Bacteria get around. They move in
sometimes mysterious ways. One of the most mysterious is via the pilus. To get
ahead using pili, a pilus is thrown forward, like a grappling hook. After
touchdown, the pilus is retracted back and the cel is dragged along. But how does
the cell make a grappling hook and pull it back in? Researchers used electron cryotomography to take many many pictures of the pilus machinery. Then using genetic
techniques they took away some components and they labeled others, mapping the location of each protein in the structure. With this new understanding of
the structure, they proposed a mechanism for how the pilus gets sent out and
brought back. When activated, a motor-like protein adds units together into a chain, which pushes out through the membrane. When the extending chain touches the
surface, the proteins change their shape– sending a message back to the machinery.
In response to this message, a different motor reverses the process–
retracting the pilus and pulling the cell along. Taking into account the size and forces at
work here, the pilus motor is probably one of the
strongest molecular motors around.