In this sample of e.coli you can see some bacteria swimming in a straight line and others tumbling around. The swimming style depends on which way the bacteria is turning its motors. Spin in one direction the helical filaments bundle together and produce forward motion, spin in the other and the filaments spread out and the bacteria tumbles on the spot. The bacteria uses this to change direction. If there isn’t much food about the bacteria will tumble more often to try and find a direction where there are more nutrients.
Motor proteins such as kinesins or dyneins carry cargoes along microtubules within the cell. A microtubule is built from αβ tubulin dimers, and its structure allows the two ends, called the + and – ends, to be distinguished: β-tubulin is exposed at the + end and α-tubulin is exposed at the - end. The local structure of a microtubule controls the direction that the motor takes. Kinesin and dyneins recognize the orientation of αβ subunits and always move towards the same end.
Video: Kinesin-covered surface transporting microtubules; the minus ends (green labelled) always lead the movement.
Molecular Machinery of life
Naturally occurring mini motors, such as the bacterial flagellar motor, are typically made from proteins. Unfortunately proteins are very complicated. They have twenty different kinds of subunit (amino acids), and the forces which determine their three-dimensional structure are exceedingly complex. This makes it very difficult to design proteins from scratch, but DNA is comparatively simple to work with, since there are only four types of subunit, which pair up in a highly predictable fashion. As a result there are a number of scientists – including several in the Mini Motors team - working on DNA motors, tiny locomotive devices made out of DNA. Here, we are taking DNA out of its usual biological context and using it to perform a different function. Rather than acting as an information store for a living organism, here DNA is the material out of which the motors are made, and also the fuel to drive them.