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David Blair

Professor of Biology

B.A. Princeton University

Ph.D. California Institute of Technology

Research

References

blair@bioscience.utah.edu

David Blair's PubMed Literature Search

Research

Many species of bacteria swim by means of flagella, thin helical propellers driven by rotary motors in the cell membrane.  The flagellar rotary motors can turn at 100,000 rpm or faster, obtaining energy from the membrane gradient of protons or, in some species, sodium ions.  The motors can turn either clockwise or counterclockwise, and by controlling the directions of motor rotation in response to environmental cues, cells can move toward nutrients or other environmental factors (e.g. temperature) that favor their survival.  Flagellar motility has a role in the virulence of many bacterial pathogens, and serves as a model for the many biological energy conversions that occur at membranes.  Our goal is to understand the structure and molecular mechanism of the flagellar motor.

About 50 proteins function in the assembly and operation of flagella.  Only five proteins – FliG, FliM, FliN, MotA, and MotB – are directly involved in rotation and directions switching of the motor.  MotA and MotB form membrane-bound complexes that conduct ions and function as the stator (non-rotating part) of the motor.  FliG, FliM, and FliN form a large (ca. 4 mega-Dalton) complex that functions as rotor.  Proton flow through the stator is believed to drive conformational changes, which are coupled via electrostatic interactions to the rotor to propel rotation.  Recent biochemical and structural studies have revealed key features of the stator and rotor proteins, allowing us to propose a model for the rotor-stator interface (figure 1).

We have recently learned how to purify the stator complexes in intact form, and future studies will seek to examine its conformational changes (the motor’s “powerstroke”) in vitro, and to determine its structure.  High-resolution information has already been obtained for a small pert of the rotor; futures efforts will focus on solving additional structures and determining how they fit together to form the entire rotor.  Switching between CW and CCW directions is thought to occur in the rotor, and a particular goal is to understand the switching process by combining structural information with information from mutational and physiological studies.

 

Hypothesis for the location and orientation of FliG subunits on the rotor of the flagellar motor. (A). Side-view. Two of the ca. 25 present in each motor are indicated. The shaded part is the C-terminal domain whose structure was recently determined (ref. 1). This part of FliG has a prominent ridge that contains the key charged residues and that we propose forms the outer edge of the rotor. (B). Plan view, showing the proposed arrangement of FliG subunits and with charges indicated on 3 subunits. Structural and mutational studies show that a large number of charged residues (ca. 125) are present at the edge of the rotor, and that this feature is essential for motor function.

References