Mechanical feedback triggers switching of a molecular motor
Researchers of the FOM-institute Amolf have developed a mathematical model that can explain the preference of a bacterial motor to switch its rotation direction after a fixed period of 0.2 to 0.3 seconds. As it turns out, switching of the motor that drives a bacterial flagellum is not only steered by messenger molecules, as was assumed until now, but also by mechanical feedback from the flagellum to the motor. The researchers publish their results in the coming edition of Molecular Systems Biology.
Despite the large number of patents, human beings are not the only species that use motors. Several types of bacteria, among which Escherichia coli (that lives in our lower intestines), move around by means of flagella, each driven by its own tiny electromotor. While the structure of the motor is largely known, the way it works is still subject of debate. In recent experiments, the motor's switching behaviour has for the first time been studied rigorously and based on those experiments a team at AMOLF developed a mathematical model for the rotation and switching of the motor
A bacterial motor has two gears: a forward and a reverse. With all motors in their forward gears, the flagella form a bundle that propels the bacterium forward. Whenever the swimming direction is unfavourable, for example as a consequence of a decreasing amount of food, messenger molecules make one or more motors switch to their reverse gear for a short period of time, causing the corresponding flagella to leave the bundle and the bacterium to tumble. The bacterium then continues to swim in the random direction it ended up in.
Bacterial motors are popular objects of research, not only because they are so incredibly small - the rotor (the moving part) consists of a ring of only 26 protein molecules - but also because the efficiency of the motor lies near 100%. The energy source of the motor consists of protons that continually move into the cell as a consequence of a pH difference. The stator elements of the motor (the static parts) change their shape by binding protons and as a consequence they push the rotor around. The rotor proteins can also change their shapes; this corresponds to the switching of the motor between the forward and reverse gear.
Recent experiments surprisingly showed that the motors are strongly biased to switch after a fixed period of 0.2 to 0.3 seconds. AMOLF researchers have developed a mathematical model that can explain this finding in detail. An important prediction of the model is that the motor has a higher probability to switch direction when a larger force is exerted on it. According to the researchers, this ingredient, in combination with the elastic properties of the flagellum, is responsible for the characteristic switching time of the motor. When the motor has just switched direction, the spiral-shaped flagellum starts to unwind and the force on the motor decreases. However, as the flagellum starts to wind up in the new direction, the force, and therefore also the probability to switch, increase. After 0.2 to 0.3 seconds, the force on the motor has increased so much that the motor soon switches direction. Motor switching is therefore not only triggered by messenger molecules, as was assumed until now, but also by mechanical feedback from the flagellum to the motor.
Reference
'The switching dynamics of the bacterial flagellar motor', Siebe van Albada, Sorin Tanase-Nicola, and Pieter Rein ten Wolde, Molecular Systems Biology.
http://www.nature.com/msb/
Contact
Prof.dr. Pieter Rein ten Wolde (Group leader Biochemical Networks) or Melissa van der Sande (Communications department Amolf). Tel: +31 (0)20 75 47 100.