30][31][32], and it has been proposed that the flagellar protein FliL participates in this process [33][34][35][36][37].…”