Research

Bacterial flagellar motility

Most of motile bacteria swims in the liquid environment or swarms on the solid surfaces by using the motility organ, named "flagella". The flagellum consists of long helical filament extended from the cell body, the flagellar motor which is embedded in the cell surfaces at the flagellar base, and the hook that connects motor and filament. The motor rotates flagellar filament by using the ion-motive force across the inner membrane (H+ or Na+ electrochemical potential) and rotation of the filament thrust cell body in the liquid to drive cell movement. Shown left below is schematic of bacterial flagella, and shown right below is the model of flagellar motor (Na+-driven Vibrio polar flagellar motor and H+-driven Salmonella motor). We study how the bacterial cell swims or swarms, especially focusing on the energy conversion mechanism equipped within the force-generating unit, the stator. The rotor-stator interaction in the motor that couples with ion flux through the stator channel generates rotational torque, and recent structural and biophysical studies provided plausible model for energy conversion in the motor. However, still details are not well understood yet.

References:

Kojima S, Blair D.F. Int. Rev. Cytol. (2004) 233:93-134.
Terashima H, Kojima S, Homma M. Int. Rev. Cell Mol. Biol. (2008) 270:39-85.
Subramanian S, Kearns DB. Annu Rev Microbiol. (2019) 73:225-246.
Wadhwa N, Berg HC. Nat Rev Microbiol. (2022) 20:161-173.

Stator activation model

The stator, also known as "torque generating unit", is known to function as energy converter complex installed into the motor. Two membrane proteins, MotA(PomA) and MotB(PomB), form heptameric membrane protein complex. The stator conducts specific ion (H+ or Na+) to conver ion motive force to the rotational torque. Multiple stator units surrounds around a rotor ring, and stator channel opens only when incorporated into the motor and anchored around the motor. This "active" stator can transmit torque to the rotor. Based on the structural and functional studies, we proposed stator activation model in 2018.

References:

Kojima S, Imada K, Sakuma M, Sudo Y, Kojima C, Minamino T, Homma M, Namba K. Mol. Microbiol. (2009) 73:710-718.
Zhu S, Takao M, Li N, Sakuma M, Nishino Y, Homma M, Kojima S, Imada K. Proc. Natl. Acad. Sci. U. S. A. (2014) 111:13523-13528.
Kojima S. Curr. Opin. Microbiol. (2015) 28:66-71.
Kojima S, Takao M, Almira G, Kawahara I, Sakuma M, Homma M, Kojima C, Imada K. Structure (2018) 26:590-598.

Stator rotation model

In 2020, cryo-EM structures of the stator complexes were reported from two groups. Stator structure revealed that dimeric B subunit axis that is anchored to the cell wall is surrounded by a pentameric A subunit ring. Together with cryo-ET in situ structure of the motor, the stator rotation model is proposed: stator A subunit rotates around the the periphery of rotor ring via gear-like coupling motion, and it seems that is the nature of motor rotation mechanism (A). We also solved the PomAB stator structure in 2025 (B) and trying to test this model and understand detailed mechanism of stator function (C).

References:

Deme JC, Johnson S, Vickery O, Aron A, Monkhouse H, Griffiths T, James RH, Berks BC, Coulton JW, Stansfeld PJ, Lea SM. Nat Microbiol. (2020) 5:1553-1564.
Santiveri M, Roa-Eguiara A, Kühne C, Wadhwa N, Hu H, Berg HC, Erhardt M, Taylor NMI. Cell (2020) 183:244-257.e16.
Chang Y, Zhang K, Carroll BL, Zhao X, Charon NW, Norris SJ, Motaleb MA, Li C, Liu J. Nat Struct Mol Biol. (2020) 27:1041-1047.
Nishikino T, Takekawa N, Kishikawa JI, Hirose M, Kojima S, Homma M, Kato T, Imada K. Proc Natl Acad Sci U S A. (2025) 122:e2415713122.

Flagellar number and placement

Bacterial cells optimize flagellar number and placement to adapt their habitat. Some bacteria has multiple flagella from cell body (lateral flagella), and others form only a single flagellum at the cell body (polar flagellum). We study how this spatial and number regulation of flgellar morphogeneis. Our model bacterium Vibrio alginolyticus has a single polar flagellum. GTPase FlhF, ATPase FlhG and polar landmark protein HubP are involved in this regulation. We try to understand how single flagellar formation occur only at the cell pole.

References:

Kusumoto A, Shinohara A, Terashima H, Kojima S, Yakushi T, Homma M. Microbiology (2008) 154:1390-1399.
Ono H, Takashima A, Hirata H, Homma M, Kojima S. Mol. Microbiol. (2015) 98:130-141.
Kojima S, Terashima H, Homma M. Biomolecules (2020) 10:E533.

Our model bacteria: marine Vibrio (Vibrio alginolyticus 138-2)

We use model bacterium Vibrio alginolyticus. It is marine bacterium and not pathogenic to human. It possesses Na+-driven single polar flagellum when grown in liquid, and swimming really fast (~60 μm/sec). When attached on the solid surface, cells induce numerous H+-driven lateral flagella from cell body (not pole). Detailed mechanism of how this surface sensing induces lateral flagella are still in mystery, and we try to address this question.

References:

Kawagishi I, Maekawa Y, Atsumi T, Homma M, Imae Y. J Bacteriol. (1995) 177:5158-5160.
Homma M, Oota H, Kojima S, Kawagishi I, Imae Y. Microbiology (1996) 142:2777-2783.
Kawagishi I, Imagawa M, Imae Y, McCarter L, Homma M. Mol Microbiol. (1996) 20:693-699.
Uesaka K, Inaba K, Nishioka N, Kojima S, Homma M, Ihara K. Peer J. (2024) 12:e17126.

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