Laboratory of Macromolecular Structure
Graduate School of Science

Professor

Katsumi IMADA

Assistant Professor Nobuhiro TAKEKAWA

kimadatakekawan16

URL: http://www.chem.sci.osaka-u.ac.jp/lab/imada/In living organisms, molecular machines
made up of biomacromolecules playessential roles in various chemical
reactions and biological functions tomaintain life. Unlike artificial machines,
biological molecular machines function
with high precision but with highflexibility. The flagellar system and proteintransport system of bacteria are typicalexamples of biomolecular machines. We
study the molecular mechanisms offunction and formation of suchbiomolecular machines through 3D
structural analysis and reconstruction ofmolecular machines.
Elucidation of molecularmechanisms of flagellar formation
and rotation.
The flagellum, a locomotive organelle ofbacteria, is the first rotational system
discovered in living organisms. At the baseof the flagellum, there is a motor about 45
nm in diameter made up of many protein
molecules. The motor is driven by theproton or sodium ion influx across the cell
membrane. The proton motor rotates atabout 300 rps and the sodium ion motor at
1500 rps. The motor can rotate in reverse
direction, and the rotation direction isswitched by a signal from the chemotaxis
sensor. Thereby, bacteria change theswimming direction. The molecularmechanism of rotation is obscure, although
it is thought that torque is generated by theinteraction of the stator and rotor coupledwith the ion flow through the stator, amembrane protein complex. Interestingly,
the stator is frequently replaced while themotor is rotating, and ion transmission
begins when it is assembled into the motor.
However, the molecular mechanisms ofstator association/dissociation coupled withon/off of ion transmission are unknown. To
solve these mysteries, we are working on
structural and functional analysis of theproteins that compose the chemotaxis
sensor, rotor, and stator, as well as theircomplexes.

Structure and function of thebacterial protein transport system
The bacterial flagellum is constructed on theoutside of the bacterium, and therefore theflagellar proteins synthesized inside the cellmust be transported to the outside of the cell.
A transport device is located at the base of
the flagellum to select only flagellar proteinsand deliver them to the outside of the cell at
the right time. The transport system not onlytransports the proteins, but also switches thetype of the flagellar proteins according to theformation status of the flagellum and regulates
the expression of the flagellar proteins. Thistransporter is a member of the type III
secretion system used by pathogenic bacteria
to deliver virulence factor proteins directly tohost cells during infection. Therefore, both
systems are thought to share a common
working mechanism. Although the molecularmechanism of transport is unknown, we haverevealed that the transport apparatus has an
ATPase unit structurally similar to FoF1-ATP
synthase, which has a rotational molecularmechanism.
Structure and function ofbacterial fimbriae
Porphyromonas gingivalis, amajor periodontal pathogen,have at least two types ofType V fimbriae for biofilm
formation and adhesion tohost cells. We are conducting
structural analysis andreconstruction experiments toelucidate the molecularmechanism of fimbriae
formation and adhesion.
As is the case with the workings of
biomolecular machines, there aremany things in the world that seemto be understood but really arenot. Please think carefully about

Department of Biological Sciences
Graduate School of Science, Osaka University
1-1, Machikaneyama-cho, Toyonaka,Osaka560-0043, Japan
TEL&FAX: +81-6-6850-5455

what you do not understand. A
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