POSTERS - BLAST X - University of Utah

BLAST X Poster #52
EFFECT OF INTRACELLULAR pH ON THE TORQUE-SPEED RELATIONSHIP OF
BACTERIAL PROTON-DRIVEN FLAGELLAR MOTOR
Shuichi Nakamura 1,2 , Nobunori Kami-ike 2 , Jun-ichi Yokota 1,2 , Seishi Kudo 3 ,Tohru Minamino 1,2 ,
and Keiichi Namba 1,2
1 Graduate School of Frontier Bioscience, Osaka University 1-3 Yamadaoka, Suita, Osaka 565-
0871, Japan, 2 Dynamic NanoMachine Project, ICORP, JST, 1-3 Yamadaoka, Suita, Osaka 565-
0871, Japan, 3 Faculty of Engineering, Toin University of Yokohama, 1614 Kurogane-cho, Aoba,
Yokohama 225-8502, Japan
The flagellar motor of Salmonella is a rotary nanomachine driven by proton motive force.
It has been shown that an increase in the intracellular proton concentration abolishes the motor
function, suggesting that the stator complex has intracellular proton-binding sites by which
intracellular protons kinetically interfere with the rate of proton translocation for the motor
rotation. In this study, to understand the coupling mechanism of proton flux with torque
generation, we have investigated the effect of intracellular pH on the rotation rate of the flagellar
motor. Intracellular pH was manipulated by adding potassium benzoate, which crosses the
cytoplasmic membrane in the neutral form and equilibrates the intra- and extracellular pH
without changing proton motive force. We showed that the rotation rates at low loads sharply
decreased as the external pH decreased in the presence of benzoate, suggesting that a high
intracellular proton concentration lowered the proton translocation rate. Also, computer
simulation by a simple kinetic model suggested that the decrease in intracellular pH interferes
with proton dissociation from an intracellular proton-binding site of the stator. We conclude that
the intracellular pH is a critical factor that determines the flagellar rotation rate and that proton
dissociation from the motor is a rate-limiting step in the mechanochemical reaction cycle.
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